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Old 12-31-2010, 06:26 AM
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Mixed plugs = Misfire & 10mpg !

Hi,

I was away for a few days over Christmas in my recently purchased 2002 4.7HO Overland and on Monday it developed a pretty bad misfire and the engine check light came on. I did the 3 on-off cycle with the key to get the code up and found out from the internet that it was a misfire on cylinder 8.

Not much I could do as I didn't have any tools with me and I didn't really want to buy new plugs, etc. as I'd already bought all the service items and had them in my workshop ready to be put in in the new year, so I just had to put up with the misfire and the 10mpg that I averaged coming home

When I got home I pulled the plugs and found 7 slightly worse for wear NGK's and in number 8 a very old, worn and tight to remove champion. I swapped in the new NGK's, started her up and got a lovely smooth idle

Moral of the story, never trust a 'full service history' and always do a service as soon as possible yourself when you get a used car!

Cheers,

Neil
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Old 12-31-2010, 10:00 AM
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Wow mixing and matching plugs. I'd go over all fluids and parts claimed to be replaced. Glad it worked out though.
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Old 12-31-2010, 10:46 AM
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Re: Mixed plugs = Misfire & 10mpg !

The only thing with a misfire condition is that you don't want to drive too far cause that is the #1 cat killer especially if the check engine light starts flashing.
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Old 12-31-2010, 11:50 AM
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Re: Mixed plugs = Misfire & 10mpg !

Wow I know that #8 plug is a pain to get out but thats not worth leaving it in! I would definitely double check any maintenance they claimed to have done on that Jeep. Since I bought mine I have changed all fluids except trans so far, all new spark plugs, air filter, wipers, wash/wax, etc.
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Old 12-31-2010, 03:21 PM
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Re: Mixed plugs = Misfire & 10mpg !

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Originally Posted by cheapjeep View Post
The only thing with a misfire condition is that you don't want to drive too far cause that is the #1 cat killer especially if the check engine light starts flashing.
The check light never started flashing which I've taken as a good thing, I was definitely concerned about the cats and unburnt fuel being chucked straight into them and did consider pulling the injector wiring connector on 8 but couldn't really get at it with the airbox/resonator still on from what I could see?

I'll be starting to tidy her up a bit more next week, first job will be the exhaust, followed by rear axle rebuild, debadging, sort a bit of rust on the tailgate, refurb the alloys and window tinting.

I'm not sure what to do with the interior as the suede outer bolster bit on the drivers seat looks tatty but the rest is in good condition, I'm not sure on the colour and think I would prefer it in black

Quote:
Wow mixing and matching plugs. I'd go over all fluids and parts claimed to be replaced. Glad it worked out though.
I think it was a case of someone couldn't be bothered due to the position of number 8 under the scuttle, but I am disgusted at the lazyness. I did an oil change last week and it was a similar story, oil level didn't even register on the dipstick an I'd say only about 3 litres drained out. I will be checking and changing all other serviceable fluids, belts, etc.

So far I've put 4 new tyres on, 255/60/17's, and replaced the bonnet and tailgate struts.

The positives are that the engine seems to still be in good overall condition (will do compression and leak down tests next week), the overall condition is fair, won't take much to be pretty good and it was pretty cheap (1,800)

Cheers,

Neil
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Old 01-03-2011, 12:07 PM
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Re: Mixed plugs = Misfire & 10mpg !

I also like to do an oil analysis too...but I totally agree with you man. Never trust the service story. Mine's had all the fluids replaced except for brake (doing that in the spring) and all the plugs changed by my own 2 hands. That #8 was a bitch though...
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Old 01-03-2011, 09:51 PM
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Re: Mixed plugs = Misfire & 10mpg !

Too funny how so many of us are doing plugs these days. #8 sucked, #7 wasn't easy and #6 was no walk in the park either. I undid the bolt that holds down the oil dipstick... I just did the oil (substituted a quart of oil for some Lucas oil treatment - anybody else do that; I've got 157k on my 02' Overland).

