1998 Grand Prix 3.8 L "Cyl 6 Misfire Detected"

I have a 1998 Grand Prix with a 3.8 L engine with 251,000 miles. It is running rough at idle (especially when warm) and the check engine light comes on after a minute or so of rough idling. The code that comes up is "Cyl 6 Misfire Detected". I just installed new AC Delco plugs and wires and I am still having the same issues. Any suggestions?

Welcome to AF.

Other things can cause the specific misfire detected other than secondary ignition like a bad or clogged fuel injector for that respective cylinder. Also, try switching out the ignition coil packs and see if the problem stays or moves to another cylinder.

Yes. I don't know the history of your injectors, but they probably could use a good cleaning. Get Chevron Techron Fuel System Cleaner (12 oz.) and put it in an empty fuel tank at the gas station, just prior to putting in only 12 gallons of gas. Run this tank of fuel down to zero and repeat only one more time. You'll need to get on it a few times to get fuel flowing through the bad injector. I have had this problem and this stuff (CHEVRON ONLY!) does the job.

Chevron? good to know!!!

I am going to switch the coil packs during lunch and see if the code follows or stays. If it stays with #6, I will go after the fuel injector and switch that with another cylinder. Thanks

The one and only. Chevron gas is all I ever use which has Techron all the time. The few extra cents per gallon is worth every penny. I haven't had a fuel injector problem in any of my cars for so long I'm getting bored. :lol:

I have a 1998 Grand Prix with a 3.8 L engine with 251,000 miles. It is running rough at idle (especially when warm) and the check engine light comes on after a minute or so of rough idling. The code that comes up is "Cyl 6 Misfire Detected". I just installed new AC Delco plugs and wires and I am still having the same issues. Any suggestions?

This happened to me one time after I swapped wires and plugs. It turned out to be the spark plug boot wasn't properly seated on the spark plug and the spark was arcing to ground. So the first thing you ought to do is re-seat the number 6 spark plug boot. The number 6 plug is located on the rear cylinder head in back of the O2 sensor.

:::::::::::::::::::::::::Firewall::::::::::::::::: :::::::

Passenger Side:::: Rear Bank: 2 4 6 :::: Driver Side
Passenger Side:::: Rear Bank: 1 3 5 :::: Driver Side

:::::::::::::::::::::::::Radiator::::::::::::::::: :::::::

While you're at it, check and re-seat the coil end of the cable.

If that doesn't work - this following procedure is how I'd systematically troubleshoot your problem:

IF re-seating the boots didn't solve your problem, check the spark plug. It could have gotten gunk on it as you blindly put it back in the hole. For GP (a pun here) stick one of the old plugs back in to rule out the NEW spark plug.

If that doesn't solve your problem check the plug wire for nicks, burn marks, etc. Swap it with your old wire to rule it out.

If that doesn't solve your problem, swap the ignition coil pack with one of the others.

If that doesn't solve your problem, visually check and gently push down on the #6 cylinder injector connector. THIS IS A SHOT in the dark. I normally don't even get close to the injector wiring when swapping out plugs and wires.

If that doesn't solve your problem, swap the ignition module.

This little procedure assumes that the problem was created after you disturbed the old wires and plugs. You did not say the car was running roughly before you got the code.


Good luck and let us forum members know what you did to resolve your problem! :grinyes:

I swapped the coils yesterday and the problem did not follow so I am ruling out the coils. The plugs, wires and associated boots are firmly seated so that is not the problem. I am going to inspect the fuel injector and electrical plug for any discrepancies. If all appears OK, then I will swap injectors to see if the problem follows.

Just to give a bit of background, the car has 251,000 miles and at about 150,000 I was getting coolant in the cylinders. I suspected head gaskets so I proceeded as such. I had the heads machined and reassembled the engine with all new gaskets only to find out that it was the infamous cracked plastic intake manifold that was leaking coolant into the engine. I replaced that and all was better with the exception of a slightly harder to start engine. Definitely nothing that concerned me since it was very minor. As time progressed, it has slowly been running rougher and within the last few months getting the Cyl 6 misfire code.

My thoughts now are:

Could this rough idle and code generation be a result from the coolant issue I had?
These are the original injectors so could #6 injector be “dirty” from the coolant issue?
Could any internal engine components be damaged?Hope this additional info sparks some thoughts.

I replaced that and all was better with the exception of a slightly harder to start engine.
Can you describe harder to start? Did it crank excessively, or did it "hunt" for idle speed, going up and down in rpm and dying.

Could this rough idle and code generation be a result from the coolant issue I had?
Maybe...

