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Rebuild 4.0 twice- 60 pounds compression all cylinders

W

waynecomanche40ho

NAXJA Forum User
Location
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I bought a 1988 Jeep Comanche about 2 yrs ago. When I first got it, it wasn't running right. So my dad and I put new rings and bearings in it. I drove it for a few months and did a compression check and it said 60 pds on all cylinders. So we thought that the rings didn't seat, so we tore it back apart and put new rings in it again. Same thing happened, drove for a few months and did a compression check, 60 pds across the board. Did a leak down test, all was good. So we thought that maybe someone put the 4.2 connecting rods in it. I torn it apart and it's got 4.0 rods. HELP!
What could cause low compression like this?

1988 Jeep Comanche

1988 Block
Sealed Power 677cp pistons
0630 head
HO intake
 
did you measure the ring gap before putting the pistons in and make sure it is still in spec since you've honed the cylinder walls?
 
don't know about the ring gap, my dad who use to build engines for Engine Masters, put the rings and pistons in.


The head looks good, I'm going to have it checked out this week to make sure it's good.

As far as the deck height, I don't know.


I noticed that the stock piston dish and pin height are different from the Sealed Power 677cp.

Stock 4.0 cast aluminium '87-'93 #83500251, '94-'95 #4773157, '96-'06 #4798329----1.601" 13.1cc

Sealed Power (Sterling) cast aluminium 677P/677CP----1.585" 17.5cc

Could this be why the compression is low?
 
Is it 60psi in all the cylinders or just one? If all of them I'd be looking at something they all have in common like the cam or timing chain.

Are you setting the valve lash correctly? If they are not closing tight they will cause this.

Try dumping a cap full of oil into a cylinder and then doing the compression test and see if you get a big jump in the number. Its called a wet compression test. Its used to diagnose bad rings. You could also do a leak down test or just hook compressed air to each cylinder and listen for air coming out of the exhaust, intake or oil fill hole.

Did you use the same gauge both times? You 100% sure its accurate? Perhaps try it in something else just to get an idea if its reading correctly.
 
A possiblility is that the cam is timed wrong and does not allow the compression to build up before opening the valve.
 
The compression is 60lb on all cylinders. As far a the valve lash, I did not know that the valves were adjustable. I already have the engine apart, so I can't do a compression check again. We checked the gauge on another vehicle, and it was showing to be accurate.

I poured water into the head to see if it would get past the valves. On a couple of the exhaust valves water was getting through slowly. Is this normal or does the valve seats need to be ground?
 
lol, I would have found a different engine 2 rebuilds ago.

You didn't answer my question. What did the compression test immediately after the rebuild?
 
Sorry. The compression was 60lb right after rebuild. So I drove the truck for a couple of months and did a compression check. 60lbs still. Did a leak down test, everything was good, no air getting past rings, valves, or crankcase. Checked cooling system, exhaust pipe, oil filler hole, and air intake.

Im going to take the timing cover off and check the timing and for the locating pin on cam that chpmnsws6 told me about.

Thanks for the reply's everyone. If you can think of something let me know. Will let everyone know what the prob is when it gets solved.
 
Got the timing cover off. Found out that the previous owner put a dual roller timing chain on it. And the dowel pin that held the cam sproket in place was very loose.


Does anyone have instructions on how to set timing on one of these. Pics would be greatly appreciated.
 
Ok, so I think that the cam timing was out. I got the two marks to line up and spun the engine over two revolutions and they still line up.

So I am going to put it back together and see how it runs and if it has better compression.
 
do you do compression checks with the throttle wide open? (I rig up a zip tie... in a pinch a screwdriver, dowel or similar long object (that won't get lost into the engine) stuck into the throttle body works. Just be sure the objects are clean, and that they are of a soft material as you don't want to ding up the aluminium of the throttle body.
 
wheeling365 said:
Is it 60psi in all the cylinders or just one? If all of them I'd be looking at something they all have in common like the cam or timing chain.

Are you setting the valve lash correctly? If they are not closing tight they will cause this.

Try dumping a cap full of oil into a cylinder and then doing the compression test and see if you get a big jump in the number. Its called a wet compression test. Its used to diagnose bad rings. You could also do a leak down test or just hook compressed air to each cylinder and listen for air coming out of the exhaust, intake or oil fill hole.

Did you use the same gauge both times? You 100% sure its accurate? Perhaps try it in something else just to get an idea if its reading correctly.
Click to expand...
the 4.0 is a hydraulic lifter motor, it doesn't have a way to set valve lash. I agree with the rest of this.

It sounds to me like it was mistimed perhaps. You set the timing correctly, to make sure you don't have it a tooth off, put the sprockets on the shafts and then place a straightedge across the center of each shaft. Turn the crankshaft till the sprocket timing mark lines up with the straightedge, make sure the camshaft sprocket's timing mark also lines up with the straightedge. There should be 20 pins in the chain section between the two timing marks on the 'shorter' side of the chain - ie there will be a lot more than 20 pins on the chain between the marks if you count going the other way.

The timing sprockets are set for a 1:2 ratio (24t crankshaft sprocket, 48t camshaft sprocket) because this is a 4 stroke motor, and only go on in one position, so basically as long as you get the number of pins between the marks correct or the marks and the centers of the shafts all line up, you are good to go.

I'm surprised it would run well at all at 60psi compression.
 
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