Low Compression

[cwestra]

I just checked compression on my cylinders today and got some variations that are concerning.  Below are the numbers.  I cranked each cylinder  about 3 compression cycles before recording the pressure, and rechecked the low and high ones again after taking the car out for a short cruise and got the same numbers.  Any suggestions on whether this is likely rings or something else?  I have numbers from a couple of years ago that are significantly different so it appears something has changed.

1 -  139 psi        2 -  121 psi
3 -  165 psi        4 -  129 psi
5 -  148 psi        6 -  145 psi
7 -  131 psi        8 -  99  psi

[jimynick]

That #8 doesn't look good. Give it a couple of good squirts from an oil can and retry it. If the wet reading comes up significantly, you can pretty well bet on it being a rings problem. You may have got lucky and burnt a valve, but you need to do a leak down test to pinpoint it. Either way, your wallet is going to take a hit. Bummer

[Strawdawg]

You held the throttle wide open so the engine was not struggling for air?

Steve

[cudabob496]

Engine Restorer restores compression (compression lost due to normal wear). I'm a believer, and been using it for 15 years.
Others don't believe it, so you should decide for yourself.


http://www.walmart.com/ip/Restore-Engine-Restorer-and-Lubricant-8-Cylinder/16783203

[cudabob496]

Maybe there is some garbage on the valve seats.
Wonder if fuel/intake cleaners added to gasoline could help?

Or maybe a lot of carbon buildup in combustion chamber, that is interfereing
with the ring seal. Might look into ways to clean excess carbon out of combustion chamber.

Besided Engine Restorer, some guys go to thicker oils, STP, or some
Lucas productss to help with compression.  I'd certainly look into these
before an engine rebuild.

When you drive it, does the engine seem less powerful??

[Chryco Psycho]

You have 4 low cylinders , with 165 as the high 10 % leaves 5 low , maybe just the heads without hard seats & using unleaded gas , as above if the compression jumps with out in the cylinders it is rings , do some testing , it may be time to freshen the whole engine

[cudabob496]

I assume you checked compression when engine was hot?

[cudabob496]

any indication of head gasket leakage near #8?

if no bad oil consupmption, maybe just need a valve job?

[cwestra]

You held the throttle wide open so the engine was not struggling for air?

Steve

Didn't think to hold throttle open during cranking but I did crank through 3 compression cycles and made sure the pressure wasn't going up any more. 

[cwestra]

Maybe there is some garbage on the valve seats.
Wonder if fuel/intake cleaners added to gasoline could help?

Or maybe a lot of carbon buildup in combustion chamber, that is interfereing
with the ring seal. Might look into ways to clean excess carbon out of combustion chamber.

Besided Engine Restorer, some guys go to thicker oils, STP, or some
Lucas productss to help with compression.  I'd certainly look into these
before an engine rebuild.

When you drive it, does the engine seem less powerful??

Seems to be rougher when I started it back up this spring.  May have happened sitting over the winter.

[Strawdawg]

Hard to fill the cylinder with air of throttle is closed and it normally takes much more cranking.  Try one of the weak ones again on  warm engine and let it hit at least six times to be sure. See if there is a change

Steve

[cwestra]

You have 4 low cylinders , with 165 as the high 10 % leaves 5 low , maybe just the heads without hard seats & using unleaded gas , as above if the compression jumps with out in the cylinders it is rings , do some testing , it may be time to freshen the whole engine

Engine was rebuilt around 4 years ago and hardened seats were installed.

[cwestra]

I assume you checked compression when engine was hot?

No.  Engine was cold when checked.  How much difference would that make and how would that explain the variation?

[cwestra]

Hard to fill the cylinder with air of throttle is closed and it normally takes much more cranking.  Try one of the weak ones again on  warm engine and let it hit at least six times to be sure. See if there is a change

Steve

Will do.  Thanks.

[Strawdawg]

Engine needs to be warm in order to have proper clearances and thus proper seal

Steve