Tampa 928s

86

Doing a leak down test today and need to know how to ID which piston is up when.

Imo000

Find the TDC for #1 (crank at O/T and cams noches are at the timing mark) and then use the firing order to go to the next one. To check the cams, all you need to do is pul the intake tubes and look down the vent holes.

Tampa 928s

I understand the TDC but I assume you will need to turn the motor over to have each cylinder in the top position. So If I do number one insert a rod into the next cylinder observe the rod go to the top will that work?

Mrmerlin

yes it will .
otherwise simply turn the crank 90 degrees for each succeeding cylinder
8 X 90 = 720 or 2 full rotations

Tampa 928s

86 S/C

I have some inconsistent readings:
Did the leak down and all cylinders are with in specifications, when I did a compression test I found the following:

Cold engine dry:
1 180
2 170
3 175
4 175
5 185
6 180
7 90 Very low reading
80 190

Any feed back

Tampa 928s

Number 7 with oil 122 Lbs

Dave928S

That's weird .... if you get a bad compression reading you should also get a bad leakdown. What were the running symptoms that led you to do a leakdown?

It's possible in doing the leakdown test that you blew some carbon around and it's now lodged under a valve, which is giving you the mysterious low comp reading.

I'd say go through the sequence again but this time with a warm engine. I recall a recent thread where the same symptoms were as a result of some loose carbon ... which blew away in a sputter of black crap when it was started again.

Dave928S

With oil improvement of 30 is not a lot above normal response to oil ... if it was a ring it'd be way more than that. Seems to point more to valves or gasket.

Tampa 928s

Well the leak down tester is gone for now so doing it again would be for another date.
Interesting on the carbon under the valve, plugs look good, motor runs fine and no oil burning.
I am adding a AFPR and thought while everything is apart to test the cylinders.

Fabio421

The threads were tight on cyl. 7 and it didn't allow for the comp. tester to screw in all the way and seat properly. #7 cyl is fine. Chase the threads and you will find that its right where it needs to be.

Dave928S

If you're down that far "really" on compression you should have noticed it was running on seven ....BTDT. I was down to 110psi terminally on one cylinder, and the 'get up and go' just 'got up and went'.

Doesn't mean you couldn't have a real issue ... but you never know your luck. A look with an endoscope could give some info on that cylinder if you know someone who has one.

An Italian tune up could be in order.

Tampa 928s

Shawn,

These readings were done this morning, Zeus said to get some GM top cylinder cleaner and soak it over night and see if the ring frees up. That is if the ring is stuck, I think I need to do another leak down test with the oil cap removed per Paul.

Tampa 928s

I put 80 Lbs of pressure in number 7 and I do not hear any air leaks!