What is the most your compression test numbers can be off due to a leaky head? Assuming the rings were fine, could the head be responsible for a drop of more than say 50? And what would the chances be of the head being that messed up across 3 cylinders out of 4?

Thanks!

By reading both of your threads it sounds like you did a compression test and the numbers came back low. The head can give low numbers if it is cracked, headgasket is blown, valve stem seals are shot, or valves are not closing (bent).

3 out of 4, i have no clue. What did u replace when u did the rebuild??

I'm hoping the head is the problem. I did the rebuild in July and should be seeing 240 or so in each cylinder. I'm only pulling down 185 in three. :( That's 8.8 CR imo. I have yet to do a leak down, but I'm looking to see just how fuckered up the head can be. What's the most anyone's ever heard of leaky vlaves or a cracked head affecting comprsison test numbers?

Yeah a bad HG can drop your compression, especially if it's FUBAR. Do you burn anything or loose any fluids.

Put some oil in the cyklinder and do the a wet test for compression and see what the numbers are.

Then the wonderful leak down.



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Wait...

I thought putting oil in the cylinders was the leakdown. :P What am I missing? Help! :o

perform a leakdown brian (compressed air, not oil) my bet is its a leaky valve on 3 or 4 (probly exhaust) lotus has a history of that on their heads.

If you have excessive leakage, use your ear to locate the cause. Pop the oil cap off and listen for a hiss (blowby from rings or worse), next listen at the exhaust pipe (exhaust valve) and finally at the intake with the butterfly open (intake valve). This will isolate your problem and direct you to what your problemo is.



***LEAK DOWN TEST***

Leak Down Test

A leak down test is more specific in pinpointing the condition of an engine, and if there are problems, it can pinpoint the needed area of attention. A leak down test is performed with a special gauge and manifold set. It consists of a calibrated pressure gauge and a pressure regulator, and adapters to connect it to the spark plug hole. Compressed air (or nitrogen) is fed into the gauge and the gauge is calibrated against a zero reference (no air leakage). The adapter is then screwed into the number one spark plug hole (with cylinder #1 at TDC compression) and the gauge is attached. As soon as the gauge is attached to the adapter air will begin to fill the cylinder. As this happens, the gauge will begin to indicate the amount of air flowing into the cylinder.

If air is leaking past the rings, or the valves, or the head gasket, we can directly read the amount of leakage, we can easily find the leak. If there is air blowing out the PCV, we have a ring leak, if its coming out the exhaust, we have an exhaust valve leak, and so on...

A leak down test is especially good at discovering leaky valves. We use much less pressure (about 80 psi) than is normally in the combustion chamber (several hundred psi), so if there is a valve that is slightly off its seat, the actual combustion pressure will help close it, the lower pressure won't put as much pressure on it.

This test is very time consuming with the engine in the car. To do the test, each cylinder must be tested one at a time, they must be at TDC compression, and the crank must be held in place to prevent rotation. A freshly rebuilt engine should have less that 4% leak down on each cylinder and if leak down is above 10%, service should be performed. All cylinders should be within 2 points of each other. NOTE: These figures are for performance engines, stock production engines may have as much as 10% right off the assembly line.


^^^ hope that helps brian.
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Originally posted by sarahdr1665
...some girls like the "nerdy" guys

My test came back really bad when i had a warped head and blown headgasket. it was like 180, 160, 121, 100 i think it was along those lines. mitsubishi priced me 1700 for that and timing belt so i told them hell no and did it my self. Now running 248 on all. I tested the other day. What is 248 all the way across acount to for ratio????

bent valves = no compresion in a cylinder unless they are bent very slightly..

bad head gasket is definatly an option.


what elevation are you at?
did you try more then one compression tester ?

When I had overheating problems, it turns out I had a bad headgasket (MLS), and when I did a compression test, it was 183psi (8.8:1 pistons) in each cylinder. I guess a bad headgasket doesn't necessarily effect compression readings. I'd recommend doing the leak down test.

Originally posted by MotoFool
bent valves = no compresion in a cylinder unless they are bent very slightly.. bad head gasket is definatly an option. what elevation are you at? did you try more then one compression tester ?

elevation has nothing to do with this test. same tester was used on multiple cars with better results. either something is fucked up or they need to redo the test with the same tester.

just remember... a headgasket keeps four things seperate. Atmosphere, coolant, oil, and compression. Any combination of these can mix if the headgasket blows.

-=B-=