Solving the mystery of the fill

Nov 27, 2023

If you dread filling the gas tank because your car will not function properly afterward, you certainly have a problem on your hands.

A few weeks ago, a first-time customer called me to say that she had multiple problems with her 2006 Audi A4 2.0 – one of the most popular cars on the Peninsula.

She first mentioned that the check-engine light was on. This alert, not to be mistaken for the maintenance or service light, is different depending on the car model (CEL, Service Engine Soon, Check Engine or simply an engine icon). The purpose of the light is to monitor your engine's performance by the use of sensors and computers.

I asked her if the car was doing anything out of the ordinary. She explained that the car drives great except for when she fills it with fuel. Every time she stopped for gas, the car would not start afterward. When pushed for further explanation, she mentioned that the engine would turn over but not start. Only after letting the car sit for 30 minutes would the engine finally run, but even then it ran extremely rough for a least 10 minutes. After that, however, the engine ran perfectly until she has to fill it up again.

This is not normal behavior for a car, so I asked her bring it to the shop.

When diagnosing tough problems like this, the first thing we do is a test-drive. Then we connect our diagnostic link connector to pull codes.

The test-drive went well. The check-engine light was on, but I did not notice any erratic behavior. We connected the car and checked engine codes. Four engine codes came up immediately, informing us that there could be four problems with the engine.

Three of the codes – P0300, P0301 and P0303 – indicated cylinder misfires. That made sense to me, because I noticed the car ran roughly after starting. So I needed to determine why. Misfires are a common problem in all engines, but they are particularly pesky in Audis. Potential causes include worn ignition parts (coils and spark plugs), lean condition (too much air, vacuum leak), rich condition (too much fuel, fuel presser, leaking injector) or a worn engine (carbon buildup or low compression).

Before I dove into the misfire causes, I decided to check the fourth engine code, P0455. It indicates a large evaporative emissions (EVAP) leak. While it was possible that the car had a leaking injector, that would not prevent it from starting after every fill up.

The EVAP system is for fuel evaporation control. The most common EVAP problem occurs because drivers forget to put their gas caps back on after filling up. The EVAP system tests itself by closing off the fuel vapor system to search for leaks. The system ensures that no fuel vapor hydrocarbons can escape into the air. Hydrocarbons in combination with other vapors cause most of the smog we see hovering over the Bay Area.

I ran through the test plan and checked that the EVAP system was working properly. The test plan revealed that everything in the EVAP system worked fine – except for the EVAP purge valve. I checked to make sure that the valve was receiving a signal and that it had proper power and ground. The vacuum hose to the valve was not leaking, either.

I removed the purge valve to discover that the gate that allows the fuel vapors to be purged from the charcoal canister to the intake manifold – and also pulls a vacuum inside the fuel system to test for vapor leaks – was stuck open. Finally, we had solved the problem.

When the customer filled up, a large amount of fuel vapor was being forced into the intake system, causing a massive rich condition. In turn, this prevented the car from starting and caused the misfire.

We changed the purge valve and re-ran a test on the EVAP system. All passed.

My customer can now see a gas station and not shake in fear that her car will not start again.

Matt Pataky owns Sunnyvale Foreign Car Service, 83 Pioneer Way, Mountain View. For more information, call 960-6988, email [email protected] or visit sfcarservices.com.

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