View Full Version : Missfire sensor

05-08-05, 12:19 PM
I thought these things are supposed to have a miss sensor or something? Mine started idling like a$$ and is missing like crazy at WOT and I get no codes?? .. Ive replaced the plugs about 40K ago, and the wires are 5K old. Any Ideas? I run it at WOT all the time, u think plugs could go that fast? Also I've noticed an arcing noise from the coils. (ticking) is this normal, or am I just noticing things that have already been like that. Tends to happen when things go wrong. Any help is appreciated.

05-08-05, 03:14 PM
No codes sounds strange, but it may be intermitten enough at this point.....I felt the misfires but didnt trip a code till it got real bad.....Hip shot....it sounds like ignition coils. (30$ for AC Delco coils on GM PartsDirect, or 28$ for aftermarket version at autozone) its a 5 minute repair on a Deville....probably same for yours too.

(still could be other things wrong....but the arcing at the coils is a hint)

check this out:


05-08-05, 03:22 PM
From Bbob,The code works by looking at the engine RPM on a continuous basis. The engine RPM in real time constantly changes with an increase in instantaneous RPM when a cylinder fires and then a decrease in RPM until the next cylinder fires. The misfire code tracks each instantaneous RPM spike and assumes that a misfire occurs if the instantaneous RPM spike that it is anticipating does not occur for any given cylinder. The code has logic so that it needs to see multiple "failures" before it sets a code so there is a countup/countdown logic for misfires that is constantly tracking misfire failures. This causes it to take longer to set the code at idle due to the longer time interval between cylinder firings.

05-08-05, 06:39 PM
Also I've noticed an arcing noise from the coils. (ticking)

Touch it and see if you notice a tingling sensation... :histeric:

No, but seriously, try the water mist technique. You might have a damaged coil or faulty plug wire or something. Do a search if you don't know how...

05-08-05, 07:28 PM
I just changed the wires a couple of months ago. Ill try misting them later tonight. Its idling like a beast now, and when u WOT it it just feels gutless and hesitates alot. Still no codes. I was busy all day puting my 3Series Angel Eyes On it and changing the belt, didnt really have time to play with the problem at hand. Plus everything is closed :(

05-09-05, 12:57 AM
There is also some logic in the misfire detection that makes it pretty hard to set the code quickly. The detection must keep detecting a misfire in a specific operating range and detect it many times. The miss may have been intermittant enough or the driving schedule such that it had not had time to make it thru all the "cross checks" before you would key off and it would then have to start over again on the next run cycle.

05-09-05, 07:51 AM
The miss at this point does not appear to intermittent. The car is grossly underpowered and it hesitates so much sometimes I dont even want to drive it. I guess I could just swap the coils from our 95 over and see if that makes a diff. I just wish a damn code would pop up so I know which one, or get an idea of whats causing it. Ill get going on it in a minute, ill let u guys know.

05-09-05, 04:07 PM
WoodShoe: Did you read that thread I linked above?

There's an easy way to check the coils right on the car, described in detail in there.

If you wanna switch em out with coils from your other car, that would work too.

Sounds like coils but it could be bad plug or wire too......did you use AC Delco plugs and wires?

05-10-05, 11:22 AM
You might want to check your fuel pressure as a matter of course...just my $.02 :wave:

05-10-05, 12:27 PM
Dang im retarded. So I swapped the coils. Checked the FPR. It seemed to be cured. So I didnt bother taking it on the highway, just a little boot around the block. Swapped the coils back, and it was fine...weird I thought. Took it on the highway, and it started missing again. Now keep in mind I just changed the plugs and wires recently, so I did not bother to check them :banghead: I took it to the stealer to hook it up to the computer. I was just not in the mood to do any more troubleshooting.... I said hook this baby up tell me whats causing the problem..."okay" he sez....plugs it in...."Missfire on 5, bad plug" he sez...."that will be 70 dollars please"......tought me a lesson.. just check the damn plugs :histeric:...good times had by all.. thanx for all your input guys.. at least I tracked down a coil pack with the controller for 100 bux at the jard.

oh..and while I was there, I was asking about disabling the traction with the Tech2 tool while he was in there....he had no Idea...."most people dont want to spin their tires" was his response. I also found out that these people have no idea that it starts in 2nd geer with the traction "turned off"....does anyone know how to do this so I can go in there and tell these morons how?

05-10-05, 12:37 PM
"Missfire on 5, bad plug" he sez...."that will be 70 dollars please"......

So the car knew, it just chose not to tell you. Interesting.

My DTS had a dead miss when I first got it and it popped up the misfire code pretty quickly. I had 2 cylinders not firing at all, so it figured it out pretty quick. It was a bad coil pack. Actually I think it was just dirty (silt, you know) but they replaced it anyway.


05-10-05, 12:44 PM
Oh it certainly knew! And it definately didnt tell me about it. I guess if it told us too much, the stealer wouldnt make any money at all. I asked him why it didnt trip the SES light, and like everyone here was saying, he said the same thing...it would have if it got bad enough....but when my gf sez 'whats wrong with your car' before the SES light comes on...u know there is issues. LOL

05-10-05, 04:17 PM
LOL....good to hear the cars back to normal though.
If youre only out 70 bucks that sounds pretty lucky.....around here its 99$/hour labor minimum.

05-10-05, 04:20 PM
What specifically was "bad" with it? Cracked insulator? Carbon tracking? Maybe a boot that wasn't seated fully????? :tisk:

05-10-05, 08:47 PM
Carbon Tracking

05-10-05, 10:06 PM
Do you mean carbon tracking on the porcelean outside the combustion chamber or down the insulator inside the combustion chamber...just curious..

I am surprised that a bad plug wouldn't have tripped the misfire diagnostic....hmmm....unless it just never ran long enough at a steady state to allow the diagnostic to test completely.

05-14-05, 04:41 PM
There was a huge track on the porcelain. I drove the car for about 3 days, while the car was running rough, it seemed like it wanted to stall the odd time. Also, a lot of surging at WOT. You could deffinatley tell there was a horrible missfire...I took the car on several highway runs about 40 miles each, also when I had it out, I would drive it for about 2 hours daily...yes I feel bad for driving it like that, but I was very busy..and its cold up here....but It should have tripped that sensor IMO..