No/low cylinder pressure.

Duber15 · Apr 2, 2016

I did a compression test last night to try and diagnose a missfire.

#1-167
#2-160
#3-158-159
#4-162
#5-15 dry and 20 wet
#6-160

My assumption is that either the rings are trashed in #5 or a valve/lifter/pushrod is bad keeping it from either getting air or pushing it out. Tomorrow after I'm back in town, I'm going to pull the valve cover off and crank the engine and make sure the valves are opening before I try to throw any cash into it. Maybe someone with a lot of experience will chime in and help.

Thanks on advance.

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enjr44 · Apr 2, 2016

I don't think it is rings. Rings don't tend to read that low. You can get that much without any rings on the piston. Don't ask how I know!!!

Cam, valve/s or anything in between!

RonD · Apr 2, 2016

I agree.

Not rings
Not a blown gasket or cracked head

valve train for sure

enjr44 · Apr 2, 2016

Well, I guess it could be a piston with a really big hole in it.

RonD · Apr 2, 2016

Guess it could but.............original question would have probably been why did my PCV valve and dipstick blow out of the engine, lol.

Duber15 · Apr 2, 2016

What would be the chances the rings could be washed out with the missfire?

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RonD · Apr 2, 2016

0 chance with that low of a compression number.

A ring issue might show 130psi when other cylinders are at 160

Adding oil would pop that cylinder up above 150psi.

At under 80psi you are pretty much looking at a valve train issue

Duber15 · Apr 2, 2016

Thanks. What's most common valvetrain failure for the 4.0L?

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adsm08 · Apr 2, 2016

I'm with Ron. Gas wash usually produces no more than a 20% loss. You are seeing 80% loss.

Next step is to pull a valve cover, make sure the valves move correctly, then do a cylinder leakage. You will hear it in the intake, the exhaust, or the cooling system. Then you will know which way to look.

For my money, with no water in the oil, I'd say it's a burnt exhaust valve. But that is just a guess.

adsm08 · Apr 2, 2016

RonD said:
Guess it could but.............original question would have probably been why did my PCV valve and dipstick blow out of the engine, lol.

I've seen a lot of different symptoms of hole-in-piston disease.

I have seen many engines with big holes in the pistons that left the PCV and dipstick in place.

I found one 5.4 once that had dropped a valve, with all accompanying damage, and the only noticeable symptoms were sounding like a tractor from the lack of a spark plug and no power brakes.

enjr44 · Apr 3, 2016

Duber15 said:
Thanks. What's most common valvetrain failure for the 4.0L?

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Most common would be push rods/rockers. But that would make a valve train tick. With compression that low. I believe you have a valve that is not operating at all.

Duber15 · Apr 3, 2016

So a 422 cam is in the near future? Any extra parts needed for the 422 cam vs the 410 or stock cam. I'll be checking for rocker action in an hour or so and will report back. Thanks guys.

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Duber15 · Apr 3, 2016

All valves/lifters/pushrods appreciate to be operating correctly. Watched the rockers while it's cranking, all move alike and look to move the same distance. No pressure pushing in coolant woth cap off. No air coming out of dipstick either.

Burnt valve?

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Duber15 · Apr 3, 2016

Can one single valve be replaced without having the head machined? Just put new valve and seal in?

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adsm08 · Apr 3, 2016

It does sound like time to pull the head. I suppose a single valve could be replaced, but if you are getting one done I'd do all, especially if you want to do a 422 cam. That requires dual valve springs, so now would be the time. I'd get the head checked while it is off anyway.

No/low cylinder pressure.

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