Thread: 1999 Pontiac Grand Prix Misfire under load

I can't figure out what's causing a random misfire under load in my GP.

It's the worst when going up a hill, I have to make it shift out of OD and hit about 3k rpms so it doesn't misfire. So any time it hits OD and the load is high, it'll miss HARD. Often times it'll miss with the A/C on at a light, or even idling as well.

Things I've changed/tried:

-TB cleaned
-MAF cleaned
-Air filter changed
-Fuel filter changed
-EGR valve cleaned, then replaced with a different used one
-Plugs/wires changed (off-brand) might try ACDelco
-all 3 ignition coils, changed, new

Perhaps a faulty ignition control module?
Fuel? I know nothing about fuel systems, so i'd need a pretty lengthy explanation on what to check.

NGK TR55

AUTOLITE Part # 97000 for the wires

No idea if it's random.

There's no code showing up, so idk if a scan tool would help.

Originally Posted by Mirraco

Scan tool should help, but it seems like you got most the stuff on the list people would say.

When i did my plugs and wires, i messed two up, my innovia 3100 abs code reader said this "Generic misfire" No cylinder, nothing else. I swapped the two around and it runs like a top.

You mean you swapped two wires? I looked over them on the coil install yesterday, but I'll check again. It'd run pretty ****ty if that were the case.

You'd be surprised how well it will run with 2 wires swapped. I did it once and the only thing I could tell is there was a small miss in it at idle, until I drove it that is then it popped the code.

Clean contact points on ICM. Seen crappy connections there on many GM V6s. Also find out if your local NAPA has an ICM tester and if so, ask them to test about 3 or 4 times.

Sent from my LG G3

First off thanks for all the tips and help so far guys, really appreciate it.

Originally Posted by FordMan77

You'd be surprised how well it will run with 2 wires swapped. I did it once and the only thing I could tell is there was a small miss in it at idle, until I drove it that is then it popped the code.

Ok, I looked over them this morning and then I wasn't positive I had the firing order right. It's the same for the S/C version of the series II as it is my version (NA)? Correct?

Originally Posted by 98bleedingaholevet

IIRC this is a waste fire ignition system so depending on which ones were switched like ford said it wont be as easy to notice.

Easy things 1st here.

swap coils around, ex. top one moved to bottom on the icm, middle moved to top, and bottom moved to middle.

go for a drive does the misfire get any better or worse? drive over to pretty much any local parts store and use scanner, every single one ive ever used says which cylinder is having a miss.

also these cars usually flash ses light then set it solid for misfires. if its not setting a code like you said then check and make sure the bottoms of the coil packs are clean and not corroded where they sit on the icm.

also plugs and wires are cheap so if your spending money, plugs and wires 1st.

I've done plugs 2x since it started and wires once. I think i'll take it to a mechanic before bothering to change them again. (don't have much free time) It'll run fine for a few drives, then it'll start again. When I swapped the coils with new coils on sunday, it ran fine one trip. It was quite enjoyable.

Originally Posted by HighOctaneRacing

Clean contact points on ICM. Seen crappy connections there on many GM V6s. Also find out if your local NAPA has an ICM tester and if so, ask them to test about 3 or 4 times.

Sent from my LG G3

I'm going to check the plug wires to see if they are swapped, then I'll clean the contact points on the ICM and take it in to get it tested at an auto parts store. I called O'Reily's, said they can test most of them.

The plug connections were all correct.

I had a code for the MAP sensor come up, I went to "u pull it" and bought a used one, changed that out. We'll see if that changes anything and if it doesn't, I'll start knocking out the rest of the possibilities. Seems to be driving a lot smoother with the acceleration, but I think I still felt a misfire.

I went and had the ICM tested, and it was found to be good.

I cleaned all the contact points and ground surfaces on the ICM, and so far, it's been running a lot better! No codes.

I didn't spend a lot of time cleaning the part, because i didn't think that was the issue. The misfire has been anywhere from non-existent to a very light and not very noticeable misfire. So I think i'll take it apart again quick and get it under some light to see if there's any spots I missed.

is the misfire always on the same cyl? (code wise) if so, swap coil positions(but move the wires relative to where they are now.) see if it follows the coil...aka misfire moves to a diff cylinder, perhaps pointing to a bad coil pack, or if it stays the same cyl misifre, perhaps the icm.

Originally Posted by ScottyDoggs

icm's test good when bad all the time. add some heat and they act up.

if need be i got one for sale.

How much? I might just hit the local dump and get one for $15 I think. Easy enough to swap to try it I suppose. The misfire came back btw.

Originally Posted by JamesIvey24

is the misfire always on the same cyl? (code wise) if so, swap coil positions(but move the wires relative to where they are now.) see if it follows the coil...aka misfire moves to a diff cylinder, perhaps pointing to a bad coil pack, or if it stays the same cyl misifre, perhaps the icm.

I am not getting any codes, so I don't know.