i have a 2000 gmc jimmy diamond ed., there is no pressure on the fuel cap when it is removed to refuel , also have low power on acceleration. on occasion after refueling, engine stalls but does not go dead. i have replaced the fuel cap with a new one.

sounds like the vent solinoid may be sutck, is there a check engin light on?

the check engine light is on.

get it scanned and let us know what codes it has, that's the place to statr.

As stated, have a scan done and post the code. Could also be a leak anywhere in the Evap system. Do you have access to a "Smoke Machine"?

you will not need a smoke mech. with a 420 code, that's a code for a cat. converter effciency problem, most times it will need to be replaced.
here's a quick run down on it:

CIRCUIT DESCRIPTION
This diagnostic test is designed in order to measure the efficiency of the Three-way Catalytic Converter (TWC) system. Catalytic convertor efficiency is a measure of its ability to store oxygen after converting the levels of Hydrocarbon (HC) , Carbon Monoxide (CO) , and Oxides Of Nitrogen (NOx) to less harmful gases. The control module is able to evaluate the catalyst efficiency once the vehicle has met the enable criteria and the vehicle is at idle instead of the steady cruise speeds used in the past. Once the conditions for running this Diagnostic Trouble Code (DTC) are met, the control module commands either a lean or rich air/fuel ratio depending on the current state of the exhaust oxygen level. The control module issues a rich command if the exhaust is currently lean, or a lean command if the exhaust is currently rich. After completion of the first command, a second and opposite command is issued.

For example, if the control module were to command a rich mixture, the upstream Heated Oxygen Sensor (HO2S) voltage would increase immediately. The rich mixture is delayed in reaching the downstream HO2S due to the conversion process occurring within the converter. The higher the efficiency, the more the delay before the rich or lean mixture is detected by the downstream Oxygen (O2) sensor. As a result of the lower conversion efficiency within a damaged or poisoned catalyst, the delay in the rich or lean mixture reaching the downstream O2 sensor is significantly shorter. This DTC monitors the amount of time required for both the upstream and downstream HO2S voltages to cross a calibrated voltage threshold in response to the rich or lean command.

CONDITIONS FOR RUNNING THE DTC

No active secondary AIR DTCs
No active CMP sensor DTCs
No active ECT sensor DTCs
No active EGR DTCs
No active Fuel Trim DTCs
No active IAC DTCs
No active IAT sensor DTCs
No active MAF sensor DTCs
No active MAP sensor DTCs
No active O2 sensor DTCs
No active transmission DTCs
No active EVAP system DTCs
No active TP sensor DTCs
No active VS sensor DTCs
No active misfire DTCs
The engine speed is 900 RPM or more for more than 35 seconds since the last idle period.
The engine has been running for more than 346 seconds and the long term fuel trim is stable.
The predicted catalyst temperature is more than 475°C (887°F) .
The system is in Closed Loop.
The BARO is 73 kPa or more.
The IAT is between -7 to 117°C (20-167°F) .
The ECT is between 75-117°C (167-243°F) .
The engine has been idling for less than 60 seconds .
The actual engine speed is within 100 RPM of the desired idle speed.
CONDITIONS FOR SETTING THE DTC
The VCM determined that the oxygen storage capacity of the catalyst has degraded below a calibrated threshold.

ACTION TAKEN WHEN THE DTC SETS

The control module illuminates the Malfunction Indicator Lamp (MIL) if a failure is detected during 2 consecutive key cycles.
The control module sets the DTC and records the operating conditions at the time the diagnostic failed. The failure information is stored in the scan tool Freeze Frame and Failure Records.
CONDITIONS FOR CLEARING THE MIL OR DTC

The control module turns OFF the MIL after 3 consecutive drive trips when the test has run and passed.
A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle.
Use a scan tool in order to clear the DTCs.
DIAGNOSTIC AIDS
The use of fuel with a high sulfur or lead content can degrade a marginal convertor's performance. Be sure to check the fuel quality.

An intermittent may be caused by any of the following conditions:

A poor connection
Rubbed through wire insulation
A broken wire inside the insulation
Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent and Poor Connections in Diagrams.

booger49,

A "Smoke Machine" is a tool that is used to find leaks in evap systems. The machine generates smoke and introduces it into the evap system. Then anywhere in the system where you see smoke exiting is where the leak in the evap system is. A couple of evap codes (THERE ARE MORE) are PO440, PO441, PO442, and for these types of codes you would use a smoke machine. PO420 is (NOT) consistent with your fuel pressure issue. Are you sure that PO420 was the only code that "Auto Zone" pulled for you?

The only other codes pulled were for the abs system, i do have a broken wire going to the right front wheel.They pulled 5 codes, four of which were on the abs.

none of the codes will give you the problem you are having with the stall.
when you are filling it, do you stop when pump stops or are you topping it up more?
i've seen the purge solinoid and a vent solinoid cause your problem with out setting codes, but it should be checked befor just replacing one or the other.

The purge solenoid is located on the intake, right? Where is the vent solenoid usually located?

booger,

The personnel who work at "Auto Zone" neither have the skills or diagnostic equipment to (PULL ABS CODES). With that being said, for now let's (FORGET) about the (ALLEGED ABS) codes that Auto Zone claims they pulled for you. If you have those codes (and you feel like posting them) then you may do so. Just understand that “ABS” codes have (NOTHING) to do with the fuel pressure issue you are describing at fill-ups.

RIGHT NOW DO NOTHING BUT THE FOLLOWING IN THE ORDER SHOWN BELOW.

Clear the code and simply drive the car.

Buy a large bottle of Vaseline. (Wal-Mart is the least expensive).

At the next fill-up before replacing the fuel cap, smear the threads of the gas cap and the gasket on the gas cap with a heavy application of Vaseline.

Now reinstall the cap, and make sure you turn it on till it (CLICKS AT LEAST SIX TIMES).

Now simply drive your car and let us know if the (SAME CODES THAT AUTO ZONE PULLED FOR YOU RE-APPEAR)

the canister vent valve is under sapre tire.