: New plugs and wires, now misfire



highonchai
01-18-06, 12:38 AM
After the recall on my '96 SLS i noticed - as many others did - that i would get a hesitation at about 40mph when i stepped on the gas. So this-evening i changed the plugs and the wires since this was the agreed upon remedy. After changing the plugs and wires, and buttoning everything up i started the car, it seemed fine, didnt purr like i was expecting and shuddered ever so slightly, at least more so than normal(I definately pay close attention to how smooth my car is) Then I pulled out and drove around the block - MISFIRE - I rechecked the gaps, made sure the wires were correct. Is there that much that i could have screwed up while changing the plugs and wires? Im throwing code : PC0300 (random / multiple misfires) and AC2255 (couldnt find this one) The engine only seems to misfire under slight load, in gear with foot on the brake it shudders, taking off from a stop it misfires badly, accelerating from cruise it misfires, but once up to speed it seems fine. I did drive it on the freeway a couple times (i now know that that was incredibly dumb) while the SES light was flashing....
Any suggestions? could i have ruined a coil?? i cant see how i could have messed too much up... Thanks a million for any help.

jadcock
01-18-06, 09:13 AM
I had the same symptom (slight miss after the recall) and new wires fixed it. The ones I bought were Autolite Professional, since I was in a crunch for time and GMPartsdirect was out of the ACDelco wires for my car. I would recheck all your spark plugs and wire connections. Plugs should be torqued to 15 Nm or 11 lb*ft. Also check to make sure you reattached the grounding strap for the ICM, if you removed it.

If you added any silicone grease to the spark plug boots, I would clean most of that out -- that can cause a misfire if there's too much.

highonchai
01-18-06, 09:34 AM
The only thing i didnt do is check the torque on the plugs -as i dont have a torque wrench-, ive replaced many sets of plugs without torquing and they have all worked fine, maybe its caught up to me...

zonie77
01-18-06, 11:57 AM
Double check that all boots are on. if they are I'd suspect a bad plug or wire...unusual but not unbelievable. You'll need to pin it down to 1 cyl either by pulling wires or getting it on a scope. The scope would be best anyway because it will pin it down to a component.

Eldyfig
01-18-06, 12:24 PM
I have had a bad wire in a new set before. It happens sometimes. Recheck the wires like Zonie said.

Ranger
01-18-06, 01:40 PM
I agree with Zonie. You may have a plug boot not seated all the way.

highonchai
01-18-06, 05:22 PM
Yep, i checked and doublechecked that everything was seated properly... Ive just broken down and decided to take it to the professionals before i screw anything up, especially something major... especially because its snowing like crazy here. i assume its a bad wire or plug, but like i said its too cold to check it right now... ill let the guys that i trust check it out and report back tomorrow after they find it. Thanks for all your help guys!!!
Per usual: You are the Cadillac of Cadillac Enthusiasts!

djb25
09-10-06, 07:50 PM
high,

Did you ever resolve this problem? I just ran into the same thing today when i changed my plugs. I'm going to dig deeper tonight or tomorrow, but if there's something in particular I should look for, I'd like to know.

Thanks,
Damion

dkozloski
09-10-06, 08:11 PM
Get an old spray bottle and fill it with water. Spray a little on the exhaust manifolds with the engine running. It should now be easy to tell which is the misfiring cylinder as the exhaust manifold for that cylinder should be cooler. Now you know which cylinder to work on. If you don't have a spray bottle just spit on the manifolds. I've done it a thousand times.

krimson_cardnal
09-10-06, 10:39 PM
dkozloski - i love it!!! just spit on the manifold YOU ARE A GENIUS MAN !!!

WHODATHUNK OF THAT K_C

weister42
09-14-06, 01:34 AM
What's the recall again? I sometimes have hestitation right round 40mph, but only if I step on the gas as far as I can without the transmission shifting from 4th to 3rd.

Ranger
09-14-06, 12:49 PM
The recall was for '95 - '97 Northstars to replace the nylon fuel line with a stainless steel line.

Greenthumms
02-18-07, 01:59 AM
Hey guys...I have a 93 Sixty Special that misfires once it warms up, which is about 20 min or a little longer. I have changed the distributor, cap and rotors, coil ignition module. Also new gas tank because the baffles were gone in the other one. Any suggestions here??? In dire need of help.

Ranger
02-18-07, 10:57 PM
Plugs and/or wires?

BodybyFisher
02-19-07, 10:07 AM
Hey guys...I have a 93 Sixty Special that misfires once it warms up, which is about 20 min or a little longer. I have changed the distributor, cap and rotors, coil ignition module. Also new gas tank because the baffles were gone in the other one. Any suggestions here??? In dire need of help.

Did you use the dielectric grease with the ignition wire boots? The FSM recommends against using it, as it can create misfires. See this:

http://i135.photobucket.com/albums/q140/Parallax1/Dielectricgrease.jpg

Misfire also can caused by factors such as worn or fouled spark plugs, loose or damaged coil connectors or terminals, wear coil, dirty fuel injectors, low fuel pressure, intake vacuum leaks, loss of compression in a cylinder, even a tankful of "bad" gas contaminated with water.

Are you getting a P030x misfire code when this happens? The most common trouble codes you’ll encounter are P0300 series codes such as P0301, P0302, etc., that indicate a misfire in a particular cylinder. A general misfire code (P0300) is probably not ignition related but is due to a vacuum leak or fuel delivery problem.

A code that indicates a misfire in a single cylinder (such as P0304), on the other hand, will usually be due to a fouled spark plug, weak coil, dirty or dead fuel injector, or loss of compression (burned valve or leaky head gasket).

Are you getting a P030x misfire code when this happens? I would check the fuel pressure when this happens to quickly eliminate that, and I would check the injectors for resistence (but as a last resort). Its possible that either the injectors or fuel pump windings are shorting out when warm..

The same goes for your ignition coils, when they are hot the insulation could be breaking down and an internal short occurs. Buy one new coil and swap it into each position until and see if your problem ceases. I have heard that you can use a stethescope and listen to the coils and hear a coil with a short but I have not experienced that yet.

I purchased Autotap last year, they have some interesting Tech papers, here is one on misfires:
http://www.autotap.com/Articles/Diagnosing_Misfires.html

Good Luck and let us know what you find out