Compression test / Cylinder leak test

[Chevy666]

Hi!

I bought an vortec engine for my 89 OBS, thinking that the vortec should be a better option then the old TBI.
But yesterday I did an compression test on all 8 cylinders and had good readings, approx. 175psi to 195 psi.
Even though the reading were good i thought i´d check the cylinders with the cylinder leak test. So started checking the 1,3,5,7 cylinders and on 3 & 7 cylinder i had approx. 20% lead which isn´t bad but on 1 & 5 i had 90% leak, and heard that on 5 I heard air flowing in the oilpan = leaking rings? and on cylinder 1 i felt air flowing though the intake = leaking intake valve?
So what´s you opinion? Should I change rings and change/fix the valves even though i had good compression? The test was made with a cold engine so the rings were also cold.

Thanks in advance!

Br. Sam

 

[Erik the Awful]

I take it this is a used motor? With those compression numbers I'd go ahead and run it. Maybe some day in the future I'd rebuild it.

 

[Chevy666]

Exactly, it’s an used engine.
I haven’t installed it in the OBS yet but because the compression numbers were fine I thinking of just running it as it is... in the plans of changing the cam for a Hotcam as well before installing it

 

[Schurkey]

I did an compression test on all 8 cylinders and had good readings, approx. 175psi to 195 psi.

Seems high. Have you tested your gauge for accuracy?

i thought i´d check the cylinders with the cylinder leak test. So started checking the 1,3,5,7 cylinders and on 3 & 7 cylinder i had approx. 20% lead which isn´t bad but on 1 & 5 i had 90% leak, and heard that on 5 I heard air flowing in the oilpan = leaking rings? and on cylinder 1 i felt air flowing though the intake = leaking intake valve?
So what´s you opinion? Should I change rings and change/fix the valves even though i had good compression? The test was made with a cold engine so the rings were also cold.

Hot or cold doesn't make much difference with a leakdown test--the rings are going to cool off when the air blows past them anyway.

90% leakage? That engine would get some inspection before I installed it.

Cylinder #1--leakage past the intake valve:
Remove valve cover, smack #1 intake valve rocker arm with a rubber mallet, straight in line with the valve stem. Ideally, you don't hit the rocker so hard that it spits out the pushrod. Just enough to open and shut the valve, doesn't have to be "fully open". You may not open the valve more than a sixteenth or an eighth of an inch. Pop it a few times. Could be a stray fleck of carbon on the valve seat/valve face holding the valve open. Verify the valve spring isn't broken. Re-test. If it's now good--you're done with that cylinder. If not, cylinder head comes off for more-intensive inspection.

Cylinder #5--leakage into the crankcase
Find or buy a borescope. Pull the spark plug, look at the cylinder wall and piston. See if the wall is scored, or if the piston has a hole in it. If it looks good, buy some GM Top Engine Cleaner, (TEC) dump 2 ounces down the spark plug hole, and turn the crank a few revolutions with the plug OUT. Then put the plug back in, finger-tight. Let it sit over night. Next morning, remove plug, drop another 2 ounces of TEC into the cylinder, roll the crank a few times with the plug OUT, and again let it sit for as many hours as you can stand it. This all works best if the engine is on an engine stand, rotated so that cylinder is straight up-and-down, not angled off to the side.

Retest. If it's better, great. If not, you MAY want to install the engine, and run it until warm, and repeat with the TEC. If you don't want to wait to run the engine...remove the oil pan, for closer inspection of cylinder wall, piston, rings, etc. You may have to pull the head to check out the cylinder wall above the piston.

Don't forget to check 2, 4, 6, and 8.

 

[Erik the Awful]

After reading Schurkey's post and thinking about it, I would suspect carbon buildup. Carbon deposits in the combustion chamber will raise your compression ratio, and if they're impeding the seats, it'll give you the leakdown as well.

 

[Lanny]

Obviously the engine isn't installed in a truck currently so if it were me i would at least remove the heads. It gives you a chance to clean everything and replace the head gaskets while you're in there. Run through the valves and make sure they're all set correctly on reassembly. Id be tempted to do an intake manifold gasket as well as they are somewhat common on these engines.

 

[Erik the Awful]

I suspect the engine is still in a donor chassis, but then again, I have seen guys start a motor laying on the ground. Kind of hard to do a compression check without it being mounted at least on an engine stand.

 

[Chevy666]

Hi and thanks for all the reply’s!
Yes the engine isn’t installed in my truck, it’s a while before I’m there because I’m rebuilding the whole car from bottom and up, the car is in molecules but it’s a slow process especially when I live in Sweden and getting ahold of parts isn’t the easiest on another subject I was about to order some new parts for the leaf springs and total price for the parts mounted up to 37$ but then 320$ for the postage cost so the build takes some time...

Yes the engine was on my trailer when I did the tests, hooked up a battery to the starter and that way did the compression test, and as you say the Readings are quite high to whats “normal” compression According google
But the leak down test got me wondering, all except 3 & 7 showed approx 20% wear which is really good in my book, so I thought that they should be worse or some indication to be worse than 20% if I have 90% wear on 2 cylinders, which lead me to doubt my precision when finding the TDC on these two, I used the flashlight on my phone and dialing in when I thought the piston was in top/stoped moving so after 1-2 tries I got the cylinders reading 17-20% so all cylinders are ok according to the leak down test.

yes I’m really tempted in opening up the intake & head just check and clean everything really thoroughly and see if there burnt oil or something like that in under the manifold (had this in my old TBI that seized due lack of oil changes) so with this engine being in such good condition (according to cheep O indicators) I feel I won the lottery got the engine, trans (4L60 computer) and transmission case in on package so I’m really satisfied with that. Now I just have to dig through all the forums to find a electrical scheme so I can hook all small electrical parts like turn key and so on. Thinking of going with the 411 computer as well in the future so I can tune the car my self, getting a tuner for a GM car isn’t either the easiest in Sweden

 

[Schurkey]

all except 3 & 7 showed approx 20% wear which is really good in my book, so I thought that they should be worse or some indication to be worse than 20% if I have 90% wear on 2 cylinders, which lead me to doubt my precision when finding the TDC on these two, I used the flashlight on my phone and dialing in when I thought the piston was in top/stoped moving so after 1-2 tries I got the cylinders reading 17-20% so all cylinders are ok according to the leak down test.

"Leakage" and "Wear" are not the same thing.

A new engine, having essentially zero wear, might have 40% indicated leakage past the rings, depending on the sensitivity of the leakdown tester used.

The construction of the leakdown tester has just as much to do with the indicated results as the actual amount of leakage. Making things worse, there's no "Industry Standard" or legal requirement governing the construction of automotive leakdown testers. (There is for aviation leakdown testers, via the US FAA regulations.)

Glad you got yours figured out. Re-testing "bad" cylinders is always a good idea.