Hunt For Low Compression (1 Viewer)

Joined
Jul 6, 2019
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Tucson
Hello All,

I have been hitting my head against a wall trying to find the reason I have low compression in all my cylinders after a fresh rebuild. after a dry and wet compression test I concluded that it was not the rings. So I thought it was either my valves or it was a timing issue. I ended up pulling the engine after 3000 miles to check if the machinist did a hack job on the valve but after I did my water and compressed air test all the valves seem like they are seating correctly, except for one.

The engine builder I took it to put in new tappets and a new camshaft from ITM, I did some reading and saw that the 2F and 3F cams have the same profile but the 3F cam is 5 degrees advanced. Is this true? I was was running a 3F cam would that cause my problems? If not, any ideas?

Another thought, with the cam advances would I need to adjust my valve lash to different values?
 
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Joined
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Tucson
yep tried 2 and was getting 110 across the board
The pistons a flat and it is the original head
 
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Joined
Nov 25, 2014
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Western victoria Australia
I suppose it's possible the 3f cam and valve timing is allowing a small amount of gas to expel at the start of the compression stroke.
Some mudders might know?
If you make the tappet clearance larger on both valves on one cylinder and the compression magically shoots up, that might give you a clue.
 
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RockDoc

I'll take Bruce Vilanch for the block.
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Head bolts have been re-torqued since rebuild? Valve lash checked prior to test? Fresh charge on battery? Throttle wide open with all plugs out while testing?

I think the cam timing difference in negligible, and if anything, advanced would up the pressure. How does it run?
 
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I know you said it’s not the rings but I’m just curious ...were the cylinder walls honed to a fresh cross hatch when the rebuild was done?
 
Joined
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All reading low compression is really strange. Are you sure the PO or owners didn't replace the head at some stage with an older head with the bigger combustion chambers? The numbers on the head should tell you info about the head.
Probably unlikely though, because the head would have to be out of a 40.
This is interesting post #24 Higher Compression
I also read somewhere that compression can be lower at low revs with some modified performance cams.
As RockDoc said, get it spinning over really fast with a good battery.
 
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Joined
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Tucson
When driving it was really slow, only able to get upto speeds around 55 when going up hill.
Also, everything had been rechecked i.e. torque, lash, and the throttle wide open when testing

The cylinder walls were honed and new pistons / rings were installed.

I am almost thinking I may have gotten 2 bad gauges at this point.
 
Joined
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I am in Tucson AZ (2389 MSL), and I was hauling my Wife and I up to Sedona (4350 MSL) haha.

I was getting 110psi to 105psi for my compression numbers across the board.

I have not check my rings yet, I need to tear it down the engine a little further to see those but do not want to unless absolutely necessary.
 

MoaByte

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I too had consistently low compression like you. Turn out I had a small leak on my tester.
 
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See if you can find another tester....and go read a vehicle you know has a good engine...then compare the two sets of results and you can then rule out or determine its a compression gage issue.

What is your vacuum when the truck is running?
 

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