Thread: Ignition Problems

Originally Posted by iflycessnas05

Ok, I guess I better start from the top. I bought this 99 grand prix se 3.8L naturally aspirated with 84,000 miles on it. The car is in pristine condition, one owner, older couple, garage kept. According to the window sticker, I should be getting 30 mpg on the highway (but I don't expect to get that much). My first tank gave me 23.5 mpg all highway. After about 2000 miles I noticed it was starting to miss during acceleration. As it gradually got worse, I got a check engine light. So, I took it to autozone for the tester and it told me I had a number 5 cylinder misfire. So I bought all new AC Declo (NGK) iridium spark plugs and a premium wire set from NAPA with my discount and replaced them.

All was well for a little while. The first tank, I went up to 25 mpg and it ran like a top. Then I noticed with each progressive tank the mpgs started to fall. Now it is starting to miss again and am averaging about 22.5 mpg. I am on my way to the store right now to purchase and replace the coil pack that controls the 5th cylinder. Does anyone have any ideas or helpful hints as to why this problem came back or what, if not the coil pack, the problem is?

Could be a bad injector.

But you don't need to buy new coil packs. Just try to change them around. The mis should move. If it doesn't, the coil packs are good.

Also do the same with the fuel injector. Change #5 and like #2. If the mis moves, then it is the suspected part.

Originally Posted by iflycessnas05

Ok, I didn't buy a coil pack because the resistance measured the same on all the packs. I also took them off and did the same to the ignition control module. Everything there seems to be consistent. The problem with changing the fuel injectors location is that the only way I knew it was a #5 cylinder miss was because the service engine soon light told me. The light is not on nor has been in quite a while, once I replaced the spark plugs, it went away. Is there any other way to efficiently check the injector before I spend 75 bucks on a new one?

Injectors are typically not the problem. But it was just something I thought I would throw out there.

If it were me and I just did a plug/wire job to fix a miss, it fixed it initially, then came back, I would pull the plugs and wires again and check everything over. Maybe a spark plug wasn't torqued and backed out. Maybe you damaged a plug on install. Maybe a wire got burnt through on the exhaust. Stuff like that. From there, I would make sure that the rest of the car is up to speed with maintenance. Like PVC valve, fuel filter, air filter, etc. Then I would try to get someone to scan it and tell me exactly what cylinder is missing. And from there, I would look into the coil packs and injectors. Possibly ICM if all else fails.