Merc 4.3 compression test and non-start issue

[CAPTahab7]

I have a 2003 Stingray with Merc 4.3 Carb 190A (Ser #M616646), just got it out of the shop (tune up/replaced plugs/clean carb/thermostat/oil). Running well until about 8 running hr later (exception, slight nock under high load). After tubing about 1 hr, put engine in idle and it stalled out. Cranks, but could not restart. Getting fuel, checked sparks and found one spark plug not firing (strange, but bad plug). Then I compression test and I get crazy (and concerning) readings: 120, 30, 65, 90, 90, 90 psi. I follow test kit (all sparks out, fuel and spark off, test each port with 3-4 cranks, record highest) but have to do it cold vice engine hot. Does this sound like a catastrophic mechanical failure or am I just not doing test right? What psi should I expect? Motor is only 110 hrs old of running time. Anay ideas on what the issue might be or what to test next?!

 

[Don S]

Re: Merc 4.3 compression test and non-start issue

Here's the service bulletin on doing a compression test. Make sure you get 4 or 5 complete compression strokes per cylinder and see if they remain the same. then do the oil test specified in the bulletin.

http://www.boatfix.com/merc/Bullet/97/97_25.pdf

 

[CAPTahab7]

Re: Merc 4.3 compression test and non-start issue

Thanks Don S. I followed the procedures with following results (all psi): Cyl 1: 130, Cyl 2: 95, Cyl 3:220, Cyl 4: 25, Cyl 5: 210, Cyl 6: 180.
I tried the air-leak check on Cyl 4 and think I can hear it coming back out of the throttle body (but not really sure). It would not come up to pressure.
Boat WAS running great, then this happened (seemly catastrophic failure). Any thoughts as to why or what to do next? Complete motor tear down? This is a 2003 motor, but with low (110) hours. Previous owner had no issues and last mechanic tuned up at 102 hrs and did not have any trouble to report. Did I get a lemon?!

 

[CAPTahab7]

Re: Merc 4.3 compression test and non-start issue

Oh, one more thing, it appears when I run on hose with out sparks in (just crank) it blows some water out #3 Cylinder.

 

[Pete104]

Re: Merc 4.3 compression test and non-start issue

Are you working on this in or out of the water?

 

[CAPTahab7]

Re: Merc 4.3 compression test and non-start issue

Out of the water. I can run it on a garden hose, but it runs poorly/rough. I just very surprised to have what appears to be major mechanical issues pop up like this after engine running so well moments before.

 

[Bondo]

Re: Merc 4.3 compression test and non-start issue

I just very surprised to have what appears to be major mechanical issues pop up like this after engine running so well moments before.

Ayuh,... That's how catastrophic failure Usually occurs...

 

[CAPTahab7]

Re: Merc 4.3 compression test and non-start issue

I just re-ran compression check with another gauge (trying to dispell my shock). Results in cyliner 1-6 order: 130,90,160,20,185,180psi. I get some water to blow out of cylinder 3 too. When I squirt some oil in cylinder 4 to check rings (per tech bulletin), it does not come up in pressure. I don't hear any 'bad' noises when I crank the engine and run (just it can run, but very poorly).
So now I'm thinking Head Gasket and/or valves?
You think I'm on the right track (any help guys)?
How big is that job for a DIY type?
Is there any recalls for this engine due to manufacturer issues?
Thanks!

 

[OldNBold51]

Re: Merc 4.3 compression test and non-start issue

sounds to me that you have an issue with the even # cylinders/head. How easy is this depends on how easy the access you have to the engine compartment is. Pull the head off the even # and check the head gasket and look for cracks in the head also. Check for scoring on the cylinder walls.
Download the manual so you can put it back together correctly.

 

[CAPTahab7]

Re: Merc 4.3 compression test and non-start issue

How about on the high side? Are pressures like 185psi, 180, 165psi in the realm of normal?

 

[CAPTahab7]

Re: Merc 4.3 compression test and non-start issue

Thanks guys. I blew a head gasket and was able to rebuild the engine (top end) to fix. Once I got the timing right (darn those distributors), it started up.