2005 PontiacGrand Am GT misfiring

hey guys, I have a 2005 grand am 3.4L, it misfires only under hard acceleration, but is starting to feel a little slugish all the time. Getting code PO302(misfire on number 2 cylinder) and PO300(random Misfire) spark plug and wires were changed a few months ago. Thinking maybe fuel filter or coil problems. Any ideas

do you have the freeze frame readings for the misfire, if so look at the adaptive readings for the fuel trims, let us know what they are, it will tell us if it's a fuel/o2 sensor cause.

it says there are no freeze frames available.

well, now you'll have to swap plug, coil and injector, one at a time from the #3 cyl. to another cylinder and see if the misfire follows them.

If you don't have access to an oscilloscope or a five gas analyzer you might be stuck using some relatively primative methods. There are a number of things that can cause this. I know you replaced the plugs and wires but they could still be the problem. Check to see if the plug is cracked. You might even wish to swap it to another cylinder for testing purposes to se if the problem jumps to another cylinder. It's also possible that you have enough wiggle room to swap wires as well You can also try inducing the problem with a spray bottle and some water. Don,t drown the whole engine comp. Just spray it on the plug wires to see if the symptom gets worse. It's not super common but over the years I have had new wires that have a breach in the insulation. Part stores drop plugs and put them right back on the shelves etc.

Chevy,

The first thing is to make sure that the fuel pressure PSI is what it should be. To do that, you have to use a fuel pressure gauge, and must match the PSI stats below.

Static fuel pressure must be 52-59 PSI

Fuel pressure at normal operating temperature and at idle must be 42-56 PSI

With return fuel line momentarily cut-off fuel PSI must be 59

If the above check out, then remember that vacuum leaks cause misfires, so post the results of an engine vacuum pressure test via a vacuum pressure gauge.

Also don’t loose track of the fact that burnt exhaust valves can cause misfires, so post the results of a cylinder leak down test.

When Big Block said momentarily he meant it. Checking the peak pressure of the pump is an important test but it can be the last straw for a weak fuel pump. Do not dead head the pump very long.

Chevy,

One easy thing to check for with Big Blocks vacuum test is to look for rapid fluctuation of the gauge. this is often indicative of a valve problem. An old shade tree and I mean a shady shade tree method is to take a dollar bill and hold it near the tail pipe at idle. If it sucks the bill to the pipe you've likely got a bad exhaust valve. You can also check at the throttle body for air blowing back out. Be very careful. If you let anything get sucked into that motor you won't be questioning a bad valve. You will have created one. Not sure that any of that will pan out for you as you said your complaint was mainly under hard acceleration. Gotta cover all the bases though.

When Big Block said momentarily he meant it.

Good point drivability.

Chevy,

A dead head pressure test checks the maximum fuel pressure of the fuel pump. With the return fuel line momentarily pinched off, the pump has gotta produce two times its normal operating pressure than it would at idle. If the pressure doesn't go up with the return line pinched off, then the pump most likely won’t be able to deliver a high enough volume of fuel at higher engine speeds. A few reasons for this are a worn pump or weak fuel pump, incorrect (low) voltage at the pump, a clogged fuel filter, a clogged or collapsed sock screen at the pump, a near empty fuel tank, or kinked fuel lines. Regarding the above an, 8x8 piece of newspaper works better than a dollar bill.

Chevy,

Does your scan tool show a data stream. The reason I ask is because if it does you can monitor the fuel trim. It would be ideal to have a capture of the freeze frame data but if you don't have it maybe you could watch it in "real time". Your scan tool may even have a record feature. Let us know.

my scan tool is pretty cheap, i am ging to try and borrow a better one from a friend and see if i can get some better results.

if the code always comes back with p0303, i'd swap out the firing componets, i think your going to have to anyway.

if the code always comes back with p0303, i'd swap out the firing componets, i think your going to have to anyway.

I agree, but you gotta understand that the above is secondary to adequate and proper fuel pressure.

I agree Fuel pressure does take priority over one single plug or wire as far as diagnostics goes. Nothing should be ruled out at this point. You could have more than one problem. You said it missfires under load but you also said it has started feeling sluggish all the time. Fuel pressure could be playing a part or be the only problem. Also, you didn't mention anything about a P0420 or anything like that but if it's been missing for a while another possible problem could be that the unburned fuel has started to restrict the cat. I'd check spark and fuel first and see how she does.

