Rough Idle

AKgmc · June 29, 2009

I bought an '05 a few months ago and I have been experience problems with a single cylinder misfiring. I first noticed it because it was idling really rough. I took it into a GM dealer and they plugged it into their computer and told me there was a random misfire in cylinder eight. Oddly enough my check engine light hasn't come on. I had them do the motervac injector clean and it seemed like it smoothed up for a week or so but now its back to its same ole rough idle. I replaced the injector today because I though I had narrowed it down to a bad injector. Guess what? Still idling rough. My other thought is the ignition control module but I plugged in a spark tester and it appeared to be sparking OK. Any ideas?

svensk · June 29, 2009

Is the obvious (spark plug/wire) already changed out?

AKgmc · June 29, 2009

Is the obvious (spark plug/wire) already changed out?

Sigh, no.... boy am I a newbie mechanic. I did purchase a spark tester however and compared it to a properly firing cylinder and they appeared the same, but I will still try a new set.

DBVille · June 29, 2009

Swap the coil & wire with another cylinder, and see if the problem remains or follows the coil.

Avoids buying a coil, if its not the problem. Have you replaced the plugs? If not, at least swap them with the coils.

bill190 · June 29, 2009

This can be caused by all sorts of things. Here is a list of things to check...

http://www.random-misfire.com/

Also is that the last cylinder on the fuel line route? Perhaps there is not enough fuel pressure or flow to get to that last fuel injector? Here is fuel system check...

http://www.asashop.org/autoinc/june98/techtotech.htm

Then there are a LOT of components to all this. Rather than "guessing and replacing", it is a lot less expensive to test these things, see what is working ok and what is not. Then just replace the part(s) which is faulty. You can do most of this testing with a GM factory service manual set, a multimeter, and a fuel pressure gauge.

camaroguy94 · June 29, 2009

I had an 2003 Yukon with something similar idle was not rough but was not smooth. Number 6 was not firing all the time. Then a like 60 or 70 MPH you could feel the truck jerk every once in a while. The dealer tech experince the same thing and they tried everything - new head, plugs wires, you name they repleace it. The where about ready to put in a new engine with some one decide to check the engine wiring harniess and found a short in the wiring for number 6.

AKgmc · July 1, 2009

Swap the coil & wire with another cylinder, and see if the problem remains or follows the coil.

Avoids buying a coil, if its not the problem. Have you replaced the plugs? If not, at least swap them with the coils.

I'll try that, unfortunately I can't tell if it follows the coil because it is not tripping the misfire sensor. My check engine light has yet to come on. Frankly I'm wondering if it even works. Got any ideas on how to test it?

DBVille · July 1, 2009

Swap the coil & wire with another cylinder, and see if the problem remains or follows the coil.

Avoids buying a coil, if its not the problem. Have you replaced the plugs? If not, at least swap them with the coils.

I'll try that, unfortunately I can't tell if it follows the coil because it is not tripping the misfire sensor. My check engine light has yet to come on. Frankly I'm wondering if it even works. Got any ideas on how to test it?

Check engine light? Run it without a spark plug for 2 minutes and it will probably come on! Seriously, it should come on before you start the engine - I guess it could be burned out. Short periods of misfires won't set the CEL. May set a trouble code that can be pulled at AutoZone or similar.