Q: Engine not starting when gas filled up

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Hi, sometimes when the gas tank is filled up, the engine couldn't start regularly without pushing harder on the gas panel. Advice to look into the reason and possible solution?

My car has 43000 miles.
My car has an automatic transmission.

What I think is going on here is what is referred to as a "hot soak".

Under most circumstances, a person is either starting their car when it is cold, or they have gone into a store for a fairly long period of time before they come back to start their car again.

In the case of a stop at a gas station, you are only stopped with the motor off for as long as it takes to fill up the tank. During this relatively short period, there is fuel accumulating in the intake manifold that creates a rich condition. Warm motors don’t like starting under rich conditions. Cold ones need rich conditions to start. When you are pushing harder on the gas pedal, you are allowing more air into the motor; the effective fuel mixture is now leaner, allowing the motor to start. If you are pushing the gas pedal all the way to the floor, this signals the powertrain control module (PCM), to go into a "clear flood" condition. In other words, it won’t allow the fuel injectors to open and spray more fuel into the motor until it actually starts.

If this is indeed what is going on, you should be able to recreate this by simply turning the motor off for a few minutes, and see if you can recreate the situation. Keep in mind, symptoms like this can be intermittent. Just do the best you can to recreate the situation that is happening at the pump. If you successfully recreate it, and the car won’t start, push the gas pedal all the way to the floor and see if it starts. It’s important to note that a good diagnosis requires the ability to recreate your hypothesis (in this case, a warm motor starting when you put the PCM into a clear flood state by putting the gas pedal all the way to the floor while cranking).

If this turns out to be the case, what you have is fuel leaking into the motor either from an injector that is stuck open, or maybe a fuel pressure regulator diaphragm has ruptured. The main thing to know is fuel is creating a rich condition to start under when the motor is warm. This won’t happen when the car sits for a longer period because the fuel will have time to dissipate and the motor will also be cooler. I would suggest having a certified mechanic inspect the fuel system for leaks.

Some other notes for those who would like to dig deeper:

Often times, you can see indications of a rich fuel condition by monitoring fuel trim data with a scanner; not a code reader that you find at your typical local auto parts store. A scanner can monitor live data that the PCM is using to run your motor.

In addition, you can rent a fuel pressure tester from most auto parts stores. What you’re looking for is fuel bleed after the motor is turned off. Once the pressure tester is connected to the fuel system, and you turn the motor off, the fuel pressure should fall more than approximately five pounds in about five minutes; in your case I would suspect it will fall quickly. If you don’t feel comfortable doing this, have a mobile mechanic come to your home or office to inspect your hard start condition.

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