low compression Merc I/O 140

I have a starcraft chieftan IV with a Mercruiser 140 I/O. I got the boat and trailer for $750 a few weeks ago. The last owner said the boat has been sitting in storage, winterized, for 4 yrs. It looks like a new carb was put on and it was overflowing. I have someone helping me work on it, and he adjusted the float and carb is now fine. But he took a compression test and said the engine has low compression, 85, 75, 75, 85. Does this mean rings are bad? Head gasket? Anyway to tell the difference without taking it apart? Or maybe compression will go up after it runs for a while? Any advice would be appreciated.

Thanks

Re: low compression Merc I/O 140

Has the engine been run yet? If not, run it first to loosen things up. Compression tests on engines that have set for a while aren't very accurate.

Check the compression tester gauge. They do go bad..

Re: low compression Merc I/O 140

The engine has run for a couple minutes. I haven't changed the oil yet either. I was thinking about taking it out on the water this evening and seeing if things loosen up and get a little better lubed. The gauge I think is OK.

Re: low compression Merc I/O 140

I would highly suspect the gauge if you ran it for even 2 minutes.
I doubt you could get it started with compression that low, and it's way to even to be valves.

Re: low compression Merc I/O 140

Ayuh,... If it runs at all, run it for a shake down cruise,....

Just bring a paddle just in case...

After you check All the fluid levels of course...

Re: low compression Merc I/O 140

Bond-o said:

Ayuh,... If it runs at all, run it for a shake down cruise,....

Just bring a paddle just in case...

After you check All the fluid levels of course...

Click to expand...

IE: check the drive's gear lube level and if it's milky

Re: low compression Merc I/O 140

I assume milky or low level gear lube is independent of low compression? What is this a sign of?

Re: low compression Merc I/O 140

Yes, independent of engine problems.

Low or milky gear lube is from a leaking drive, lots of possible places for it to leak lube out or water in. Running a drive with low lube can eat the upper gears($$$) and is far worse than running it full of milky lube, though I wouldn't run it with milky lube either. Milky lube = water contamination.

Re: low compression Merc I/O 140

I let the engine run several times for approximately 5 min each time. It seems to run smoothly. I went to the autoparts store and rented their compression gauge. Compression after warming up is 110, 96, 94, 105. I squirted oil in the cylinders 2&3 and pressure went up to 135 and 125 respectively. I don't plan on water skiing with this boat but use it once in awhile for fishing in Baha this winter (in a bay of the sea of cortez). Is there any concern about running this engine that has borderline minimum acceptable level of compression? Anything to do improve this a little bit without engine overhaul (sounds like rings and pistons are the problem)?

On another note, and perhaps a different thread: after carb adjustment we couldnt get the motor running again. There was no spark coming from the coil and the coil was quite warm. There was another coil in the boat (I just bought the boat used and there are spare parts in it), so we swapped them out and it started right up. While testing the compression, the ignition coil got hot again (cable from ingition coil to distributor was unplugged). When I reconnected all cables, I couldn't get the motor started again. Again, it turned over, but no spark coming from coil. After I let it sit for an hour, it started right up. Does it sound like the coil is not wired correctly? Maybe if I put the old coil back in now that it is cooled, it would work? Just get a new coil?

Re: low compression Merc I/O 140

It's worn out, time for a rebuild over the winter.

Re: low compression Merc I/O 140

You cannot leave power going to the coil when engine is not running or it will get hot and get cooked. Secondly it is possible the coils are wrong and need an external resistor.

Now, the amount of compression you raised with the oil, does indicate the rings are leaking. And yes I would agree the engine is tired out, but I also think you could try a valve lapping and see if you can get some more out of it. With a little time, the rings will loosen up. Will it be like new, definatly not, but they are not all that uneven.

It is also my experiance that if you pull the valve cover and see all kinds of black/brown varnish on the springs and surfaces, that the rest of the engine will look the same. If you find this, likely the rings will NOT loosen up. Its too late.

Re: low compression Merc I/O 140

Perhaps, but I just took it out for a spin and it seem to run great. Smooth, able to get on plane at half throttle, and up to 35 mph. I didn't notice any blue smoke or rough idling. If I hadnt taken compression reading, I wouldn't have known based on performance.

Makes me wonder if I did the test correctly. I did the test on a warmed up engine, disconnected ingition coil wire, removed spark plug and inserted gauge. But I think I misinterpreted open throttle plate completely for open baffle valve completely. I also didnt use a fully charged battery. I will redo the test tomorrow.

It did take on a lot of water, which I suspect is coming from the transom assembly, because hull is fine.

Re: low compression Merc I/O 140

What size boat?

Shift cable bellows is a problem point. Also the exhaust tube can drip and you will never notice.

Most compression check procedures have you block the choke plate full open, throttle handle full open, pull all plugs, and a fresh charged battery with a booster on it only when you go to crank. Do not leave on booster setting while not cranking.

You will be surprised at the increase over your last test.

Re: low compression Merc I/O 140

how do I keep power from going to coil? Is there power going to this when the engine is off but the key is still turned to on? Does a coil stop working when it is hot, then start working when it is cold? Or if is cooked, it is cooked for good?

I am going with your reasoning on the engine compression. Won't be like new, but probably good enough for me, and maybe I will try a valve lapping at some point. I will take a look under the valve cover for reality check to see how much I want to invest in this boat.

Thanks

Re: low compression Merc I/O 140

boat is 18'.

I am thinking it is not an exhaust tube since it didnt leak when the hose/muff was going while running the engine on land. But I don't really know what I am talking about.

Sounds like improvement in test technique will lead to higher compression readings. Given I am fine with the performance, I am going to assume it is 'good enough'. To put into perspective what I was going to use as a fishing boat before I saw this was a whitewater raft with 10hp power motor (I do a lot of whitewater and not much flatwater). But this boat was the same price as the 10hp outboard so what the heck (I will borrow a kicker for backup though).

Next job and post is to fix leak in gimbal housing or whereever. Also trim up and trailer up and down doesnt work. I hope if I fix these two things I will have a boat that runs, floats, and allows me to catch a tuna.

Thanks all for your replies. I will start learning more and asking less rudimentary questions.