Hello Ccortiz,

My name is Helene. Big Block 409 is my boyfriend, and over the years I’ve learned a quite a number of things from him. A few of those things have had to do with auto mechanics and repair as well (LOL). Don’t be fooled for one second sir. Everything you have ever heard about a Marine is more than true, and Jason simply personifies what it means to be a Marine. Big Block 409 (aka Jayson) thought it would be interesting if I posted my opinion, regarding your problem, so I spent some time reading the answers and comments. All I can add, is that you shouldn’t think (or believe) that your misfire is being caused by a fuel injector problem. Many individuals make that mistake, and swap out fuel injectors, in the same manor that they would swap out spark plugs and ignition cables, in the hopes that the misfire will follow. It simply doesn’t work that way. In your case a defective fuel injector is not setting PO302 or PO300.

Thank You
Helene

Helene,

Enjoyed reading your post and agree that the injector is most likely not the problem but Ccortiz owns the 2002 Tahoe with the knock sensor question. This is chevyman on this post. Big Block (now known as Jason) and you must be having some fun today. Big Block got him a lady with some car sense. I like it.

i'm going to have to disagree, a plugged or slightly clogged injector will cause a misfire code and may not set a injector code, it may not be hurting the cat., so no flashing light, but still a misfire.
here's a bulletin from gm on this car that will show prove this.

Fuel System - Driveability Issues/MIL/Multiple DTC's

TECHNICAL

Bulletin No.: 03-06-04-030G

Date: April 22, 2009

Subject:
Various Driveability Symptoms Due to Clogged Fuel Injectors, MIL/SES DTCs P0171, P0172, P0174, P0300, P1174, P1175 (Clean Fuel Injectors and/or Perform Injector Test With AFIT CH-47976)

Models:
2005-2009 GM Passenger Cars and Light Duty Trucks
2005-2009 HUMMER H2
2006-2009 HUMMER H3
2005-2009 Saab 9-7X

Equipped with Engine RPOs listed in the Table above and MULTEC(R) 2 Fuel Injectors

Attention:
GM does not support cleaning injectors on any engines that are not listed in this bulletin. Engines other than the ones listed in this bulletin that diagnosis indicates having restricted injectors should have those injectors replaced.

Supercede:
This bulletin is being revised to update the model year to 2009 and to provide applicable engine RPO table. Please discard Corporate Bulletin Number 03-06-04-030F (Section 06 - Engine/Propulsion System).

Condition

Some customers may comment on any of the following various driveability symptoms:

- Extended Crank Time

- Hard to Start

- MIL/SES Illuminated with DTCs

- Hesitation

- Lack of Power

- Surge or Chuggle

- Rough Idle

- Light or Intermittent Misfire

Cause

Due to various factors, the fuel injectors may become restricted. Extensive testing has demonstrated that fuel related issues are the cause of clogged injectors. At this point, no specific fuel, fuel constituent, or engine condition has been identified as causing the restriction. The restriction causes the engine to operate at a lean air fuel ratio. This may either trigger the MIL to illuminate or the engine to develop various driveability symptoms.

Correction

Fuel injector restrictions, deposits can be cleaned on the vehicle using the following procedure. Under NO circumstances should this procedure be modified, changed or shortened. As a long term solution, and to prevent reoccurrence, customers should be encouraged to use Top Tier Detergent Gasoline. For further information on Top Tier detergent gasoline and fuel retailers, please refer to the following Corporate Bulletin Numbers:

- 04-06-04-047G (U.S. Only)
- 05-06-04-022D (Canada ONLY)

Notice
GM UPPER ENGINE AND FUEL INJECTOR CLEANER is the only injector cleaning agent approved for use with General Motors fuel system components. Other injector cleaners may cause damage to plastics, plated metals or bearings. General Motors has completed extensive laboratory testing of GM Upper Engine and Fuel Injector Cleaner, and can assure its compatibility with General Motors fuel system components, as long as the cleaning procedure is followed correctly.

