Compression

Bugman

NAXJA Forum User
Location
N. Calif.
I've got a stock 1988 4.0 with 120,000 miles in my Comanche. Compression ranges from 130lbs to 110lbs. Are these numbers acceptable for the 4.0? I also have a 1995 4.0 with 85,000 miles out of my Cherokee that I might swap into the truck. I can see that I'll have to change the fuel injection and the distributor but is there anything else? If these compression ranges are acceptable then I'll just stick with the same engine. Any feedback?
 
Should be 120-150 psi with no more than a 30 psi difference between cylinders.

Just to make sure, was this with the engine at operating temp, all plugs out, throttle wide open, cranking over like mad?
 
Yes, all of the above. I put a little oil in the cyl's on the 2nd go 'round and no dramatic change so the rings seem to be alright. Thanks for the reply.
Steve
 
Your compression is a little on the low side, but the fact that they are pretty equal between the cylinders indicates that if you can tolerate maybe a little blow-by and a little oil consumption then you don't need to worry about an immediate rebuild.
 
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