2000 Gran Prix 3100 misfire

[Scottyjr]

Hi all,

Just finished replacement of the LIM gasket on this machine. Seems to be a misfire problem now. I'm pretty sure it's on cylinder four since disconnecting the #4 plug wire at the coil pack doesn't alter the engine sound at idle. We (buddy of mine and I) were very careful when removing the push rods, making racks and removing them one at a time. Same with installation. I could see making an error here if it were just me but with both of us being watchful, I think they are correct.

Today I swapped the #4 cylinder coil pack with one from a 98 olds that is soon going to Purple Heart. No change. Also installed another plug wire from the same vehicle. No change. All new plugs were installed

Although the vehicle ran well before the LIM gasket replacement, I'm suspicious that it may be a fuel injector; only because I can think of no other cause affecting one cylinder. Something we may have done wrong was to take the LIM to a car wash to blast it clean. When we did spray it, it was upside down to avoid spraying the fuel rail side of the injectors.

I poured some injector cleaner in the fuel. Maybe that'll do the trick but frankly, I'll be surprised if it does.

What do you alll think? Is there a way to further troubleshoot this problem?

Do you all use code readers for this type of engine performance troubleshooting?

- Scotty

[pocket-rocket]

If you think it's an injector, try swapping that injector with another one and see if the dead cylinder follows it. Code readers can help in these cases, but it's probably only going to tell you that you have a misfire (duh), and maybe a lean condition on one bank (from the lack of fire and it pumping normal air into the exhaust). Injector cleaner by itself probably won't do any good since it has no way of getting in the injector itself if it isn't opening and flowing fuel.

[Scottyjr]

Guess it's not the injector. I got a used one off ebay and put it in #6 cylinder and put the one from #6 into #4 cylinder. The misfire still seems to be #4 cylinder since when the plug wire is removed there is no difference in the way the engine runs. Anybody got any ideas as what to try next to troubleshoot this problem? Thanks.

[johnnyc]

My wife has a 2003 Grand Prix and had a missfire twice. The first time a valve spring broke and the second time the rockerarm stud pulled out of the head.

[Scottyjr]

Got it fixed

Turns out it was one of the plug wires on the rear bank. I took a three wire set from another vehicle and hooked them up and the miss was gone. Wish I had done them one at at time so I could have pinpointed the culprit. Then I had the idea to stick the original wires in one at a time to identifiy the bad wire. Well, I got all the original wires in and the engine ran smooth; no miss. I think the problem was the connection on #1 cylinder plug.

What bothers me is that this problem was pretty much seen as caused by #4 cylinder. Three times the #4 plug wire was disconnected from the coil pack. Everytime the engine sounded the same as when it was connected. I don't think I'll be able to rely on that method of troubleshooting, if needed, in the future.

Thanks for your help.

- Scotty