Low compression in two cylinders that are directly opposite of each other.

ok. my main problem is i have low compression in 2 cylinders. here is the deal. I have 170 psi in all but 2 cylinders. they are not on the same side, so head gaskets are ok. the 2 i have a problem with are 3 and 4. they are the 2nd cylinders back on either side. the #3 on port side is 130psi and the #4 on starboard side is 100 psi. when i saw the first one low i thought it might be a sticky valve. then when i found the 2nd one low exactly opposite i am wondering if there is an intake gasket blown or something else. it seems weird the 2nd cylinder back on both sides are low. I was told to do a leak down test, but I do not have a leak down tester. The best case scenario here is the heads have to come off anyway correct?

If you haphazardly pull the cylinder heads without doing a leak-down test, you'll not know where the culprit is......... valves or piston rings.

It's not completely uncommon to see two opposite side cylinders low on pressure. It's the luck of the draw, so to speak.


There should be no correlation to the intake manifold gaskets that I'm aware of.

BTW, the SBC cylinders #3 and #4 do not share an intake manifold plane. (see attached thumbnail view)


Maybe you could rent a tester.

Ok. Thank you Ricardo. I will get a leak down tester. I was just wondering if maybe I could get away with not taking the heads off. I was hoping someone had seen this before and it was a easy fix. I knew it would not be. But I was hoping.

Bluecollerboater said:

Ok. Thank you Ricardo. I will get a leak down tester.

I was just wondering if maybe I could get away with not taking the heads off.

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Barney and I use Dynosuar urine in our cylinders to free up sticky rings.




Well, unfortunately..... there's just no realistic way to check or correct low cylinder pressures without getting into the engine.

If the cause is valves..... heads need to come off.
If the cause is in the cylinder/piston/rings, then these too must be removed.




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if only i hadn't used the last of my dinosaur urine last week on the dishwasher. Thanks ricardo. i knew the answer i just didn't want to believe it.

Bluecollerboater said:

if only i hadn't used the last of my dinosaur urine last week on the dishwasher. Thanks ricardo. i knew the answer i just didn't want to believe it.

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Sorry to be barer of not so good news. I do know what you mean, however.

hi there i just registered here great site. what i need to know is i just bought a aq 130 motor to replace my old one it is fresh water cooled can i remove system and just go raw water cooled due to space in engine compartment thanks for any input