3.0L LX zero compression on #4

[DBlume]

My 1997 Suntracker pontoon has been running okay all season but it would not reach WOT. First outing this season it would only reach about 3100-3200 rpms, figured it was a carb issue or fuel filter. Purchased a new fuel filter and plugs at the same time, when I went to install both I found the #4 plug was melted or fried (the pin exiting the plug was missing). So I figured this was my main issue for lack or rpm's. After replacing fuel filter and plugs, boat ran better but still only reached 3600-3700 rpms on last two trips since swapping them. After this last trip watching rpms slowly decrease to 3200 from 3700 on return trip home I purchased a compression test kit. Just tested last night and was surprised to find out #4 gave me nothing on the dial. Other readings were 1-145, 2-140, 3-150, and 4-0. The hour meter only shows 53 hours, I purchased this 1.5 years ago. While on the boat last season and this season I never remember seeing it overheat and I am pretty diligent about watching gauges on the water. What could have caused this?

A family member recommended trying a wet test by squirting in a little oil and trying again. Reading still showed 0 psi after inserting oil. I was told to try a leak test next and listen for leakage from oil cap and exhaust. I understand the oil cap but where do I listen for the exhaust side?

 

[Bondo]

What could have caused this?

Ayuh,...... Welcome Aboard,..... Detonation, most likely, 'n probably from a lean condition,....

Bad fuel, water in the fuel, 'n crud in the fuel are common causes,..... ethanol makes it worse,....

The damage is so severe, ya might as well just pull the head, 'n Look,...
I'm sure the damage will be obvious with 0 psi compression,...

For the pressure test,....
If the exhaust valve is tuliped, you'll hear the air escape through the exhaust,....
If the intake valve is beat, you'll hear it through the carb,.....
If the rings are pooched, or the piston holed, you'll hear it in the oil fill, 'n vent,....

 

[DBlume]

Bondo, thank you for the response. I have been creeping here since I go the boat but have not had to post until now

For the pressure test, where should I listen for the exhaust portion? What is the worst and best case scenario for leakage, just so I know how many beers to drink after hearing it leak? I am no mechanic but handy, should I tackle removing the head or have a professional do it?

It does not have a inline fuel water separator, the previous fuel filter I replaced was on old stone filter. After getting all this taken care of, I take it I should have one installed.

 

[Bt Doctur]

with 0 compression you either have a tuliped valve, a burnt valve or a sticking valve. best thing is to pull the head and look.
Most cases are caused by a bad riser leaking water into the exaust port

 

[DBlume]

with 0 compression you either have a tuliped valve, a burnt valve or a sticking valve. best thing is to pull the head and look.
Most cases are caused by a bad riser leaking water into the exaust port

Thank you Bt Doc, I will try to remove the head and get some pictures.

 

[DBlume]

Well air is leaking from carb, did not hear any from exhaust or crank case. Still have not removed the head, looking to see if I should just pay someone as I will have to pay someone to fix it anyway.

 

[Eric 3.7LX Alpha 1]

Detonation can cause overheating, damage to the spark plugs, pistons, and/or valves.

Use of low octane gasoline is one of the most common causes of detonation. Other causes are:

1. Over-advanced ignition timing.

2. Lean fuel mixture at or near full throttle (could be caused by carburetor or leaking intake manifold).

3. Cross-firing spark plugs.

4. Excess accumulation of deposits on piston and/or combustion chamber (results in higher compression ratio).

5. Inadequate cooling of engine by deterioration of cooling system.

(from Service Manual 13)
http://boatinfo.no/lib/mercruiser/ma...ser13.html#/98

Here is a video of Detonation/pinging on a car... you can hear it well at about the 3-minute mark:
https://www.youtube.com/watch?v=JA0cH_vRABg


Pulling the head off is not a bad job. The manual I have linked above is for your engine. I think Chapter 3 covers it. Give it a read and see what you think.

 

[stonyloam]

Well air is leaking from carb, did not hear any from exhaust or crank case. Still have not removed the head, looking to see if I should just pay someone as I will have to pay someone to fix it anyway.

I suspect that if you had a hole in the piston you would have gotten a ton of oil smoke when it was running, and you are hearing air out of the carb, so I suspect a valve problem. IMO you should just go ahead and pull the head. If it is a piston you will know right away. Just taking the valve cover off you might see what is going on. Pull the plugs and rotate the engine, watch the #4 valves. See if one is not moving. If it is a valve, you should be able to take the head to a machine shop and have it rebuilt. From that point you should be able to do it all yourself. You will just need gaskets, some Permitex aviation sealant and a good torque wrench. Should be able to save a ton of money doing it yourself. Good luck.

 

[Benny67]

0 psi on a compression reading and the motor is running would lead me to believe it's an exaust valve issue.

Even the worst rings would give some sort of compression if the valves were solid and if you had a hole in the piston you would have some smoke happening in the exaust..to say the least.

Also, if it was an intake valve you would probably see some sort of serious compression coming back out the carb when running especially under load...maybe even some fire out of the carb.

Keep it simple and don?t get ahead of yourself. Have a look at the valve train before committing to tearing the head off.If the valve is tuliped or burned you should see the top of the valve sticking up slightly higher than the other valves.

 

[DBlume]

No Title

Well got the head off. From the looks of it, looks like the intake is tuliped as most of you guessed. Everything else looked okay as far as the other valves and top of the head. The piston was much more discolored than the others, but walls looked clean and it moved well.

Machine shop next? Anything else I should do?

 

[carrier82]

I had similar issue with a 350. The engine was running too lean. New intake valves, gaskets and install plus a carb tune worked for me.

 

[stonyloam]

Yeah, find a good machine shop for a valve job, ask about the condition of the guides and seals (should get new seals) and you should be back in business. Here is a parts diagram (with part numbers) of your head: https://www.mercruiserparts.com/Sho...bnbr=100&bdesc=Cylinder+Head+and+Rocker+Cover I am a fan of OEM head gaskets, IMO worth the extra cost, the other gaskets aftermarket should be OK. Good luck.

 

[DBlume]

Yeah, find a good machine shop for a valve job, ask about the condition of the guides and seals (should get new seals) and you should be back in business. Here is a parts diagram (with part numbers) of your head: https://www.mercruiserparts.com/Sho...bnbr=100&bdesc=Cylinder+Head+and+Rocker+Cover I am a fan of OEM head gaskets, IMO worth the extra cost, the other gaskets aftermarket should be OK. Good luck.

Just got back from dropping it off at a machine shop my boat mechanic recommended, about $150 to redo all of the valves. Thanks for the link, I will see if I can get my hands on the OEM gaskets. I will also be having the carb rebuilt with the parts kit I got from Mike's, having my mechanic do it so maybe he can diagnose what caused the lean condition in the first place.

Thank you for all your help everyone

 

[stonyloam]

Your local Mercruiser dealer or these guys https://www.mercruiserparts.com (where the parts diagram came from).