Injector Cleaning Procedure

The following tools, or their equivalent, are required:

- CH-47976 Active Fuel Injector Tester (AFIT)
- J 35800-A Fuel Injector Cleaner
- J 37287 Fuel Line Shut-off Adapter
- J 42964 Fuel Line Shut-off Adapter
- J 42873 Fuel Line Shut-off Adapter
- * One bottle of GM Upper Engine and Fuel Injector Cleaner, P/N 88861802 (in Canada, P/N 88861804)
- * One bottle of GM Fuel System Treatment Plus, P/N 88861011 (in Canada, P/N 88861012)

Active Fuel Injector Tester (AFIT- CH-47976)

Some dealers may not have an Active Fuel Injector Tester (AFIT- CH-47976). Dealers can contact to order an AFIT- CH-47976. Dealers still can test the fuel injectors without an AFIT. Refer to Fuel Injector Diagnosis (w/ J 39021 or Tech 2(R)) in SI.

Important
As mentioned in the AFIT User Guide, vehicles that are not listed in the AFIT menu can still be tested with the AFIT. Depending on the model, it may be possible to enter the previous model year and proceed with testing using the DLC connection. If this is not possible on the model that you are working on, it will be necessary to use the direct connection method outlined in the AFIT User Guide (See Pages 17-31).

General Motors recommends that the Active Fuel Injector Tester (AFIT) be used in testing fuel injectors. If the SI diagnostics do not isolate a cause for this concern, use the Active Fuel Injector Tester (AFIT - CH-47976) to perform an "Injector Test" as outlined in the AFIT User Guide.

The AFIT "Injector Test" measures the flow characteristics of all fuel injectors, which is more precise when compared with the standard Tech 2(R) fuel injector balance test. As a result, the AFIT is more likely to isolate the cause of a P1174 DTC (for example: if it is being caused by a fuel injector concern).

The CH-47976 (Active Fuel Injector Tester - AFIT) can also be used to measure fuel pressure and fuel system leak down. Also, as mentioned in the P1174 SI diagnosis, if the misfire current counters or misfire graph indicate any misfires, it may be an indicator of the cylinder that is causing the concern. Refer to Fuel Injector Diagnosis (w/CH-47976) in SI for additional instructions.

Training (U.S.)

To access the training video on AFIT, take the following path at the GM Training Website:

1. After logging into the training website, choose the link on the left side of the page titled "web video library."

2. Then choose "technical."

3. Next, within the search box, type in September course number "10206.09D.

4. This will bring up a link with this course. Scroll through to choose "feature topic."

5. At this point, the seminar can be chosen to view or the video related to the AFIT.

Additional training is available from the training website. Please see TECHassist 16044.18T2 Active Fuel Injector Tester and also see 16044.14D1 GM Powertrain Performance for more information on GM Upper Engine and Fuel Injector Cleaner.

Also, dealers can now download software updates for the AFIT at GM Dealer Equipment (GMDE).

Training (Canada)

To access the training video on AFIT, take the following path at the GMPro LMS Training Website:

1. After logging into the website, choose the link on the left side of the page titled "Catalog."

2. Then choose "Catalog Search."

3. Next, within the search box, Select Course Number - Contains - "T" then select search.

4. This will bring up a list of TECHassist courses. Scroll through to choose "Active Fuel Injector Tester" and select "View."

5. At this point, a new window will open and the program can be Launched.

Also, dealers can now download software updates for the AFIT at GM Dealer Equipment (GMDE).

Techlink

Additional information can be found on AFIT (June 2006 Edition) and GM Upper Engine and Fuel Injector Cleaner (November 2006 Edition) in Techlink. To access the articles, take the following path:

1. Go to GM DealerWorld (U.S.) or the GM GlobalConnect (Canada).

2. Click on the Service Tab in DealerWorld (in Canada, click Technican Resources in the Service Library of GM GlobalConnect).

3. Click on the GM Techlink Hyperlink.

4. Click on the Archives Hyperlink at GM Techlink.

- Click on 06-2006 in the Archives Section and Click on the Active Fuel Injector Tester Link in the June 2006 Techlink Article.

- Click on 11-2006 in the Archives Section and Click on the GM Top Engine Cleaner Replaced Link in the November 2006 Techlink Article.

Injector Cleaning Procedure

Notice
GM UPPER ENGINE AND FUEL INJECTOR CLEANER is the only injector cleaning agent recommended. DO NOT USE OTHER CLEANING AGENTS AS THEY MAY CONTAIN METHANOL, WHICH CAN DAMAGE FUEL SYSTEM COMPONENTS. Under NO circumstances should the GM Upper Engine and Fuel Injector Cleaner be added to the vehicle fuel tank.

Do not exceed the recommended cleaning solution concentration. Testing has demonstrated that exceeding the recommended cleaning solution concentration does not improve the effectiveness of this procedure.

