888

My new to me came 98 4.0 GC equipped with a CEL and I read the codes yesterday. I knew there was a problem with the cat so the 02 codes didn't surprise me. What did surprise me was the PO 306 cylinder 6 misfire code. Checking around on the web, it seems that this is usually thrown by something with either the wiring or the injector itself rather than the plug, and the plugs were recently replaced.

Does anyone know whether this is usually the wiring or the injector?

If the wiring, what exactly goes wrong? The wiring looks undamaged, I can take it apart and spray the plug with contact cleaner, but other than that.....

I guess I can swap injectors between cylinders and see if the code follows.

Any comments would be welcome.

Thanks

Bustedback

Start simple, look at the plug and spark plug wire first. The boot could have come off the plug, the plug could be defective, the wire could be defective or rubbed through. Pull the distributor cap off and look for carbon tracking or cracks. Don't jump right to the injectors or injector wiring. Injectors don't fail very offten like some would have you think. Other mechanical things can cause a misfire too, like a valve that is not sealing, or a blown head gasket. You have to do some DIAGNOSTICS first to determine what the actual problem is. Yesterday I had a 2001 Taruas in my stall that had a P0302 misfire code. The plugs and wires were just done by another shop, but I started looking at the #2 plug first. I removed the plug and it was fuel fouled. That told me two things, there is fuel getting to the cylinder and there is no spark. So I traced the spark plug wire and found a spot where it was resting on a hot EGR tube. The "tech" that did the tune up did not route the wire correctly and left it to burn on the EGR tube. So NEW does not always mean GOOD. Start simple and don't over think the problem.

CherockeeXJ

I would pull the plugs out and make sure they are all gapped properly. The PO on mine gapped 7 out 8 to spec and left the 8th ****ing tiny as hell which caused a misfire.

I went ahead and replaced all 8 plugs and while I was at it did the wires, distributor cab and rotor.

It's cheap (and easy) enough that I would replace all the plugs, wires, cap and rotor. You rotor/cap could have build up or something causing a bad contact, wires could be bad, and the plugs could have build up on them.

Fixed my misfired. I had 3 by the way. One was bad gapping and two where from bad contact being made between the rotor and two contacts by the looks of the parts I pulled out.

888

Thanks for the replies. I did pull the plug wire for #6 and noticed a small bit of corrosion on top of the plug, used Scotchbrite to clean that up. I looked down inside the boot, it didn't appear to have anything that looked corroded. Plugs were recently replaced according to the service records, I can pull #6 and check the gap/condition of the plug.

The plug wires weren't inserted in the wire separator so I fixed that. No idea when the wires were replaced so I ordered new ones.

dave1123

From the sound of your "corrosion", I think it wasn't installed completely and was arcing there. That would cause it to not fire at times or with a weak spark. You need VOLTS to fire plugs under cylinder pressure.

Also, check your cap and rotor. If that plug wasn't firing, all that voltage had to go somewhere, usually to ground in the cap. I've seem GM HEI rotors with holes burnt in the rotor because it wanted to ground to the shaft.

888

Thanks for the reply, I changed the cap and rotor in general principles, I saw corrosion on the rotor tip that I didn't like. The contacts in the cap were not even in appearance either. The plug wires are on the way from Rock Auto. #1 looks like fun, being behind the AC compressor.

The front end is still in the air, I want to regrease and double check everything before having it aligned.

I did check the #6 spark plug, it is an Autolite Platinum recently installed and it did not appear different than the other plugs I pulled to cross check.

Hopefully, something I've done has cured it. I'll reset the code and see if it comes back.

Bustedback

Get rid of those platinum plugs and install some Champion copper core plugs.

pdowty68

That's right, basic engines like the 4.0 don't like fancy *** plugs, just champion coppers... I thought that was strange when I did my tune up, but there cheap, and run great

888

Will do on the copper plugs, after I sort out the misfire.

Thanks

Bustedback

The plugs will help, or possibly fix the misfire.

888

Ok, thanks for the clarification, I took the posts to mean that the Jeep may run a little better with coppers, not that the recently installed platinums might be behind the CEL. My bad, sorry.

I'll install some good quality coppers and see what that brings to the rodeo, I'm not a Champion plug fan going back to issues I had with them 25 years ago.

Bustedback

I bought the Accel tune up kit from www.summitracing.com for my last tune up, I've been very pleased with how my rig runs since. It was about 65bucks for the cap, rotor, plugs, and pretty red 8mm wires. All very good quality parts.

CherockeeXJ

They only have pretty blue wires and cap with rotor for $60 or ugly cap with rotor and wires with plugs for $120...

dave1123

Maybe I'm prejudiced, but I haven't had good luck with Autolite parts in Mopars or GM's. I don't do Fords, so I couldn't tell you how they work there. I put Autolite wires and plugs on a Dodge K-car 4 cylinder and they lasted a week! Crap!

Basic rule;
Champs in Mopars
ACs in GM
Autolite in Fords
NGKs in Nissans
etc.

888

Funny, plug threads on gas car sites seem to sometimes start similar debates that motor oil and K&N versus stock air filter threads do on diesel sites.

I've just never had good luck with Champions, they never seemed to last, if something I bought came with them, I took them out. The last straw for Champion spark plugs came when I was removing them from my new 94 Dodge Shadow in favor of some thing else and one of the Champions left the barrel stuck flush in the head. Just sheared right off for no reason. I was able to get it out but I've never used them since.

I'll install something copper in there and see what happens. Thanks for the replies.