Advice on Compression Test Results

SirDoopYJ

SirDoopYJ

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I did a dry compression test on my 4.0L 6cyl. I did three tests on cylinders 3 to 6, but was only able to get one test on cyl 2 and two on cyl 1 due to issues with my AC compressor in the way.

What should I do about my 6th cylinder, the results were low compared to the others.

Cylinder 1: 155...155

Cylindet 2: 155...155

Cylinder 3: 155...160...160

Cylinder 4: 150...150...145

Cylinder 5: 150...150...150

Cylinder 6: 110...110...110
 
Generally speaking low compression comes from a leaking/burnt valve, or valves, worn or broken ring,or rings, a failed head gasket,cracked head, which will rear its uglyness with other problems like coolant loss or over pressuring the cooling system. Be sure you're getting correct results across the 6 cylinders. Hows oil consumption?
 
Im pretty sure I am getting correct results. My oil consumption is what led me into wanting to do a compression test. I use full synthetic. I have to fill my oil every so often. I don't notice any signs of head gasket. Before my compression test my initial thought of where it is leaking is from valve cover or rear main seal.

20210619_122153.jpg
 
You're results are probably correct and I'm with @Gilaguy23 it's most likely a valve issue.
You're not going to know for sure unless you do a tear-down.
I'm actually surprised the numbers are as high as they are on the other cylinders.
Years ago I had an 11:1 compression 4 banger that ran about 185 psi.
Maybe these newer motors run higher compression, if nothing else this is interesting...
 
Well if it's a cracked valve, it could blowtorch out a pizza slice from the valve and damage the seat, or even the head.
Did that myself on the fore-mentioned 4 banger and it cost me a ported head.
But I was pushing it Really hard on a road trip back from CA. LA traffic had become "Fight or Flight".
I chose "Flight" and the speedo in my Pinto was pointing straight down (about 130mph) 'til I burnt the valve in Indio.
I Don't ordinarily drive like that!

But it's not as simple a just fixing a valve, if you gotta pull the head it should have a complete valve job by a competent machine shop.
You might be able to just hand lap the valve if it isn't cracked, but you'll be doing the job again sooner than you would like.
I have a shop do valves, and I'm an experienced mechanic.
They have the special grinding tools for dressing the valves & seats and installing guides if needed.
And from the mileage on your's, it might need those too.
 
JeepJoe is right on the money with his words. Been my experience when a valve burns it leaves a kinda pie shaped divot missing. The head needs to come off and a valve job performed to do it right. valves lapped,seats cut if needed or replaced,that kinda thing. Not sure about just buying a rebuilt head as an option. Again, generally speaking the 4.0's are pretty indestructible and 180K isn't that high of mileage, but it really depends on how its been maintained and used since day one. if your losing oil, and looking at the wet underside you should be able to judge if that's where its going. If the engine is tired,rings,bearings, freshening up the top end does little good for a weak/worn bottom end and oil consumption may increase. Kinda up to you pending funds and what you need it for. The only vehicle DD or a toy that gets romped here and there.
 
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I am going to have a shop do a 2nd compression test sometime this week and after that depending on the results I will be calling machine shops near me for quotes on inspecting the head and do a complete valve job. Hopefully the head is good and I hope the pistons look ok too. I guess for the machine shop to really tell is that they will have to pull the pistons out and measure everything? I would like to save the money and just do the head inspection and valve job, but if it is better to just go ahead and do a complete rebuild while everything is torn into, then I don't mind doing that. I want it done right.
 
Dont let anybody convince you a "in frame" rebuild will be worth while. This leaves the block and crank in the frame. No bore job, no new crank/cam bearings Its a half A'ed way to do a half A'ed rebuild. If they pull the head and drop the pan and determine cylinders are scored, a ring(s) or piston skirt is broke, the only way to do it correctly is to remove the engine and do a full rebuild or install a long block or low mileage used. Not sure about dropping in a later 4.0, as in 99+. If so be aware of the 0331 head castings.
 
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