Important
Vehicles with less than 160 km (100 mi) on the odometer should not have the injectors cleaned. These vehicles should have any out of specification injectors replaced.

1. For 4, 5 and 6 cylinder engines, empty two of the 30 ml (1 oz) reservoirs of the GM Upper Engine and Fuel Injector Cleaner container into the J 35800-A - Injector Cleaning Tank then add 420 ml (14 oz) of regular unleaded gasoline. If you are using any other brand of cleaning tank, you will need a total of 60 ml (2 oz) mixed with 420 ml (14 oz) of regular unleaded gasoline.

2. For 8 cylinder engines, empty two of the 30 ml (1 oz) reservoirs of the GM Upper Engine and Fuel Injector Cleaner container into the J 35800-A - Injector Cleaning Tank then add 420 ml (14 oz) of regular unleaded gasoline. If you are using any other brand of cleaning tank, you will need a total of 60 ml (2 oz) of Upper Engine and Fuel Injector Cleaner mixed with 420 ml (14 oz) of regular unleaded gasoline. This procedure will need to be repeated for a second time for an 8 cylinder engine (8 cylinder engines receive 960 ml total fluid - 120 ml (4 oz) of Upper Engine and Fuel Injector Cleaner and 840 ml (28 oz) of gasoline.

3. Be sure to follow all additional instructions provided with the tool.

4. Electrically disable the vehicle fuel pump by either removing the fuel pump fuse or the fuel pump relay and disconnecting the oil pressure switch connector, if equipped.

5. Turn the ignition to the OFF position.

6. Relieve fuel pressure and disconnect the fuel feed and return lines at the fuel rail. Plug the fuel feed and return lines coming off the fuel rail with J 37287, J 42873 or J 42964 as appropriate for the fuel system.

7. Connect the J 35800-A to the vehicle fuel rail.

8. Pressurize the J 35800-A to 510 kPa (75 psi).

9. Start and idle the engine until it stalls, due to lack of fuel. This should take approximately 15-20 minutes.

10. Turn the ignition to the OFF position.

11. Disconnect the J 35800-A from the fuel rail.

12. Reconnect the vehicle fuel pump relay and oil pressure switch connector, if equipped.

13. Remove the J 37287, J 42873 or J 42964 and reconnect the vehicle fuel feed and return lines.

14. Start and idle the vehicle for an additional two minutes to ensure residual injector cleaner is flushed from the fuel rail and fuel lines.

15. Pour the entire contents of GM Fuel System Treatment Plus (P/N 88861011 [in Canada, P/N 88861012]) into the tank and advise the customer to fill the tank.

16. Review the benefits of using Top Tier Detergent gasoline with the customer and recommend that they add a bottle of GM Fuel System Treatment Plus to the fuel tank at every oil change. Regular use of GM Fuel System Treatment Plus should keep the customer from having to repeat the injector cleaning procedure.

17. Road test the vehicle to verify that the customer concern has been corrected

Big Block got him a lady with some car sense.

Yeah, but I had to practically beat the sense into her. She's really smart, and she asks alot of questions, and alot of good questions. She likes swapping out CV joints. That's her thing. Go figure. One hour tops.

Most misfire codes are electrical relates (plugs, wires coils, ect.) swap the coil that supplies power to #2 cyl with another coil--see if the misfire code follows.

If not, run a compression test on that cylinder (wet and dry compression checks).

chevyman_396,

one other thing, if the coil doesn't fire at the new position, try the third coil to see if it fires at that position. if each coil fires at another position be neither will fire when you swap it to the #2 cyl position suspect the ICM (ignition control module) is faulty. They can be found pretty cheap at those pullapart type salvage yards.

Bfree has a good idea on the diagnostic end but beware of junkyards. They say you get what you pay for. If you want to buy a solid piece of metal with no moving parts or something that does not have electronics they can be a good deal. Buying computer components that have been exposed to god knows what for god knows how long can be a bad idea. Even if they apply a short warranty period, how many times do you want to replace it? What happens when it fails shortly after their warranty expires. You've already got a used part. That is you're problem. Don't trade on for another. You buy junk you get junk.

you're right. but consider this, my last ICM from a junkyard cost me $10 (you might expect to pay $20-$25 for one). If you purchase a new unit for around $150 and that doesn't cure the problem you can't return it because it's been connected into the circuit and therefore considered used. (autoparts store wil not allow you to return electronic parts whch have been 'hooked-up in the circuit).
A least with pullapart, if it's bad you can return it for full store credit.You don't get the $10 back, but you have $10 credit at any pullapart in the nation.

