Have a Car Question? Ask a Mechanic Online.
It runs pretty well. The only problem I have (other than the misfire code)is that it will stumble mostly after a restart: Ex: runs fine when first started in morning, after a quick stop for cofee, it will stumble and hesitate with no power under load, until the gas pedal is released for a few seconds, then it clears out and runs great with good accell. This condition has gotten better since I repalced the fuel pump (original) and regulator.
I use a SnapOn scanner and after I clear the code the misfires will come back slowly at idle and under power. Here's the thing that's got us baffled (I got a few people helping on this problem)...ONLY cyl. 1-8-5-6 misfire. Every other one in the firing order and across and diag. in the banks!! the other 4 cyl never misfire. All coils show NO Faults on the scanner.
Another strange thing: I recently cleared the code after cleaning the injectors and not having the code reappear for about 30 miles, well the "service engine" light came on again after about 30 miles....BUT...the next day it was OFF (I didn't clear it),,,SO...I took a chance and had it tested at the local inspection shop but the P0300 code is PENDING, meaning it's still there without the light!! Still no inspection sticker!!
I haven't checked the vacuum.
The code will come back if it's idling or running. But the light doesn't come on until many misfires. I've replaced the evap vent sol. and hose recently and fuel pump module, I didn't check for water in gas, but I use inj cleaner often and had the inj pressure cleaned last week.
The only cyl misfiring are 1-8-5-6 and the coils show OK.
Have we scoped the crank sensor?
Did you say that you can actually feel the misfire?
Crank sensor replaced.
It has a rough idle, been that way since new. But when it's going down the road I can't feel the misfire. Misfires are about 25-32 per cyl. per 100 cycles. The light comes on when they get into the thousands range. Now the light shut off itself WTF..but the code is still there as PENDING!!!
AND...this is the thing that leaves everyone I talk to about this problem confused: ONLY cyl 1-8-5-6 misfire. The other 4 cyls never misfire.
I will check the ground wire more closely tomorrow..I've had a look at it before.
This is very odd.
I thought you said it misfires when accelerating?
But if it idles rough, then maybe we have a vacuum leak? CHeck the freeze frame on your scanner and see what RPM the engine is at when the misfires occurs.
ALso, disconnect the EGR valve somehow and see if this helps at all.
In addition, just a a side note, if I remember correctly, I think GM has a update for a new crank sensor and shim kit just for this type of problem. It pulls the crank sensor away from the flywheel just enough so that there is no more contact between the two (if there was before).
I guess what I am try to get at is that IS THAT THE ENGINE ACTULLAY MISFIRING when the computer says it is, or is it just the computer that thinks it's misfiring? Thats why I was recommending to scope the crank sensor.
with the scanner on the engine. the 1-8-5-6 cylinders misfire at idle, no accell, acell, highway speeds, etc. The engine seems to run fine, I would drive this truck anywhere, no problems with accleration, etc. BUT........the ECU sees those 4 cylinders misfiring!!!
I just want to clear the code so that I can get an inspection sticker. I've driven around for 2 years with this light on and the truck runs fine. But now I'd like to have it inspected (I'm in MA.) so that I can sell it.
Would the things you said make just those 4 cylinders misfire....that's the BIG question?? Or are the cylinders NOT misfiring and the ECU has a glitch and thinks they are??
Well thats what we've gotta figure out.
If those cylinders are misfiring at idle only, I would be looking for a vacuum leak. You might want to smoke the intake with a smoke machine.
Also, check engine vacuum to be sure that its nice and steady.
I don't know how you are going to pass this inspection by only clearing the computer and not fixing this problem unless you time it just right! I mean by clearing the codes on the way to state inspection. Hopefully the readiness monitors will reset enough so that you will pass.
those 4 cyl misfire all the time, idle and driving. I can drive the truck for 40 miles on the highway with the scanner hooked up and watch them misfire.
I can't clear the code and get a sticker because it comes back before the readiness is done. I went today for a sticker because the light wasn't ON...but there tester said that the P0300 code was PENDING!! meaning it's going to pop the light.
BTW: the light went out by itself this time without clearing the codes!!!????
It's driving me crazy.
SO when the computer says its misfiring when you are driving on the freeway, can you feel it misfire?
NO...I don't feel it misfire. The scanner shows maybe 25-32 misfires per 100 cycles, which I guess is not enough to feel anything on the highway with the vehicles ineartia and all!!
The truck runs fine.
Well this leads me to believe that we should be looking close at that crank sensor to verify that it is actually misfiring or not. A scope is the best way to tell.
I have heard on these motors that the valves stick and causes low compression at times on certain cylinders. But because this is happening at idle on your motor, leads me to go in a different direction. But a vacuum test should prove me wrong or right.
We then to scope it, you would need to get your hands on a scope, either borrow or buy a scope, or have someone scope it for you.
The wave form will need to be studied very close to determine if the engine is actually misfiring or not.
But before we do that, lets check engine vacuum at idle.
Let me know!
Did we get a chance to check engine vacuum?
No, I can find a vacuum gauge...
BUT,,I just had the scanner on it.
I cleared the code and at idle---NO MISFIRES (actually #6 cyl shows a couple)
If I race the engine in neutral to about 1,000rpm's the MISFIRES come in and the light comes ON.
I thought I could check the Cranksensor with the scanner, but the only thing I see close is Cam HI to LOW and Cam Low to HI---which I assume is just the opening and closing of the valves as they follow the cam??? Beause this valve just increases as the engine runs...Am I right about this??
What do you think?
I won't be around my computer until sometime Friday....have to go home from work now...
The cam signal from low to high and high to low is new to me. This must just be how that particular scanner interprets the cam signal. I don't believe this is in relation to the valves because while some are closed, some are open. They are not all open or closed at one time. I believe there is a low and a high reference wire at the cam sensor and this is what the computer is reading.
Let me know when we can discuss this further. I will be back in front of my computer on Friday as well.