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Compression Test Numbers...I'm scared Now

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ayleeuss

Contributing Member
2001 5.0 Mercruiser Alpha carbed in a 22' Stingray. Boat just wasn't running quite right the last two times i had it out. In the slow speed zones (about 1400 rpm) it was surging slightly. Seemed ok at cruising speed. At WOT it did not have the power it normally does. I could normally get 4900 RPM at 50 MPH (GPS speed), it would only get to 4400-4500 RPM at 45-46 MPH with a slight poping, like a backfire out of the carb.
I decided to do a compression test on the motor hoping it would give me some piece of mind. Well, now i am scared. Engine warmed up, throttle open. Here's what i got:
#1-180 #2-120
#3-180 #4-170
#5-160 #6-140
#7-180 #8-130

I repeated the test several times. numbers would change 10-15 psi on the lowest cylinders, would change about 5-10 psi on the highest cylinders. But the high ones stayed high, low ones stayed low.
What does everyone think?
 
Ayuh,.... While yer compression numbers ain't the greatest,....

Yer runability issue is Bad fuel delivery, it's goin' Lean at higher rpms,...

Probably crap in the carb,...
 
ayleeuss said:
2001 5.0 Mercruiser Alpha carbed in a 22' Stingray. Boat just wasn't running quite right the last two times i had it out. In the slow speed zones (about 1400 rpm) it was surging slightly. Seemed ok at cruising speed. At WOT it did not have the power it normally does. I could normally get 4900 RPM at 50 MPH (GPS speed), it would only get to 4400-4500 RPM at 45-46 MPH with a slight poping, like a backfire out of the carb.
I decided to do a compression test on the motor hoping it would give me some piece of mind. Well, now i am scared. Engine warmed up, throttle open. Here's what i got:
#1-180 #2-120
#3-180 #4-170
#5-160 #6-140
#7-180 #8-130

I repeated the test several times. numbers would change 10-15 psi on the lowest cylinders, would change about 5-10 psi on the highest cylinders. But the high ones stayed high, low ones stayed low.
What does everyone think?
Click to expand...

A typical suggestion is that all cylinders be within 10% of/from the highest pressure cylinder compared to the lowest pressure cylinder.
Your readings show that you are well out of this safe range..... (I.E., 180 psi -vs- 120 psi)
This is not good.

Question:
Are you cycling each cylinder through several compression strokes?
The pressure within the test gauge is accumulative only until the hose the burden tube equalize.
Once after that, you could spin the crankshaft 100 X's, and the reading will/should be constant.

Conversely, if you do not spin it through at least 3 compression cycles, your reading may not be accurate.


With exception to #4 cylinder, it's somewhat interesting that your Stbd bank is on the high side, yet the Port bank is on the low side.



.
 
First your gauge could be off way, way off.

What would i do... leave the good bank alone and set up the low bank again.

I use the running method. Check the studs first maybe one or 2 pulled up, look at the springs see whats what before you set up the bank again.

It's going to get a little messy but the oil is good for the metal have a bunch of rags.

Post back.
 
I was thinking the runability issue is fuel related. Filters were replaced 3 months ago as yearly maintenence. I dumped out the canister into a jar and it was clean. Pulled the little one out of the carb inlet and it was clean also. I use it often enough that i dont think it would be a fuel quality issue but i need to run it off a seperate tank to be sure.
As far as the compression numbers, i would crank it till the gauge wouldnt go up any more. i found it intresting that, for example, #1 would test at 120, then i would test another cyl, go back to #1 and it would get 130.
I plan to squirt some oil in the cyls and redo the comp test. I would also like to get another gauge to confirm the results.

What is the "running test"?
 
Also, as far as the WOT running issue. It did not pop everytime i ran it WOT, only the first time i tried to run it at WOT. Later in the day i opened it up again and it ran smooth, but would not get up to the RPM and speed it normally would. Almost felt like the timing was slightly retarted.
Seems ive got a mess on my hands with compression numbers the way they are. I'm wondering if it will ever run right with the compression so off.

A theory i have is that maybe the stbd exhaust manifold is leaking water into the engine causing all of my problems. They are original. 2 yrs ago i had to pull em and unplug em.
 
Took the exhaust manifolds off and it was what i suspected. Stbd side riser was in 2 pieces. Exaust ports on the head were rusty but does not seem to be any water in the oil. Port side was leakin too but not as bad. One exhaust port was rusty. I'm wondering how bad it would run w those compression numbers if i tried to put manifolds on it and run it. The voice in my head says i have to pull the heads.
What does everyone think?
 
if you use the boat a lot or take it offshore, fix it right, now...no more issues for a while and you'll recoup the repair costs with improved fuel economy...one call to SEATow should be more than enough to get the head straight at the machine shop...

if you don't use it much, it will run for a few more hours but you know the head will have to come off before too long.

Before you make the final decision you may want to do a leakdown test to ensure only the head needs to be reworked...
 
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