Bfree,

That is true but you shouldn't replace a part that you haven't confirmed to be faulty (at any price). They say test don't guess. I know there are times in certain flow charts that they say " replace with known good part" but this particular problem is a pretty simple problem to figure out.

Sure, they say test don't guess.
the test for a bad ICM involves using "ICM test equipment" to determine if the circuits in the ICM are good and able to produce the switching action needed for the ignition system to work.
In the absence of that ICM testing equipment, using the process(s) mentioned in the eariler posts perform basically the same tasks.

Consider this, if each coil fires properly on two of the three ICM slots, but neither of them fires at all on the third slot, what's likely the problem?
Having said all of that, of course, if a shop has the ICM testing equipment you could simply take it there and have it tested. However if that's not available to the "do-it-yourselfer" he has the alternative mentioned in the previous posts.
May not be the easiest TEST, but still a decent way of checking the ICM.

I would agree with bfree that the likely problem in his given scenario is obvious. The ICM testers are often available at part stores. It's unfortunate thet they appear to condemn modules more often than not (almost as if they're designed to sell modules even when they are not bad rather than test them). I was just trying to say that replacing parts (even at a junkyard price) can get really expensive. Not to mention the diagnostic nightmare it can cause if they buy a bad part. If someone is a do it yourselfer and does it because they like to (regardless of price) that is awesome. If they are doing it to save money they have to make a decision. When do they draw the line and take it to a pro?

Chevy,

I'ts only been 4 days. What progress have you made? Have you ruled anything out?

I have swapped coils and the missfire stays with #2 cylinder, I also changed the fuel filter because it probably needed to be changed anyway. I have ordered a better scan tool and it should be in soon. Hopefully that can give me some more information on what is going on in there.

I have swapped coils and the missfire stays with #2 cylinder.

Swap the number 2 plug with another plug. Swap the number 2 fuel injector with another injector. Check the O2 sensors. Check for an exhaust valve problem. Vacuum leaks cause misfires, and can cause single cylinder misfires as well. Use an engine vacuum pressure gauge to check for vacuum leaks and post the results. Make sure there aren't any exhaust leaks, or ambient air problems.

also use the vacuum guage to check for proper valve operation and timing. engine should have about 17-20 in of vacuum with the needle steady. a bad valve would make the needle jump between about a 2-3 point spread at a rhythm consistant with the idle speed of the engine.

Chevy,

Is the number 2 plug fouled? Pull it and describe the electrode burn to us.

how's the diagnosis going?

Chevy,

Let us know how it is going.

considering the fact that you've checked everything that we've suggested and to no avail, please let us know what you did to fix it (assuming its' fixed). This could help us out in the future.

If its not fixed, let us know hwat you've tried and we'll assist further.

not fixed yet, hopefully have more time this week to look at it. I will let you know when I figure something out.

chevy,

not fixed yet, hopefully have more time this week to look at it. I will let you know when I figure something out.

No problem man we're here. Can help you move this along quicker if you tell us what (if anything) you ruled out so far?

ok, i got my new scanner today! Mac tools ET99. It shows fuel trim levels, but i'm not totally sure what i am all looking at on there. I am a John Deere mechanic so i am new to some of this automotive stuff. I noticed that the fuel trim on bank 1 is always much lower than on bank 3. Is this normal? Can you guys explain what fuel trim is?

To much to explain here. Best thing to do is to go to Google and type in "fuel trims explained". What have you ruled out so far? Are the basics checking out or not? Is the misfire still PO302? If it is, tell us what the swap test revealed? If the misfire has spread, and it's no longer just PO302, let us know that as well.

Chevy,

Big Block is right there is a lot to be discussed. You'll want to read about it.

You asked about fuel trim. The easiest way I can put it. Is when the fuel trim is low (-) the computer is trying to take away fuel. If it is high(+) it is trying to add fuel. You will want to look at both the LTFT (long term fuel trim) and the STFT (short term fuel trim). STFT is basically the computers immediate response to the O2 sensor input. LTFT is an overall reading of the adjustments the computer has made in the recent past.

You said bank 3. I'm guessing you meant bank 2. Were you looking at LTFT or STFT and what were the readings? What are the LTFT readings?

