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1996 Bonneville runs rough after stopping and restarting

I have a 1996 Bonneville SSE. Over the past year, when I drive the car for a longer distance (10-20 miles) and the engine heats up, stop for 15-30 minutes, then start the car again it runs extremely rough and the check engine light comes on. If I stop for several hours and the engine cools down I do not have that problem. I have even had that I can not start the car and need to wait for a while to try and start it again. I friend told me that it’s because of poor gas, but I switched for a while to name brand gas and it still happened.

Its not poor gas.

The next time the engine light comes on drive into one of the auto parts chains in your area. Most will hook up a code reader and pull your error codes for free. Write down the exact error code (like “P1234”) and post it along with all the info you have on the car - mileage, and state of maintenance.

If I has to make a first guess I’d say that you may have a leaky fuel pressure regulator or fuel injector sending gas into the engine while it is shut down.

The car is in very good condition (for 15 years old), has 136K miles on it and the code is P0300.

Maybe not bad gas but maybe low fuel pressure. Is it possible to hook a fuel pressure gauge up at the time its acting up?

“The car is in very good condition” doesn’t say anything. How old are the spark plugs & wires? Fuel & air filters? What other kinds of maintenance have been done, if any?

I’m still going to guess at a flooding condition created by fuel leaking from a bad pressure regulator or fuel injector. The regulator would be my first guess since a P0300 is random/multiple cylinder. I’d not put high odds on all 6 injectors are leaking - so that would be more likely to affect specific cylinders.

Spark plugs,wires and air filter are in good condition. Fuel filter probably needs to be changed. Last year I replaced the manifold (got antifreeze in the cylindars), but the problem existed before then.

If it were the fuel filter, wouldn’t the symptoms happen all the time? Why just when the car is hot and stands for a while. No problems when I start the car in the morning or when I come home from work.

To help confirm a leaking fuel pressure regulator, the next time this happens hold gas pedal to floor. If this does the trick then fuel pressure reg is the most likely culprit. You can pull the vacuum line going to regulator and check for the presence of fuel. Should be none.

“If it were the fuel filter, wouldn’t the symptoms happen all the time?”

Things like fuel filters are not parts that you replace to repair a problem. They are basic maintenance items that should get replaced on regular schedules so that you avoid problems. The question about maintenance wasn’t about those very specific parts. When you ask someone about a car problem, more often than not knowing how the car has been cared for can tell you a lot. The general state of maintenance items should be a part of any report of a car issue.

I still think you probably have a bad FPR. In addition to what pete p noted, if you have access to the FPR you can pull its vacuum line after a hot shut down - if there is any fuel in the vacuum line then replace the FPR.

Even though I find it unlikely, I’ll give you a fuel filter theory anyway: clogged fuel filter overworks fuel pump; fuel pump getting weaker and weaker; the hardest it has to work is when its hot (has been running recently) and on an initial start up - thus a hot restart problem. It is true that if this was a problem you would likely have other symptoms. But we also have no info about how you drive or if you know enough about cars to recognize other symptoms. It would not necessarily happen on cold restarts. See, now if you say that the fuel filter on this car has never been replaced at all over 15yr/136K then all bets are off and who knows what kind of issues you’re having.