My genpu has a rough idle where it makes a small sputter every few seconds, I also get a p0300 random misfire code that comes on every few cold starts. I have done everything I can think of to fix this issue. I've changed the spark plugs, cleaned the valves, rebuilt the HPFP, changed the o2 sensors, replaced the pcv valve, and replaced my timing chain/timing components. compression is good. can't find any vacuum leaks. The only thing I can think of next would be coil packs but I've never gotten a misfire code for a specific cylinder, it's always random/multiple misfire. what on earth could be the cause of my sputters?
chasing p0300 and rough idle
- Thread starter dangitbobby
- Start date
I have pretty much the exact issues for the last 3 years. Ive done most of what you said, as well as changed injector seals, and injectors. I'm going to clean my coil packs and see if that helps any. Now I just override the cold start by slightly pressing the gas pedal on start up, it stops the random misfire from coming up. I really miss the cold starts though lol but if I let it cold start, it sounds like I have a VTCS delete. Annoying.
I just recently noticed that if I pump the brakes multiple times while idling, my AFR will climb up to 20, STFT to +25 and the sputters become more noticeable. I checked for vacuum leaks around the brake booster (and everywhere else) and couldn't find any but I think this confirms my problem is a vacuum leak somewhere.
LTFT is usually close to zero at idle, and -7 at the most when cruising. I'll double check that ground for sure
Have you verified the booster itself isnt the issue...
How do I know if my brake booster has a vacuum leak?
Apply vacuum to the booster with the pump to bring it back to 20HG. Depress the brake pedal and hold it down for 30 seconds. You should see booster vacuum drop a little and then hold steady for the remaining of the 30 seconds. If vacuum drops considerably, replace the brake booster.
How do I know if my brake booster has a vacuum leak?
Apply vacuum to the booster with the pump to bring it back to 20HG. Depress the brake pedal and hold it down for 30 seconds. You should see booster vacuum drop a little and then hold steady for the remaining of the 30 seconds. If vacuum drops considerably, replace the brake booster.
messed with the car some more today, my brake booster seems to hold vacuum no problem, I disconnected the brake booster from the intake manifold and the car still sputters. Also, if the car is not fully warmed up then pumping the brakes does not mess with fuel trims/afr nearly as much. But at operating temp, I can easily get STFT up to +25 by continuously pumping the brakes. Here is an idle log where I give the brakes a couple series of pumps.
Ran the car for a few minutes and it held vacuum for at least 30 seconds afterwards. One other thing I just noticed today is I could not suck air through the booster vacuum line, but I could after running the car. Could this indicate a malfunctioning check valve?
Did you ever figure out the problem?