Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I have Subaru legacy misfiring at idle but when i press the gas ,its run fine.no trouble code at all.checked ignation coil,sparks plugs,injector nozzles and are ok.nids some help!
Hi there,
Do you have access to your fuel trim numbers? Giving us an idea of what the ST and LT numbers are doing at idle and 2500 rpms would provide us with some very relevant information. Very well could be a vacuum leak of some sort...though I've also seen a blown head gasket leaking coolant internally cause idle misfires, particularly on first start when the engine is cold.
Thanks!
Can see ltft and stft but i see a/f learning 1 % and fuel correction -23% at idle.air fuel sensor # 1 0.75 hope this is equivalence ratio.lambda sensor voltage 2.5 to 2.3 and sometimes 0.95 to 3.0 volts.
-23% fuel correction at idle...so the computer is taking fuel away. At idle are you smelling fuel at the tail pipe? I suppose I'd want to know if this truly is a rich condition or if it's a false rich condition. If there's a strong smell of fuel, particularly at cold start up (when the engine is in open loop), it's likely that there really is an issue with the fuel delivery system (for example, a ruptured fuel pressure regulator that's leaking fuel into the intake). If there is no smell of fuel from the pipe, I'd look at your AF and lambda's and try to prove that they're responding properly, namely by creating a big vacuum leak and watching to see that the voltage readings drop really low.
Also, does that fuel correction number do when you raise the rpms to 2500-3000? Does it stay really negative or does it get closer to zero?
Checking the fuel pressure regulator on these engines is really easy. it sits on the fuel rail below the air intake duct and has a vacuum line attached to it. Unplugging that vacuum line when the engine is running and looking for fuel spitting out will tell you if the diaphragm in the regulator is ruptured. A quick, easy to perform test.
Another vote for mechanical problem. Personally seen these engines burn valves, or have sticking exhaust valve guides.
A relative compression test would give you great direction, before pulling the plugs and doing compression testing. Even if you don't have a scope, you can disable fuel and just listen to the engine cranking.
The Jeep 4.0L timing problems video has a perfect example of this. Listen to the engine around 7:42.
If you hear something like that during cranking, then I'd say you can move directly to compression and leak down testing.
Still very happy you came up with a fix, sir. What led you to checking the EGR valve?
In retrospect, the fuel trims being way negative could have steered us towards a stuck open EGR, oh well. Why the PCM didn't pick up on that would probably depend on how it monitors the EGR position, and what the code set criteria is. If this uses a potentiometer in the EGR valve, then it may only code when the position exceeds a certain point. Knowing what a known good closed position value is might be useful for the future.
Hey Cyrus, did you happen get a look at the fuel trim numbers after the fix? I'd like to know if if you still have excessive negative trims. It would really help for a discussion going on in the Book Questions and Discussion section. Thanks man, good job on the fix by the way.
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