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Solving Engine Hesitation Issues - Diagnosis

Jag x-type engine

We started this series talking about engine hesitation to describe what it is and what the most likely causes for it would be. Now that we know the most likely causes, lets look at the best ways to diagnose the true cause so it can be repaired.

It should go without saying that pulling the engine error codes should be your first step. If your scan tool is capable of pulling sensor response codes (PIDS), get those as well. Specifically we're looking at the mass airflow sensor and throttle position sensor. Be sure to also check fuel trim readings.

P0171, P0174 Codes

If your engine's computer is throwing these codes, your engine is likely running lean. A check of fuel trim readings (short-term and long-term) would confirm this.

A lean engine mixture is usually caused by a vacuum leak. Check for a vacuum leak quickly by taking a trim reading at idle (usually around 8, plus or minus a point or so). If the reading is above 10, you probably have an issue. Rev the engine to around 2,000 RPM. If the trim reading stabilizes to a normal reading, you definitely have a vacuum leak. If the reading does not change, you likely have a fuel delivery issue or a bad sensor instead.

If the problem is a vacuum leak, check engine vacuum to confirm and then narrow down the leak to fix. You'll find instructions on doing so at this link.

If it is not a vacuum leak, check fuel pressure. We talked about doing that here.

Also check the EGR Valve, PCV valve, MAF, pull and clean the fuel injectors, and check the fuel pressure regulator and pump if fuel delivery is your problem.

Next in our hesitation series, we'll talk about engine hesitation that isn't the engine, but is instead the transmission.

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