Chevy,

I was thinking about what Big Block said and just checked real quick to see if there was anything you could look into for a quick tutorial. I found this and watched it. It's only about 3.5 mins long and doesn't go into full detail but seemed to be accurate. You might want to do as Big Block said and read more about it, as this link doesn't cover everything but for 3.5 mins. it's a pretty good start. Here it is:

http://www.youtube.com/watch?v=Y4EAyYI67Cw

Hope it helps. Don't forget to post back what your LTFT readings are.

what does the adaptive fuel trim memory read, low readings are normal, a high readind, 25 or more are abnormal and means the computer is at the limit of it's adaptive control, so even seeing a stft that adds and subtracs fuel, doesn't mean it's working correctly, it's just trying to compensate and keeep a fuel ratio around 14.7 to 1.
you'll need to reset the adaptives and monitor them, now you'll be reading adaptive learn, any tests/adjustments will now show in real time.

chevy,

Only been a couple of days and haven't seen a response from you. Know it's summer and you could be on vacation. On the other hand, could that fact that we haven't seen a response from you be that you are "lost"? If so let us know. Blowing your mind with our answers and gettin you lost is not why we are here.

thanks guys,not lost, just dont always have time to work on it. I will let you know what I find out when I get to it again

just dont always have time to work on it.

Understood Chevy, and thanks, but I just wanna remind you that you gotta keep us up date. No pressure man. But anything you rule in or rule out you gotta let us know.

Thanks for responding chevy. Glad we haven't lost you or gotten you lost. LOL. Take your time and keep us posted. I'd like to hear how this turns out.

lokk forward to hear from you next time.

ok, i had a bit of time to check the long term fuel trims. For some reason my scanner lists it as bank 1 and bank 3, but i am assuming that is bank 2.
on accelleration long term trim is 0 on -1.6 on bank 1 and 93 on bank 2
when i let off the gas bank 1 goes to 0 and bank 2 goes to -64. I am going to take a geuss that there is something wrong in bank 2? Also I am probably going to try swapping injectors tommorow. I can see how to get to the injectors, but just wondering how they come off of the fuel rail. Any tips for swapping injectors?

Chevyman,

Yes, bank 2 is what appears to be the problem in your last post. It looks like it s going lean on accel. It is (oddly enough lol) the bank that cyl. #2 is on (P0302).

With readings like the previous post reflects I would lean towards a fuel problem but for the sake of other readers a high fuel trim on a given bank can be caused by ignition problems as well.

To answer your question on the injector swap, there are little metal clips that hold the injectors to the fuel rail. They pop off pretty easy. You might want to consider replacing both the upper and lower o-rings on the injectors so you don't create a another problem during this test swap.

Here is a link that might help:

http://www.autozone.com/autozone/repairinfo/repairguide/repairGuideContent.jsp?pageId=0996b43f8036e7d8

If you are going to swap injectors make sure not to swap injectors on the same bank. For example don't swap # 2 with #4. Swap #2 with #1.

Let us know what happens.

Chevy,

Forget about trims and readings. At this point they mean nothing. You're not where you should be at this time for trims and readings to make a difference. Still waiting to hear from you about the basics. Are they in order or not? Always always always basics first. If basics check out then you look for a bigger problem, and turn attention to trims and readings. Basics are.

Problems with fuel pressure
Problems with fuel regulation
Problems with vacuum leaks
Primary and secondary ignition systems
Cylinder compression problems

Things to check for single cylinder misfire. (Generally speaking, as it applies).

Spark plug wires for damage, carbon arcing and resistance (a maximum of 3,000 ohms per foot is allowed)

Presence of Spark and Spark Plug condition

Fuel Injector Operation, (+12vdc direct from the FI fuse, the ground is switched by the PCM). Measure 11-14 ohms across each injector coil (out-of-circuit). Measure source voltage with the key ON, measure pulses on the ground while cranking.

Ignition coil, inspect for carbon arcing and measure the resistance of the secondary windings, (should be 5,000 - 8,000 ohms).

Remove FI fuse, disconnect Ignition Coils from the Ignition Control Module (ICM), connect a test lamp across the primary circuit terminals of the ICM, crank the engine. If the lamp blinks, suspect a bad coil, if it doesn't blink, suspect the ICM.

Chevy,

I know you said you were going to swap the injectors and have already swapped the coils. You also said that you had changed the plugs and wires a few months ago but did you test them and swap them as we suggested? I was curious if you had ruled this out before you go through the time to swap the injectors.