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Ive been pulling my hair out trying to figure this jeep out. Its a 2005 jeep grand cherokee laredo with a 4.7 roughly 105,000 miles. Its has a misfire throws a p0300 only code it throws. The misfire only happens on startup after the engine is hot it doesnt happen on a cold start. So once you drive it and it gets warm and you shut it off for a while then come back and start it up it will sputter and misfire check engine light flashing etc for a few seconds then you either pop it in neutral and rev it up or get on the gas hard in gear and it clears up and runs fine till you repeat the process again. When it throws the p0300 and you look at freeze frame data the STFT are pegged negative -32 both banks every time and its always within the first 15 seconds of engine run time then it clears up and runs flawless. So i hooked up my scan tool and graphed the up stream 02's and the STFT's and when it acts up on startup it pegs the STFT's negative 32. And both up stream o2's drop to about .035 volts and hold there till it clears up. After it acts up the jeep has full fuel control on both banks and the o2's run perfect. Ive attached a kinda crappy picture of the event on my scan tool you can see the drops in the beginning of the graphs. So far ive ohmed out all the coils and injectors all good. its has a brand new mopar egr valve on it. new map sensor.Throttle body has been cleaned. Idle air replaced. tps replaced. up stream o2's replaced factory mopar. The ecm seems to have full control of coils and injectors. The only thing ive noticed is on the freeze frames for the p0300 is sometimes one bank or both will still be in open loop. Im kinda leaning towards a ECM being bad not sure where to go with it anymore any help would be great.
Does fuel pressure hold or does it bleed off? Also it could possibly be a head gasket. Try pressurizing the cooling system see if you get any coolant in the cylinders
Smell your oil and see if it smells like gas is getting in it. If the 02s are cold and not up to the correct temp they won’t tell you if it’s rich on startup.
What is ECT and IAT on a cold soak before starting?
Making Pressure Differential Sensors (PDA Sensors) for pressure pulse diagnostics.
Currently servicing Central Texas.
Cooling system holds pressure I also used the lisle combustion leak detector kit everything came back normal. Pulled the plugs last night as well they have about 10000 miles on them and they look fantastic.
John Curtis wrote: Smell your oil and see if it smells like gas is getting in it. If the 02s are cold and not up to the correct temp they won’t tell you if it’s rich on startup.
What is ECT and IAT on a cold soak before starting?
I changed the oil last week and it didn't smell like fuel at all had around 4000 miles on oil. I'll get the cold soak ECT and IAT in the morning it's was just driven a few hours ago.
It might be vapour lock in the fuel rail..... You could try the following:
With the hot engine not running carefully release the fuel pressure through the schräder valve (watch out for vapour). Then cycle the key several times to on position in order to build pressure in the rail. Next start the engine and verify the condition.
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
juergen.scholl wrote: It might be vapour lock in the fuel rail..... You could try the following:
With the hot engine not running carefully release the fuel pressure through the schräder valve (watch out for vapour). Then cycle the key several times to on position in order to build pressure in the rail. Next start the engine and verify the condition.
Great idea ill give it a go tomorrow and let ya know. Thank you
John Curtis wrote: Smell your oil and see if it smells like gas is getting in it. If the 02s are cold and not up to the correct temp they won’t tell you if it’s rich on startup.
What is ECT and IAT on a cold soak before starting?
I changed the oil last week and it didn't smell like fuel at all had around 4000 miles on oil. I'll get the cold soak ECT and IAT in the morning it's was just driven a few hours ago.
Ok this morning outside air temp was 69 degrees, IAT 72 degrees and ECT 75 degrees.
juergen.scholl wrote: It might be vapour lock in the fuel rail..... You could try the following:
With the hot engine not running carefully release the fuel pressure through the schräder valve (watch out for vapour). Then cycle the key several times to on position in order to build pressure in the rail. Next start the engine and verify the condition.
Ok I was able to complete this today. And after bleeding the rail out and represurizing the event still happened.
Take some crushed ice (like snowcone ice) and run engine till if misfires then put the ice on each injector and see if it still happens while engine is hot. If it works and engine runs without misfiring then it's an injector issue. "Shorted internally". If no change and still misfires probe the injectors and capture the wave forms for sticking, shorting, seized injectors
It’s unlikely but, What are the emissions monitors ready status before startup? I’ve seen ecm’s reset continuously and cause startup issues.
And is can you have someone else start it when it’s gonna misfire and go back to the exhaust with a tissue or dollar bill and hold.it in front of the tail pipe and see if it sucks back in? If it sucks in to the tail pipe, it’s a real misfire (lack of combustion).
Making Pressure Differential Sensors (PDA Sensors) for pressure pulse diagnostics.
Currently servicing Central Texas.
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