random misfire

[redcar]

My 03 impala 3.8 w/120K has a random misfire and mild rough idle. At cruising speed the car rides fine. I've had the random misfire rough idle, diagnosed as a problem with an exhaust valve, with what I remember as a 10lb difference in the cylinder compression test. My question is, this causes the cel to intermittently go on for a few days then go off, is there any additive that might correct the cel, so it isn't on so often. (when the cel is on the idle is noticeable, while when not on idle is more smooth) The car otherwise runs great, and I hate to have a valve job done at this time. Also the car will not pass emissions test with the cel on. Thanks for any advice.

[Iflylow]

Fixing the problem will keep the CEL from coming on.
Aside from that, you could also need new plugs and wires if they haven't been changed already. That could fix your misfire.

[manicmechanix]

Yes in this case actually there is an additive that might help. A lot of times an occasional random misfire code and rough idling can be cause by dirty and partially clogged fuel injectors. A bottle of Techron fuel system cleaner that treats 20 gals might cause an improvement. I heard Toyota dealers sale a good fuel injector cleaner that's not too expensive and GM does as well. Use Techron or one from those dealers, not the cheap stuff. Although sometimes injectors need to be cleaned with a professional kit.

Other than that it might need a tuneup. It doesn't sound like you have a bad exhaust valve.

[redcar]

Thank You for the advice, I've installed new plugs and wires and the injector system was cleaned and a new injector for the faulty cylinder. I'll try some Techron today. Thanks Again.

[manicmechanix]

Originally Posted by redcar

Thank You for the advice, I've installed new plugs and wires and the injector system was cleaned and a new injector for the faulty cylinder. I'll try some Techron today. Thanks Again.

So you had already gave it a tuneup and there was a bad injector that was replaced and the injectors were cleaned? I didn't know that, but that still could indicate that the injectors could still be dirty.

[redcar]

I had the work done to cure the random misfire and rough idle, then the compression test revealed the difference in the cylinder that has the misfire. I was just hoping to find a band aid for the bigger job of replacing the valve. Thanks Again

[manicmechanix]

Originally Posted by redcar

I had the work done to cure the random misfire and rough idle, then the compression test revealed the difference in the cylinder that has the misfire. I was just hoping to find a band aid for the bigger job of replacing the valve. Thanks Again

OK I see. So after that work your random misfire turned into a specific cylinder misfire I wonder if the misfiring cylinder is the one that got an injector. anyway a misfiring cylinder can wash the out out of a cylinder and read a lower compression sometimes. When you said there was only a 10 psi drop in one cylinder that didn't sounds too unusual or like a bad valve. But maybe when they tested it they did a leak down test and it was 10 % and identified the bad valve. Good luck.

[redcar]

Yes, the injector went to the misfiring cylinder in hopes of curing the problem, with no luck, then to the compression test.

[manicmechanix]

OK got it. So you still have a specific cylinder misfire code and not a random cylinders misfire code and do you know which cyliner it is? Keep in mind that you could still have an electrical problem at that injector (the wiring) or there could be a problem still in the ignition (coil, ICM). I'm just saying to really diagnosis a bad valve would take a leak down test, which might or might not have been done, and not just a compression test.

Did you try the Techron?

[redcar]

I've been running Techron and super unleaded this week and the CEL has reset, not sure which caused the rest but keeping my fingers crossed the combination will work. The idle is smoother also, Thanks for the feedback.

[manicmechanix]

Good news, thanks for the update. I was thinking you probably had a dirty injector and not a bad valve. hopefully that's the case.

[wafrederick]

Another thing it could be is a bad coil and there are 3 of them.Some scan tools including the Tech 2 have a missfire graphic built in to find out.