I have a 2003 malibu and in the morning after sitting all night it starts just fine but when I drive it somewhere and want to start it an hour or so later it turns over just fine but has trouble starting. Could this be a bad oxygen sensor causing the fuel/air mix to be wrong?
This sounds more like it’s related to engine coolant temperature than the O2 sensor.
I’m gonna take a guess here and ask if your check engine light is lit. Let us know.
Why ask us You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code not just their translation into English and post it back here. Note: codes may be stored even if the CEL is not lit.
it’s definitely not the O2 sensor. They never work until the engine is running for about 30 seconds.
My check engine light is not on. I will have someone read the codes.
I would test pressure in the fuel rail immediately after the engine is shut off. If pressure drops, than you may have a fuel leak in one of the injectors or in the pressure regulator. (A gasoline leak in the pressure regulator could go into the vacuum control line and into the intake manifold.) In either case you would be flooding the engine after a warm turn-off. (I think fuel pressure initially increases after a warm turn-off, because the gasoline in the fuel rail absorbs heat from the engine as it is no longer recirculating back to the gas tank as it does under engine-running conditions.
I had the codes read and no codes were stored.
Here is a little more information that may be useful.
If I drive it somewhere and want to start it again in five or 10 minutes it starts just fine. When I drive it to work and leave it sit for eight hours it starts fine. If this were a leakage problem wouldn’t I have the same starting problem at eight hours as I do at one hour of engine off time?
The O2 sensor helps the computer fine-tune the fuel mixture when driving at steady speeds mostly, so I say no, it is not the O2 sensor.
If this were a leakage problem wouldn’t I have the same starting problem at eight hours as I do at one hour of engine off time?
In eight hours the excess fuel has a chance to evaporate and diffuse throughout the intake manifold. Also, cold starts require more fuel anyway, so the excess evaporated fuel would be less of a problem during cold starts.
But first I would check the coolant temperature sensor voltage with a VOM when the engine is cold and when it is hot. If there is no change in voltage, your problem is with the sensor. If that is not the problem, I would go back to measuring the fuel rail pressure.