4.0L Compression Test

[WillCherokee]

So, I had a compression test done on my 91 XJ after an engine rebuild, the results were (cyl 1-6) 50, 20, 25, 20, 75, 110. Everything was tightened to spec, new head and valve cover gaskets but I did reuse my head bolts. What can be problem(s)?

[4.3LXJ]

You should use new bolts. Are you getting pressure in the cooling system?

[WillCherokee]

Not too sure...how to check for that?

Sent from my SAMSUNG-SM-G900A using Tapatalk

[4.3LXJ]

With numbers like you have, there should be a lot of pressure in the cooling system. It would blow the coolant out the radiator cap very noticeably if the fuel/air was getting past the head gasket

[WillCherokee]

Hmmm....never thought of that. Imma check and see

Sent from my SAMSUNG-SM-G900A using Tapatalk

[OrangeXJ]

Steve you think maybe the timing chain is installed wrong and a bunch of valves are open?

[denverd1]

how involved was the rebuild? top end or full?

new head bolts are easy. I'd start there. so is pulling the timing cover and checking the chain.

if the chain is off a couple teeth, it doesn't explain normal compression on #6

[OrangeXJ]

Just throwing this out there maybe the chain was installed with #6 at TDC instead of #1

[Roqon>]

Sounds like the marks on the timing gears are not lined up.
Head bolts can be reused one time. Unless the bolts are just finger tight or they all snapped they will not cause those compression results.

[denverd1]

Just throwing this out there maybe the chain was installed with #6 at TDC instead of #1

if the valves are out of sync on 1 cylinder, they'll be out of sync on all of them

[OrangeXJ]

if the valves are out of sync on 1 cylinder, they'll be out of sync on all of them

On a 6 cyl engine 1 and 6 will be at TDC at the same time one is on the compression stroke and the other will be on the exhaust stroke. I'd still check it. The one on the exhaust stroke will have an exhaust valve partly open to scavenge air just as it starts the intake stroke.
To be honest I've never installed a timing chain 180 deg off so I don't what would happen to the compression on all cylinders.

[Pookapotamus]

If you did new rings, was the engine broken in properly when it was finished, kinda sounds like a ring problem, not seating, who did the rebuild? It is also probable that the rings were installed wrong.

Give us more info please


Having 4WD means getting stuck in more inaccessible places!

[4.3LXJ]

I hadn't thought of the timing chain, but yes it would be good to check that at this point too

[OrangeXJ]

A easy old school way to see if there is a ring/cylinder problem or a valve/head gasket problem is to put about a table spoon of oil in each cylinder then check compression. If it goes up more than likely it's a ring/cylinder problem. If it stays the same than more than likely it's a valve/head gasket problem.

[XJ Wheeler]

A easy old school way to see if there is a ring/cylinder problem or a valve/head gasket problem is to put about a table spoon of oil in each cylinder then check compression. If it goes up more than likely it's a ring/cylinder problem. If it stays the same than more than likely it's a valve/head gasket problem.
Never tried it myself but heard wd40 works too.

Sent via messenger pigeon. I talk, he types.