Thread: Low compression and good leakdown... what does it mean?

Also... im not.sure how sensative.hydrolic lash is but YES if you re shimmed.everything.too tight.the theres a.chance a.valve.can be hung open. its.something i deal with more on the gtir engine.

I might be wrong here and if so someone please correct me. My understanding of the hydraulic lifter is that it makes up any minor difference in rocker arm height via oil pressure keeping it extended. When shimming valves with hydraulic lifters you shim the 2 valves each rocker arm sits on to the same height so that the cam contacts the rocker arm evenly.

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Not sure if it's relevant or not but the crank case is pulling vacuum when idling. Not sure how much but it definitely pulls vacuum at the t from the valve cover.

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Ran through another compression and leakdown test this evening. Also did a block tester test.

Compression 1-4 158, 40, 145, 80 (previously 150, 30, 150, 50)

Leakdown % 1-4 2.5, 15, 9, 15 (previously 5, 17, 5, 15)

Engine was fully warmed up this time.

Blocked tester detected no exhaust gasses in the coolant. No smoke coming out the exhaust either. I'm pretty sure the head is sealed.

I noticed on the cylinders with the lowest leakdown had the most pressure coming out of the t on the valve cover. The cylinders with higher leakdown had less pressure coming out of the valve cover t and there seemed to be a slight change in sound on the two bad cylinders when opening and closing the throttle body. makes me.think the rings are good and the pressure is leaking by the valves and.not.the rings. Sound logical? I'm going to adapt my leakdown tester to just pump air straight in to the cylinder without going through a 1 mm orifice so I can get more airflow to detect exactly where it's leaking from. Am I going in the right direction? I think I am...

Also, the lifters in this engine were ran without being full of oil. They were not bled correctly by me They had pretty much no oil in them when I pulled them and rebled them a few days ago. Is it possible they might be permanently damaged. I have some in another engine in the garage I can swap in if needed.

Any and all comments, andvice, suggestions are appreciated and welcomed!

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one way to rule out the valves is to take the valve cover off and when you put the cylinder on todc of the compression stroke check the clearance between the rocker and the cam of the cylinder your checking

if you have the correct gap and its not bound up then you know its not a shimming issue

I just finished up a high compression build so I can share some findings.

First, don't worry about the ultimate numbers for the tests, instead pay attention to differences between the cylinders. In your case, the differences are a concern.

The problem with hydraulic lifters they can fool you because they can either bleed down or pump up. When they bleed down or have air inside of them, the compression test reading will be high because a bled down lifter is like having a small cam. The full cam profile is not followed closely.

The opposite situation happens when the lifters are pumped up. Some time the valves never close. Your leakdown test might point to this. To double check, do a leak down test for the suspect cylinder. Now, slowly start loosening up all the cam caps. You don't have to fully loosen up the cam caps but just enough so the cam moves up, then take a reading. See if it changes for the better. This is just one process of elimination.

Thanks! That's a good idea about loosening the cam caps. I didn't think of that.

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Verdict is in! not satisfied with the numbers from the leakdown test I modified my tester to just pump as much air as possible into each cylinder. I did it with the cams out so there would be no question the valves should be closed. Cylinders with bad compression had a fair amount of leakage out of the exhaust valves. Gremlins put to rest... anyone know of a good machine shop in phoenix that can match the valve work already done to the head. I don't really want to ship this thing anywhere...

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Ah! Good to know the culprit. Now it can get fixed! I've used B&B in Mesa. Only had a standard valve job done with one of my engines. Might see if they can accommodate it.

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Sounds like someone fubar'ed the S4 cam install..