Did my air cleaner. What fluids should I do? Tranny, transfer, rear end, brakes??

Ohhh, one question: I only found 1 ground strap on my plugs; #8. I thought both sides were supposed to have a ground strap. I was surprised after, changing my plugs with the original equipment Champions, that the idle didn't make a big improvement. The plugs I took out were freaking' horrible. I made sure to gap all the new ones at .040 - but was shocked that most all of the old ones were soooo shot the gaps were about .070. Surprised it ran at all. The idle was lopy before ...and again, surprised that it didn't feel brand new with the new plugs. Any thoughts?

Thanks. I've never owned a Jeep before and absolutely love this rig. Pretty great condition!
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Old 01-03-2011, 10:06 PM
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Re: Mixed plugs = Misfire & 10mpg !

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Originally Posted by jpilk99 View Post
Too funny how so many of us are doing plugs these days. #8 sucked, #7 wasn't easy and #6 was no walk in the park either. I undid the bolt that holds down the oil dipstick... I just did the oil (substituted a quart of oil for some Lucas oil treatment - anybody else do that; I've got 157k on my 02' Overland).

Did my air cleaner. What fluids should I do? Tranny, transfer, rear end, brakes??

Ohhh, one question: I only found 1 ground strap on my plugs; #8. I thought both sides were supposed to have a ground strap. I was surprised after, changing my plugs with the original equipment Champions, that the idle didn't make a big improvement. The plugs I took out were freaking' horrible. I made sure to gap all the new ones at .040 - but was shocked that most all of the old ones were soooo shot the gaps were about .070. Surprised it ran at all. The idle was lopy before ...and again, surprised that it didn't feel brand new with the new plugs. Any thoughts?

Thanks. I've never owned a Jeep before and absolutely love this rig. Pretty great condition!
Hi,

I got your pm, but I've not made enough posts on the forum yet to reply, I'll get some pics of the tyres tomorrow and post them up

There where 2 earth straps on mine, no actually just 1 but it had 2 tags on, 1 for cyl 8 and then continued to cyl 7 if that makes sense?

I put NGK plugs in, not sure if they're a bit better at idle than the champions

I'll be doing the trans, transfer, front and rear axles, coolant and brake fluid.

Cheers,

Neil
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Old 01-03-2011, 10:16 PM
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Re: Mixed plugs = Misfire & 10mpg !

Thanks. I only found the one ground wire on #8. Guess I'll look into that more. Looking forward to those pics. If you can't post em' send me an email at jay@midsixtiesmuscle dot com
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Old 01-04-2011, 12:53 AM
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Re: Mixed plugs = Misfire & 10mpg !

Are you noticing any coolant loss or smell from the exhaust. Sometimes a misfire is a clue to a blown headgasket. they couldve changed that number 8 plug in hopes of fixing the misfire...when in fact it had a blown gasket in that chamber causing coolant to leak in and foul the plug.
If the misfire continues...I would pressure test the coolant system with the plug removed. Coolant would leak in when the engine is off and then be burned off by combustion and the cats.
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Old 01-04-2011, 05:56 AM
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Re: Mixed plugs = Misfire & 10mpg !

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Are you noticing any coolant loss or smell from the exhaust. Sometimes a misfire is a clue to a blown headgasket. they couldve changed that number 8 plug in hopes of fixing the misfire...when in fact it had a blown gasket in that chamber causing coolant to leak in and foul the plug.
If the misfire continues...I would pressure test the coolant system with the plug removed. Coolant would leak in when the engine is off and then be burned off by combustion and the cats.
Hi Scott,

no coolant loss or smell from the exhaust and the misfire hasn't re-occured since putting in the new NGK's.

The Champion plug was definitely older than the NGK's and in pretty tight so I think that it was just a case that they couldn't be bothered or couldn't get it out.

I'll be at my workshop tomorrow so will pressure test the cooling system and do cylinder compression and leak down tests as a precaution, but I think all is ok

I've got a pretty busy day tomorrow with other cars but hope to get a chance to start on the exhaust, axle, etc.

Cheers,

Neil
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