Interpretation/thoughts on compression test results

[hoowahfun]

Engine is a 1987 3.0, S/N B515218.

Engine is currently not running, and wanted to do a compression test as a start to some diagnostics I want to do on the overall engine. Engine was seized when I got it, but freed it up after soaking the pistons for a few days and changed the oil.

Engine dose not run so test was done on a cold engine. Compression test went as follows:
1) Removed plugs and grounded the wire coming from the coil.
2) Throttle at neutral position. Would not crank at forward position so I kept it at neutral (honestly I don't see why throttle position has an effect on the test...maybe someone knows?).
3) Went front to back with the tester and cranked it over a few times (4-6) until the needle on the gauge didn't move. Dry test results:

1-160
2-50
3-90
4-130

4) Tried to isolate cylinder #2 to see if the low pressure could be from a blown head gasket. Dry test with plug in 3: 50 psi (no change). Dry test with plug in 3 & 4: 50 psi (no change).
5) Tried to isolate cylinder #3. Dry test with plug in 2 & 4: 90 psi (no change).
6) At this point I thought either cracked block/head, stuck valve, or stuck rings. So I started with a wet test. Pumped in 3 squirts of oil from my oil can into cylinders 2 and 3. Put all the plugs back in, cranked it over a few times. Removed the plugs then tested 2 and 3. Wet test results as follows:

2-80
3-125

I was encouraged at this point that it could possibly be stuck rings causing low compression, thought number 2 concerns me a lot. Still could be other issues, but other tests will hopefully reveal that. Service manual calls for 140 psi as normal compression. I went back and also did another test on #1 since I thought 160 was pretty high. Final dry test on #1 gave me 120 psi, but my key almost broke off in the ignition so I had to stop the test after a couple of revolutions. This number makes more sense to me given the age of the motor. I'm going to try to have a couple of other keys made today and try to repeat the test this weekend.

Initial thoughts???

Thanks!

 

[Scott Danforth]

two scenarios

motor was overheaded and there is a warped head between cylinders 2 and 3
manifold/riser joint leaks and valves/rings are rusty in cylinders 2 & 3

either way, drive is coming off, motor is coming out and needs to be torn down.

and yes, water in cylinder #1 will give you a high reading

 

[hoowahfun]

I'm thinking it could be overheating. I noticed that every plug wire looks bubbled and slightly deformed where it connects to the plug. That makes me think this engine overheated at some point...which means I need to find the cause and what damage was done.:blue:

 

[harleyman1975]

most likely cause was bad impeller and damage is either warped head/blown head gasket or rings lost their temper or scored pistons.

 

[Bondo]

Ayuh,.... It sounds like a rebuildable core motor,...
Not a motor that'll ever run, Reliably, without a full on rebuild,....

 

[QBhoy]

Most likely what the above experts are telling you, but a few thoughts if I may...
Without checking online, I would imagine that 160 psi is a figure above what a brand new engine would have. How certain are you of the gauge ?
Also, I was recently lucky enough to buy a boat with an outboard that was written off due to low compression on 1 of its 3 cylinders. They were 120, 70 and 118 when I first tested.
I left oil down the bores for 2 days....got the engine running. Sounded ok so did another compression test warm.
120, 118 and 120.....bargain !

The idea with the throttle open is to allow as much air to be inducted as possible during the test. Must admit, it's never made much difference in any compression test I've done.
Check the gauge I'd say before you start ripping things to bits.

 

[hoowahfun]

Thanks for all the tips. Part of me thinks a rebuild would be kind of fun and would give me a chance to get to know the motor and tinker around. Would be my first rebuild of a full size motor that's not an ATV or small engine...but part of me just wants to get on the water and I don't have a garage big enough to pull the boat into or an engine hoist so I may just sell this whole thing at a loss and move on. Another option would be to get a running donor motor and swap out the engine.

As far as the gauge goes...I think it's okay. It's an Autozone rental so not the best, but it looked brand new when I opened the case. I'm going to retest each cylinder a couple more times just to try to rule out any flukes.

 

[Maclin]

Regarding the Throttle, sounds like you did not have the lever in throttle only mode.

 

[achris]

I have never trusted cold tests. You need to get the engine running and re-test. And yes, there will be a button on the control lever that you push in, or maybe the whole lever pulls out slightly, to release the shift cable and advance 'throttle only', and ALWAYS have the throttle at full open when compression testing. As already stated, allows a full belly of air in.

Chris..........

 

[hoowahfun]

Thanks everyone. A lot to think about and do. With remnants of Irma coming through the area in the next couple of days I may have to wait a few days before I can get back to it.

 

[hoowahfun]

I have never trusted cold tests. You need to get the engine running and re-test. And yes, there will be a button on the control lever that you push in, or maybe the whole lever pulls out slightly, to release the shift cable and advance 'throttle only', and ALWAYS have the throttle at full open when compression testing. As already stated, allows a full belly of air in.

Chris..........

I wasn't able to do much today on the boat. Mostly just covered it and tied everything down. I did find the button you mentioned. It was disguised as an emblem of some kind where the throttle rotates. Push it in and that's throttle only.

I did discover what could be the cause of the melted plug boots. The previous owner had 4 new plugs in the glove box which I assumed were correct. Silly me. I checked with the service manual and sure enough they were a hotter plug in the same series. My assumption is that's what he used before I bought it.

My question is this: could this cause any damage related to a low compression condition?

 

[Scott Danforth]

a hotter plug wont melt plug boots. an over-heat would.

a hotter plug wont cause low compression. an over-heat would

see post #2

 

[hoowahfun]

a hotter plug wont melt plug boots. an over-heat would.

a hotter plug wont cause low compression. an over-heat would

see post #2

Good to know, thanks.

 

[harleyman1975]

He tried hotter plugs because the two dead holes...it didn't work.