help diagnosing 2001 Chevy Silverado

1

1quick6

Shadetree Mechanic
Joined
May 24, 2001
We've been chasing a problem on a rough running 2001 Silverado. The truck generates a code p0300 random multiple misfires detected. We changed all the plugs and wires and there is no difference. The plugs were all fine with the exception of one cylinder. That plug was black. The coil pack on that cylinder is producing spark, but I am not sure how strong it is. We just let it spark against the exhaust manifold. What else could it be? Should we replace that coil pack on the cylinder even though it produced spark? I had also thought it might be a bad injector, but would that cause the computer to generate a misfire code?

I just pulled the new plug on the questionable cylinder and it is already black. The computer still reports multiple random misfires.
 
Check compression on the cylinder in question. You may have a broken exhaust valve spring, or if you do have a leaking injector, I'm sure the rings are pretty washed out.

If I remember correctly, the fuel rail is pretty easy to pull off of those engines. Pull the fuel rail (with the injectors still in it), and pressurize the fuel system. If you've got an injector leaking bad enough to blacken a plug that quickly, you should be able to see it dripping.
 
Thanks for the replies. I don't have a scantool for this vehicle. I am just pulling the code with an Actron scanner that can read and clear codes. I will check the injector on that cylinder since I am really thinking it is stuck, especially after reading the replies. :) The plug on that cylinder was black after about 4 minutes of idling while I had the code scanner hooked up. I will try a compression check also. The truck has 77k miles on it and is fairly well taken care of. It is my brother in law's truck, so I was trying to help him out. I kind of feel bad because I thought the plugs and wires would take care of it. He doesn't have a lot of money, so I now feel bummed. At least the brakes I put on work.
 
Can you feel the miss? You need to get a scan tool. Might just need a crank variation re-learn. Fuel pressure? Injector drop test? Do you know someone with a scan tool? Phil.
 
Leaking intake manifold gaskets are pretty common on that engine. It will eventually set a P0171 or P0174 if it is an intake leak. But, hard to tell with out data.
 
I will borrow a scan tool for this vehicle since all I have is an Actron code puller and Turbolink. :). The miss can be felt. The truck can be hard to start and you can smell gas. I pulled a few random plugs after changing them and letting it run a few minutes and they are all clean but one. The one cylinder is still black.

The one thing about it is, the code P0300 is in there as a pending code, which I am guessing the ecm must see it multiple times in a time period to trip the engine light. I can erase the code and it will set another pending code within a minute or so. As far as I know, the check engine light is rarely lit from what he tells me.
 
If you can isolate which cylinder the miss is coming from, you can try switching injectors to see if it changes to the other cylinder. Phil.
 
Drop the exhaust pipes down at the manifolds . I've had alot of converters go down and cause a randon misfire. I have no clue why it only effects 1 cylinder but i've seen it many times.
 
Code 300 usually means more than one cylinder is misfiring. Make sure that most items that affect all cylinders are good: pump pressure and volume, crank sensor, thottle body leaks etc.....
 
another possibility...MAF. I have seen them start to go and cause multiple missfires. if all else comes back ok. a tech-II should be able to tell you without having to pull things apart. good luck!
 
leaking intake and bad fuel pumps are notorious on the newer style trucks. I'd look at the FP first. Hows it idle?
 
Quik note on fuel pumps:
The aftermkt pumps have a serious reliability rep. IE: Airtex, etc.
My parts guy, [NAPA], sent me to the stealership, saying they no longer sell the a/m junk...too many comebacks.
NOT CHEAP, either. You buy the complete assy.
Online sources have the GM units for less than the dlr.
 
Chuck Leeper said:
Quik note on fuel pumps:
The aftermkt pumps have a serious reliability rep. IE: Airtex, etc.
My parts guy, [NAPA], sent me to the stealership, saying they no longer sell the a/m junk...too many comebacks.
NOT CHEAP, either. You buy the complete assy.
Online sources have the GM units for less than the dlr.
Click to expand...

That's good to know. It's always a gamble with aftermarket pumps as far as I'm concerned. I'd rather get one from the junkyard before I'll put a aftermarket one in. I've seen them last the life of the car and then there's others that leave you stranded out in 5 oclock traffic in the middle of the road. I had a friend that always used FPs from the junkyard and they were GM ones and never failed. I went through 3 aftermarket pumps in 2 years before I finally bought a GM one.
 
Thanks for all of the advice, guys. We are going to dig into it more this weekend as I just got back from vacation. We'll go through the usual checks recommended by you guys. I am still thinking it is an injector stuck open since one spark plug is black and the rest are clean, but we'll go through the checks. He mentioned he smells gas after startup too, but I didn't notice that myself.
 
pdzz said:
If you can isolate which cylinder the miss is coming from, you can try switching injectors to see if it changes to the other cylinder. Phil.
Click to expand...

do this.

i'd start by swapping the coil to another cylinder to see if the miss moves with it. if not, then move the injector.
 
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