Cracked spark plug caused slowly decreasing mileage

daddio · **Posted:** Tue Oct 04, 2005 9:24 pm

I was tracking a steady decline in gas mileage for over a half year, about 1MPG decrease per month. As we got near 12MPG, repeated trips to the service department were still turning up nothing except blame on my roof rack and my 'oversize' 225/70 tires.

As we eventually touched single-digit(!) MPG, one cylinder started to miss. This was the first time there was noticable rough running. The service department still couldn't find anything, as the on-board computer detected nothing wrong and was still logging no errors.

The Tech then did a temperature measurement and found abnormally hot temparatures near one rear cylinder. He pulled the spark plug, the one hidden at the right rear of the engine (the one I couldn't easily reach and check a couple months earlier). It had cracked it's ceramic insulation, and the crack was progressively widening, as the spark apparently burned its way out. The overheating had melted the coil o-ring and allowed water in to rust the plug badly. And the coil unit was heat damaged. From the amount of distruction there, we figure it had cracked all those months ago, or went in cracked from the factory, and was responsible for the slow MPG decline.

The Libby is now back up in mileage where it used to be, and holding steady and reasonable for a couple months now. I got a chance at the engineering roundtable at Camp Jeep to suggest making the on-board computer a little more comprehensive.

LibertyN8 · **Posted:** Tue Oct 04, 2005 9:36 pm

Nice work man! i think i should change my plugs before the snow hits, and it may help my mileage too thanks

Guest · **Posted:** Thu Oct 06, 2005 8:27 am

All repairs covered under warranty I hope??

All joking aside, I think your tech is a dipstick if he can't diagnose an OBVIOUS problem with one cylinder without the computer telling him that there's a problem. :shock:

Luis

Trodo · **Posted:** Thu Oct 06, 2005 8:35 am

Daddio, how many miles do you have on your 04?

freetolio · **Joined:** Tue Aug 09, 2005 11:47 am **Posts:** 38

The first thing I'd think with a misfire would be plugs followed by timing problems.

Onboard comps can have a hard time diagnosing such things. For Example, an 05 model Aston Martin is the first car to ever have computer-controlled misfire detection and engine shutdown. To do it required engineering a neural network chip that could use specialized processing for individual sensors so that it could identify a misfire through heuristics. Luckily the specially designed chip is only $12. Perhaps it is easier though to sense problems with a plug itself.

Skyjump136 · **Joined:** Tue Jul 05, 2005 6:50 pm **Posts:** 1633

Its amazing what a difference plugs make with the mileage. Just changing mine out at 30,000 miles gave me a 2.5 mpg increase.

daddio · **Posted:** Thu Oct 06, 2005 10:24 pm

RE: [ All repairs covered under warranty I hope??]

Yes, once they figured it out, they seemed unusually certain for some reason that it was cracked right from manufacture, and all was covered under warranty. What I had to pay for was all that gas pissed away for all those months. Imagine every time you filled up, you drove home and let 25% of the tank leak out, and did that for a few months. Certainly that cost a lot more than the new plug and coil. Luckily I also had a Grand and a TJ in the garage to tool around in while I was mad at the Liberty.

I have around 12,000 miles on the Libby now.

Cracked spark plug caused slowly decreasing mileage

Who is online