What does this indicate?

[FFRSpec72]

Did a compression test, 1, 3 and 4 were 150PSI and 2 was 142 PSI. Then did a leak down test @ 90 PSI and 1, 3 and 4 were 8%, which is fine, but as soon as I had the gauge on 2 pressure went to 75 PSI and difference was about 10% but why the drop to 75PSI?

Both tests were on a cold engine as engine is out of car

[Racebrewer]

Warm would be best. Was the butterfly wide open?

Did you repeat compression test with a little oil in the cylinders? Rings would seal better with a bit of motor oil.

Re the leakdown test, did you listen at the throttle body, exhaust and crankcase for the tell tale leak sounds?

John

[FFRSpec72]

I have to do tests cold as engine out of car (not ideal but it is what it is), I did not have it on WOT for compression test (I forgot that part) but not sure why its required. The compression test seem OK.

The leak down an the #2 cyl, bothers me as I set the standard pressure from compressor to 90PSI connected the hose to the leak down pressure fitting and the pressure at the standard pressure gauge went from 90 PSI to 75 PSI and the leak down pressure gauge was at @ 70 PSI. I have never seen this before. the other 3 cyls had about an 8% leak down pressure (the standard pressure remained @ 90 and 1, 3 and 4 cyls were at 84PSI).

[Tocoti]

Warm would be best. Was the butterfly wide open?

Did you repeat compression test with a little oil in the cylinders? Rings would seal better with a bit of motor oil.

Re the leakdown test, did you listen at the throttle body, exhaust and crankcase for the tell tale leak sounds?

John

RB, i went to Wellsville for automotive and that was one good tool to learn. Just to add, don't just listen while it's leaking down, keep the pressure on. Use the stethoscope with just the open hose. You'll find it. It might not tell you what it is but it will give you a direction.

[Racebrewer]

FWIW, Having the throttle plate WF open allows the motor to suck air freely, so there are no complicating factors to affect the readings. A closed throttle plate (especially if inconsistent) will lower the compression test reading.

IMHO, In regards to the leakdown test, the results can vary depending upon condition of the intake valve and seat, exhaust valve and seat, and ring seal. The ring seal can vary depending upon where in the cylinder the ring is sitting at the time of the test (TDC, BDC, middle of rotation, etc, etc......). The ring may be sitting in a spot where there is some slight damage. Let's say there is a scratch at BDC. It'll leak more when the ring is at that spot. A wear spot or a distortion at some spot in the bore will do the same. A lack of oil in the motor will do the same. (Oil seals nicely). So if you do a leakdown test while the rings are in a bad spot, it'll read lower than in a good spot, but if its at BDC, who cares.

A cold, dry motor will make the readings lower than a warm, oiled motor. Consistency in how you do the test can remove some head scratching.

John




I have to do tests cold as engine out of car (not ideal but it is what it is), I did not have it on WOT for compression test (I forgot that part) but not sure why its required. The compression test seem OK.

The leak down an the #2 cyl, bothers me as I set the standard pressure from compressor to 90PSI connected the hose to the leak down pressure fitting and the pressure at the standard pressure gauge went from 90 PSI to 75 PSI and the leak down pressure gauge was at @ 70 PSI. I have never seen this before. the other 3 cyls had about an 8% leak down pressure (the standard pressure remained @ 90 and 1, 3 and 4 cyls were at 84PSI).

[FFRSpec72]

FWIW, Having the throttle plate WF open allows the motor to suck air freely, so there are no complicating factors to affect the readings. A closed throttle plate (especially if inconsistent) will lower the compression test reading.

OK, I understand the WOT issue, but the immediate drop in standard pressure makes no sense to me.

[skullandbones]

I hate mysteries. So since your engine is out anyway, I would pull the head. Put gas in the chamber to see if it leaks out (burnt seal/valve or bent valve). Then pull the piston and check for damage or proper stagger. I know you assume the rings are installed correctly but maybe not. JMHO, WEK.