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Head gasket leak (and ash fouled plugs) but good compression

2K views 9 replies 7 participants last post by  jay-h 
#1 ·
So I've searched all sorts of threads and can't seem to find the answer on this one. There are lots of good threads about some of these issues, but not exactly what I'm experiencing.

1986 CJ7 4.2L 258 6I
Carter BBD (not nuttered)

Leaks:
Leaking from the head gasket on passenger side of the engine
Fixed the oil dipstick tube at the engine connection
Replaced the gasket on the distributor
Checked the fuel pump (no leak)
Checked valve cover (no leak)

Rebuilt carb

Getting blowback out of the oil cap and PVC valve cover tubes into air cleaner

No smoke at all from the tail pipe

New spark plugs ash fouled (only on 1/2 of the point though) after 1 day
The old ones were really ash fouled. They were in for over 2 years.

Cylinder Compression
Dry Wet
#1 119 160
#2 120 162
#3 119 160
#4 120 162
#5 120 162
#6 120 162

The data isn't conclusive if its just a head gasket, cylinder rings and seals, cracked block or just a PVC issue without ripping into the engine.

I'm planning on doing a leak test next, but the wet compression test seems to indicate that its not a cylinder leak that is the issue.

Am I up for a complete rebuild, just a head gasket change, PVC overhaul, nutter, new carb. Trying to narrow down the options.

Any suggestions are much appreciated.
 
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#3 ·
Looks like the rings are worn, which would also explain the blowbye. I did also experience the leaking out the passenger side of the headgasket. It was leaking through the lifter valley, not pressure, so it was just seeping. Its actually common, the gasket may not seal as good there. I actually replaced my head gasket to fix it, used spray sealant to build that area of the gasket up.

kov
 
#4 ·
I would guess piston rings as well.

We did a similar on a friends Jeep with a 4.2L and the results look very close to what you have.
Pulled the head and and pistons and changed all the rings after honing while the engine was still in the Jeep. It took about two dedicated weekends to complete.
After careful end gap measuring of the new rings and filing to be within spec, the engine was put back together with new head gasket and new cylinder head bolts. Worked out well and eliminated his blow by that he was experiencing. That was back in 1982 on his 1975 CJ-5.
 
#5 ·
Great advice from everyone.

I added 1-2 ounces of engine oil through the spark plug hole and re-ran the compression test to get the wet compression PSI. And its leaking oil from the head gasket.

Whats weird is that I have such good compression even though it looks like the cylinders are leaking. At least enough to get oil into the cylinder chamber and then leak. It really gets bad when I run it on the freeway. So the engine is working harder at that point, to more oil vapor is escaping into the chamber. I plan on doing a leak test which should confirm the compression leak.

Based on the suggestions, I'm going to pull the top of the engine and look at the gasket first. For sure replace that. If the rings are bad on the cylinders, I'll move to a rebuild (rings and honing) like Keith suggests.

Thanks so much for the suggestions.
I'll keep you posted on the progress.
 
#6 ·
Also, any suggestions on the best rebuild kit?

I'm planning on taking the block to a machine shop to hon the ports. So a few questions:

1. Do I need to do that before I buy the rebuild kit to make sure the pistons and new rings fit?
or
Do I need to even buy new pistons?

2. Will honing and the new rings do the trick? And if so, how do I make sure i get the right rings so everything seals properly?

First time on a rebuild, so thanks for the patience.
 
#10 ·
Also, any suggestions on the best rebuild kit?

I'm planning on taking the block to a machine shop to hon the ports. So a few questions:

1. Do I need to do that before I buy the rebuild kit to make sure the pistons and new rings fit?
or
Do I need to even buy new pistons?

2. Will honing and the new rings do the trick? And if so, how do I make sure i get the right rings so everything seals properly?

First time on a rebuild, so thanks for the patience.
You won't know for sure what you need to do til you open it up and measure the bores. (It may even have been rebored in the past).
 
#7 ·
I just had the internals of my engine rebuilt/replaced. I did some calling around and asking in person and most of the shops didn't even want to touch it if I had brought my own parts. I didn't go for anything custom, I just told them that I wanted it rebuilt with new pistons, rings, honed... etc.

They said that they didn't even want to put their name on it, if I supplied my own rebuild kit.

That could have been because they didn't trust the parts I would bring, or they simply wanted to "up-sell" me the parts that they get a good price on. Either way, was easy just to have them do it (since I wasn't going high-performance).
 
#8 ·
Since the head gasket is leaking, I would first try just replacing that by purchasing one from Quadratec. It may not be necessary to do all the other work involved for the rings and block honing if a replacement head gasket will fix your problem.
 
#9 ·
I think that engine is too evenly worn out to have a blown head gasket causing it. I think the specs are 115 psi means dead engine, 150+ for a new one.
 
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