All cylinders misfire after driving a few miles
05-17-2023, 03:24 PM
#63
RL Community Team
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05-21-2023, 01:19 PM
#65
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Thread Starter
No change trying that method.
I drove the car about 75 miles yesterday, 4 separate trips of letting it cool (one hour stop, 5 hours stopped), not letting it cool (10 min stop) and noted that when the car doesn't cool and have to regain operating temps, the misfires didn't recur.
I'm also noting a pattern when the misfires may just start to stumble, if I can rev the engine to over 3,500 RPM, they sometimes, but not always, go away. There have been a few times where they do come back, but only briefly, during longer driving periods. This is maddening, have I mentioned that?
Maybe starting to think it's something with fuel delivery? New fuel pump, just ran a full bottle of Techron injector cleaner with only 8 gals gas and no change. HELP!!!
I drove the car about 75 miles yesterday, 4 separate trips of letting it cool (one hour stop, 5 hours stopped), not letting it cool (10 min stop) and noted that when the car doesn't cool and have to regain operating temps, the misfires didn't recur.
I'm also noting a pattern when the misfires may just start to stumble, if I can rev the engine to over 3,500 RPM, they sometimes, but not always, go away. There have been a few times where they do come back, but only briefly, during longer driving periods. This is maddening, have I mentioned that?
Maybe starting to think it's something with fuel delivery? New fuel pump, just ran a full bottle of Techron injector cleaner with only 8 gals gas and no change. HELP!!!
05-21-2023, 02:17 PM
#66
RL Community Team
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The reason I'm curious about the RPMs is because the Variocam plus kick in around 3,800 rpm.
06-07-2023, 11:48 PM
#67
Misfires
From everything I am seeing, I’m inclined to say you have troubleshooted the cold start circuit.
I also have the same car, same motor.
sadly mine is getting rebuilt due to bore scoring.
I have a few older aircooled cars….but this issue you describe seems the same.
1) it runs ok when cold
2) it starts misfiring when/after it warms up
3) if left off for 10 min, it still misfires
4) if left off for a long period, it runs fine until warmed up again.
im punting on a bit of this, but the ideas are the same.
any cold ICE motor needs more fuel and less air when cold starting.
in old cars, that is a choke on the carbs
on newer FI cars it was ADDING fuel to change the AF ratio.
in the early FI systems that was done with a cold start injector which sprayed more fuel until engine temps hit XxX then the power to that injector shut off
in newer FI systems (guessing) I suspect a longer fuel cycle for each injector does the trick.
with your engine, the temp sensor that tells the ECU to turn down the fuel to the normal cycle is not sending the signal properly.
new rebuild, maybe used sensor just moved over and not replaced?
it certainly sounds simply to be a problem in the cold start system not turning off when temp is normal.
this is like trying to run a carbved car with the chokes on all the time, or the cold start injector always running
without a complete car I cannot go see or test that system for you, but maybe others know the details of the cold start system on the 997 3.8 liter from 2005-2006….
I also have the same car, same motor.
sadly mine is getting rebuilt due to bore scoring.
I have a few older aircooled cars….but this issue you describe seems the same.
1) it runs ok when cold
2) it starts misfiring when/after it warms up
3) if left off for 10 min, it still misfires
4) if left off for a long period, it runs fine until warmed up again.
im punting on a bit of this, but the ideas are the same.
any cold ICE motor needs more fuel and less air when cold starting.
in old cars, that is a choke on the carbs
on newer FI cars it was ADDING fuel to change the AF ratio.
in the early FI systems that was done with a cold start injector which sprayed more fuel until engine temps hit XxX then the power to that injector shut off
in newer FI systems (guessing) I suspect a longer fuel cycle for each injector does the trick.
with your engine, the temp sensor that tells the ECU to turn down the fuel to the normal cycle is not sending the signal properly.
new rebuild, maybe used sensor just moved over and not replaced?
it certainly sounds simply to be a problem in the cold start system not turning off when temp is normal.
this is like trying to run a carbved car with the chokes on all the time, or the cold start injector always running
without a complete car I cannot go see or test that system for you, but maybe others know the details of the cold start system on the 997 3.8 liter from 2005-2006….