Spark plug problem with my '96 Cherokee Sport (Page 1/1)
Quad Raider APR 26, 07:01 PM
I've been having trouble with my Cherokee running poorly for the last couple of months.

Background: it's a high-mileage Jeep. I switched out the engine with a salvage yard one last summer after discovering a crack in the block near the water pump. After the swap, the engine would idle OK, but at highway speed in OD it would sputter a bit. After replacing other parts, I replaced the spark plugs and that fixed the problem.

It ran fine until about six weeks ago. The problem was similar to the one last year, but worse in some ways, including backfiring during acceleration. I replaced many parts and sensors, because I was convinced the plugs couldn't be at fault since they were so new. Last night I finally pulled two plugs and guess what I found. What could cause this?



It looks like the tip has been sanded down, not broken off. I used Champion plugs, the exact kind recommended by the Jeep forum.

[This message has been edited by Quad Raider (edited 04-27-2016).]

thesameguy APR 27, 06:25 PM
I have seen that sort of wear happen on rotary engines which have really high combustion chamber temperatures - they will eat spark plugs in just a few thousand miles. Is it possible you are running really lean and have super-high EGT?

Here is a *really* massacred spark plug from a 13B:

http://s17.photobucket.com/...ia/DSC07761.jpg.html

You get the idea...

And, check out crazy spark plug designed to survive rotaries!

[This message has been edited by thesameguy (edited 04-27-2016).]

Quad Raider APR 27, 10:55 PM

quote
Originally posted by thesameguy:

I have seen that sort of wear happen on rotary engines which have really high combustion chamber temperatures - they will eat spark plugs in just a few thousand miles. Is it possible you are running really lean and have super-high EGT?

Here is a *really* massacred spark plug from a 13B:

http://s17.photobucket.com/...ia/DSC07761.jpg.html




Yikes. I guess it could be running lean. Maybe the injectors are worn out. The Jeep has 235,000 miles on it.

[This message has been edited by Quad Raider (edited 04-28-2016).]

thesameguy APR 28, 11:00 AM
If it's one or two plugs like that, I'd look at injectors. If it's all of them, I'd look at the fuel pump. Or maybe the fuel filter. Or a big vacuum leak post-measurer.
Quad Raider APR 28, 10:43 PM
I pulled the other plugs. One was missing its tip,, all were roasted. With the new set installed, she purrs again.

I put a new fuel filter on last summer. I have never tested the injectors. As for the fuel pump, I always thought when they went bad they went all the way bad.
thesameguy APR 29, 06:21 PM
Pumps can definitely die slowly. I'd check fuel pressure first.

But you also need to look at mixture (air leaks or MAF/MAP/whatever) and ignition timing. One plug could be a manufacturing defect - all plugs is a problem at the system level.

[This message has been edited by thesameguy (edited 04-29-2016).]