For a couple of months, I've been having trouble with my car starting. It's fine when I drive it, the problem is with the idling. It takes several attempts before the car will turn over and start. It has very rough idling and sounds like it's going to quit, but doesn't. Giving it gas gets it going, but there is a P0300 code. There's a small bit of fuel coming out of the vacuum line when I checked the fuel pressure regulator. I thought that was the problem, but no. Turned on the key, but no fuel came out. Can you help me know what's the problem?
Code P0300 is for a random misfire, which indicates there is a problem affecting all cylinders. This could definitely be caused by a faulty fuel pressure regulator, as could the hard starting.
If there is any fuel at all in the regulator vacuum line connection, the regulator has most likely failed. Fuel doesn't have to be squirting out to indicate the regulator is bad. So from what you have told me, I would think that it is closer to the regulator.
The regulator can be further tested at fuel pressure gauge and a handheld vacuum gauge. The fuel pressure should decrease by 3 to 10 PSI with 12 in Hg applied to it. If not, the regulator has failed and should replaced, of course, there are many other potential causes for the symptoms your vehicle exhibits. I suggest you have a trained professional, such as one from YourMechanic, diagnose your rough idling issue and find a repair for your vehicle.
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