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Been trying to figure this out. It's a 05 grand Cherokee with a 4.7. Once the engine is hot and you shut it off let it soak for 10ish minutes and restart it then go to drive it falls flat on its face no power foot to floor no power. Then it will clean it's self up in a few seconds and runs great until you repeat the above cycle. I've checked fuel pressure, injector drop test,spark etc... It throws a p0300 and the only thing I see on the freeze frame data is the Stft is pegged negative when the event occurs. So I pulled up some pids and caught the event with trims and o2 data see pic when the trims are pegged negative especially bank 2 it's missing running like crap. I'm leaning towards the exam chasing this. The o2s are working beautifully when the event occurs and it has 2 brand new OEM up stream o2s in it. Where do I go from here in diagnosing this? Any ideas?
Hardtopdr2 wrote: What does the fuel pressure read at idle and during the acceleration event? Possibly fuel pump/regulator... at least thats the direction i would start at.
Fuel pressure at idle is 59 psi. On a throttle snap it will for a split second drop to 58 ish psi. And it will hold the 59 psi for at least a hour (as long as ive left the guage on it for). I drop tested all the injectors was at max of 1 psi difference between some of them. Ive also had the fuel pressure guage on when it was acting up and the pressure remained at the 58 to 59 psi mark throughout the event. Fuel was my first thought as well but i cant find anything though testing that leads me that way? Ive thought about checking the pulse width on the injectors with a scope when the event is happening but im not sure what thats gonna gain me? It will only happen after the engine is fully warm then shut off for ten or more minutes and then restarted and either brake torque it or start driving it (almost like it needs a but of engine load to act up).
Ok how much did the pressure drop when testing injectors? Did you test all of them?
Kinda makes me wonder if an injector is shorting out or sticking open when hot. Check resistance of injectors when cold and when hot should ohm out around 11 ohms. If you have a way to see injector on time in milliseconds see what each bank says when cold and when hot.
Hardtopdr2 wrote: Ok how much did the pressure drop when testing injectors? Did you test all of them?
Kinda makes me wonder if an injector is shorting out or sticking open when hot. Check resistance of injectors when cold and when hot should ohm out around 11 ohms. If you have a way to see injector on time in milliseconds see what each bank says when cold and when hot.
Well as far as what the injectors dropped it depends on the pulse setting my tool was on for the test I did and what the pulse setting my tool was they dropped to 32/33psi. I also ohm out each injector hot and cold and theinjectors were all well with in the spec I got out of service info which was 10.8-13.2 ohms and no big variances between them. I also checked the coils hot and cold they were in spec as well and put new oem plugs in since I was there. If it was a stuck open injector id think I'd see it in the fuel pressure testing that was done? Or the o2s would reflect it and they don't seem to. If you look at the pic in the original post the o2s are are working perfectly but the stft drops way pegged negative when the event happens. I do have a scope so I can look at the injector patterns ill do that this weekend and see what I can see. I'm missing something just not sure what. It almost seems like a computer input issue but I'd like to prove it first.
Well the next spot i would look at is spark plugs to see if any have carbon tracks or electrode gap bridged and general look of plug itself (i look for a slight off whiteish color to plug to slightly brownish color. If any cylinder plug is wet make a note of which one and see if your injectors have individual drivers or grouped together. Then see where the wires go to pcm and test the driver circuit's for function with test light first key on engine off and then while cranking to see if testlight lights when hooked to battery positive. If it lights when key on engine off driver is bad or you have a short somewhere. Other than this check your rpm to desired rpm if its higher or lower you will need fo check the idle air controller solinoid. If its ok then i would check maf or map sensor to see if it is over reporting air flow as well as tps and app sensors sending higher than actual throttle position. You will need a dvom that can read in hertz/frequency (for maf) for this and a hand vacuum pump(for map sensor).
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