These are the original injectors so could #6 injector be “dirty” from the coolant issue?
I'm not a mechanic, but I don't think so. The injectors are upstream from that issue.

Could any internal engine components be damaged?
How much coolant got into the engine and how long was it in there?

Like I said, I'm not a mechanic, but if you have compression, fuel, air and spark, all timed properly, you're going to get ignition. The slowly deteriorating smooth idle is indicative of dirty injectors. :2cents:

Can you describe harder to start? Did it crank excessively, or did it "hunt" for idle speed, going up and down in rpm and dying.

Maybe...


I'm not a mechanic, but I don't think so. The injectors are upstream from that issue.


How much coolant got into the engine and how long was it in there?
Like I said, I'm not a mechanic, but if you have compression, fuel, air and spark, all timed properly, you're going to get ignition. The slowly deteriorating smooth idle is indicative of dirty injectors. :2cents:
Hard to start is just maybe a crank or two more than before the incident. Prior to the intake going bad, it would start instantly. It does not go up or down in RPM or hunt for idle. Very minor but just a slight change that I noticed.
Coolant was not there long but a couple cylinders had to be drained and it was pooled up on the top of the motor when I removed the plastic intake housing.
I am leaning towards the injector/injectors being dirty.

Someone can correct me if I'm wrong, but here's what I think. If the engine misses on a particular cylinder it will set the code and the "Service Engine Soon" light will go on. If it is constantly missing, meaning that the fault is there all the time, the "Service Engine Soon" light will flash. I think that can be interpreted as: "You better check it really soon!"

I went somewhere once when it started getting cold out and was inside for a while. I went out to my car late and got in and started it and it was idling really rough all of the sudden. The "Service Engine Soon" light was flashing. I left where I was and started heading home and I could tell it was missing on one cylinder. I drove normal until it warmed up, all the time the light was flashing. I then said "enough of this" and I really got on it, pedal to the metal. The car was running rough and then all of the sudden, it cleared up. Simultaneously to this, the light went from flashing to steady on. I checked the code and it was a particular cylinder that was miss-firing. I put the cleaner in that I mentioned before in this thread and eventually the light went out on its own and never came on again for about a year. Then it did it again. Did not check the code, just put in the cleaner and again, it cleared up. Now I try to just run the cleaner through once a year to prevent it from happening.

Just to update this situation, I swapped Cyl #6 and Cyl #4 fuel injectors and the "Cyl #6 - Misfire Detected" is still indicated even after I erase all codes. It is still running rough at idle particularly when warm. Not sure how to proceed from here other than keep driving it as long as it does not strand me somewhere.

Now you want to take out the sparkplug in number six. Inspect it for carbon tracks and for cracks. Also use an ohmeter on the #6 wire. Not sure what the spec is, but it shouldnt be any higher than 10k ohms. Less is ok. As you have your plug out. Since all other cylinders are ok, unplug the ignition module electrical connector, and test the cylinder for compression.

Now you want to take out the sparkplug in number six. Inspect it for carbon tracks and for cracks. Also use an ohmeter on the #6 wire. Not sure what the spec is, but it shouldnt be any higher than 10k ohms. Less is ok. As you have your plug out. Since all other cylinders are ok, unplug the ignition module electrical connector, and test the cylinder for compression.

I just replaced the plugs and wires with AC Delcos a few weeks back. I have changed plugs multiple times during this problem and have never had any visual problems with the plugs. The one thing I have not done is check compression as you stated.

It's very possible, if the coolant leak was bad enough, that you hydrolocked cylinder 6 and bent a rod. This will cause low compression and a miss in that cylinder, due to the piston not coming all the way to the top. I've seen this scenario happen before where other than the miss, it will run fine. No knock, unusual noises or excessive oil consumption. Do your compression test and let us know what you find.

FYI, acceptable resistance in a spark plug wire is 500 ohms per inch of length. Always remember, just because the part is new, doesn't mean it's good.

Is it possible that he has spun a connecting rod bearning, I did this and had to have the engine replaced. The car was running hotter and was also very hard to start was pulling much more current to start the engine.

My 2000 GTP had a similar issue, it idled and maintained speed fine but had a bad miss when accelerating at any speed over 30 mph. Changed plugs, wires, coil packs, PCV, air & fuel filters, and had the injectors cleaned. Did some of the things twice but all to no avail. Problem was finally solved when I found the ohms reading on the old coil packs (5.7ohms) to be different than the new ones (8.3 ohms). The old coil packs are back on and she is her old self again (careful what you get from the aftermarket retailers!).