Low compression in cylinder #5

what can be some causes for this?
Here are the compression test results in PSI:
#1 -119
#2 -124
#3-123
#4-129
#5 -100
#6-123

I'm think it's the valve guides.

Thanks!

 

the valve guides would be one of the lowest chances of causing the drop in compression.

a burnt or bent valve, bad rings on the pistons, a cracked piston, a lobe going out on the cam, loose rocker arm. rocker stud coming out of the head, restriction in the intake runner, or a lot of other stuff.

the next step in testing is to do a cylinder leak down test. that will help eliminate most of the possibilities and direct you where to find the problem.

how did the spark plugs look in comparison on that cylinder to the rest?

 

were you having a problem or just checking... ...those #s don't look bad.

 

Do a wet test to figure out if it's the rings

 

well the engine smokes like all hell and the head and block are both fresh rebuilds. The spark plugs all look the same (lean). I was just concerned cause the shop that did the head has a bad reputation apparently so I was concerned about how far off that one is.

 

hook up an air compressor line to 5 and see if its leaking ito the intake or block, or exhaust.

 

You did notice this thread is 2.5 years old??

 

maybe this will get us an update...

 

ha,no just was reading posts.. my bad. I just did a compression test on my engine and they are within 3 psi of each cylinder. lucky day