Merc 3.0LX Compression Numbers

[maxumized]

3.0LX / Alpha One will not reach WOT RPM of 4500-4800. Only about 3000.

Compression readings on all 4 cyilnders are within 2psi of each other, however, they all read approximately 100psi... right at the minimum listed in the Merc manual. Is this compression bad enough to be causing 1500 lost rpms at WOT or have I additional problems? I've checked for exhaust obstructions and gone over the ignition and prop issues. All that seems to be left is the carb / compression.

 

[JustJason]

Re: Merc 3.0LX Compression Numbers

go through your fuel system starting with the fuel filters.
and adjust the valves and see if you can get some of that compression back.

 

[craze1cars]

Re: Merc 3.0LX Compression Numbers

Just for comparison sake, my 1992 3.0L (Not an LX...has different heads than yours) pulled 140 to 145 psi compression on all 4 cylinders last time I checked it.

 

[Bondo]

Re: Merc 3.0LX Compression Numbers

Compression readings on all 4 cyilnders are within 2psi of each other, however, they all read approximately 100psi...

Ayuh,.....

That motor is just plain Worn Out...........

 

[maxumized]

Re: Merc 3.0LX Compression Numbers

Worn out is what I'm hoping against. I am going to try the wet test to see if its rings or heads.

captJason, I'll probably go through the filters, too.a

This engine has a jasperengines.com serial plate on it. If it's been rebuilt already, what's the tolerance of these blocks? If it's .030 already, is the block about maxed?

 

[Bondo]

Re: Merc 3.0LX Compression Numbers

If it's been rebuilt already, what's the tolerance of these blocks? If it's .030 already, is the block about maxed?

Ayuh,... I believe you can buy .060 over pistons....

 

[maxumized]

Re: Merc 3.0LX Compression Numbers

Hmm... interesting. If I have to rebuild, I wish I could up performance. I really wanted to do a turbo project but that's before I realized how the exhaust was liquid cooled, some rubber pieces, etc... Just when you think you can transition quickly from auto to marine, its more different than you believe.

 

[Robj]

Re: Merc 3.0LX Compression Numbers

100 psi and all within 2 psi of each other. That could be worn out, but from a worn out engine I would expect greater variance than 2 psi. Are you doing the compression test correctly? Are you cranking the engine until the gauge no longer changes? I know Don has a post on how to do it. Or could this be a timing issue, the chain jumped (assuming it has a chain not a belt). I would do a little more digging before I concluded that the engine is toast.

Have a great day,

Rob.

 

[maxumized]

Re: Merc 3.0LX Compression Numbers

What's bad is that there's no hour meter, so I'm not sure how old this motor is. Like I mentioned, I'm assuming its a reman since it has a jasperengines.com plate on it unless they were building engines for Maxum/US Marine.

One thing I'm fairly certain of, though, is that someone has overheated the motor in the past, hence the melted and missing exhaust flapper.

I did the compression check as advised in the Merc bulletin. And all reached right at 100psi before or at 4 compression strokes.

 

[MikDee]

Re: Merc 3.0LX Compression Numbers

1500rpm loss, I would look elseware for issues. Don't worry about compression right now, once you fix whats wrong, Then I'd run Seafoam through the motor, then check compression again.

 

[a70eliminator]

Re: Merc 3.0LX Compression Numbers

I'm with Mikdee on this one, compression just seems too uniform and jasper is known for good quality around my area, you don't get Jasper rebuilds at autozone you know. I would look close into the distributor and carb specs, I hade an issue with my 351 not quite getting up to max rpm so I rebuilt the carb and found 66 jets when they should have been 73's, I swaped jets back to 73 and wow what a difference at WOT, glad I didn't drop the pitch of my prop now. I later found out that the previous owner had it re-jetted in an attemp to save gas hmmm. Oh by the way the jet swap was good for about 500 RPM's

 

[maxumized]

Re: Merc 3.0LX Compression Numbers

Thanks for the advice everyone. Will give it a shot. Hopefully won't need a rebuild...

 

[45Auto]

Re: Merc 3.0LX Compression Numbers

Compression does sound very uniform to be a worn engine. Sure you did the compression check right? All plugs out, throttle wide open?

 

[maxumized]

Re: Merc 3.0LX Compression Numbers

Yes. I'm sure the procedure was correct. Now is the gauge accurate? Maybe not. I guess I need to borrow another to make certain the numbers are right. I'll double check to make certain and do the wet test at the same time.

 

[45Auto]

Re: Merc 3.0LX Compression Numbers

Did it use to reach 4500 RPM at WOT? Is this a gradual thing where it went from 4500 to 3000 RPM over a period of 10 years, or was it just one day it would only go to 3000 when it went to 4500 the day before? First case I could believe gradual engine wear, 2nd case would be something changed dramatically, like the timing chain jumping a couple of teeth. Or is it a new boat to you where you have never seen anything but 3000 RPM at WOT?

 

[maxumized]

Re: Merc 3.0LX Compression Numbers

The boat is new to me, but even the previous owner mentioned it never surpassed 3000rpm. They'd had it in the shop at least 7 times and the shop never mentioned an issue. They even re-propped. That's why I'll never return to that particular shop. They had it out on the water 7 times for $75 test runs and never mentioned the problem.

It wasn't until I bought it and mentioned the low rpms to the previous owner did they say "Well that's normal, right?"

 

[Don S]

Re: Merc 3.0LX Compression Numbers

Sounds like it's time to start looking at all the possibilites one at a time. And make sure your tachometer is correct.

List of possible causes of low WOT.

Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation
• Remote control cables and linkage for proper travel to open throttle plates fully.