MerCruiser 5.7L Low-ish Compression

[jamesm113]

Helping a friend with a boat he's inheriting. Boat's a 24' sea ray from the late 70s, has been sitting for 4+ years. After a carb clean and new plugs, we got it fired up the other day. It seems to start and run fine in the driveway.

Today we did a compression test, don't have exact numbers, but two non-adjacent cylinders on the starboard side were considerably lower than the other 6. The good 6 cylinders were between 150 and 168PSI. The two suspect cylinders were 100 and 130psi. Spec appears to be 100 PSI min with no more than 30% off from the highest cylinder, so obviously it falls short.

Wet test on both the suspect cylinders made no difference in PSI. We had time to hook up a compressor to one of the suspect cylinders. Heard air escaping out the carb.

Question is mainly - can we run it? Any fixes we can try that do not involve pulling the head? I don't think my buddy wants to invest a great deal of money or effort into the boat, it needs a good amount of TLC all-around.

 

[jimmbo]

Welcome to iboats

When doing the Leakdown test(Compressed into Cylinder) you had verification that the Valves were Closed? A bit of something is likely keeping the valve(s) from Seating, and thereby Leaking. You can try a few Applications of PowerTune, or Engine Cleaner. Myself I would pull the Valve Cover and spray the Heck out of the Valve Stem sticking out of the Valve Guide, and also to verify if the Valves are Closed when using the Air Compressor.
Running it with a Leaking Intake Valve is great way to have all kinds of Combustion in the Induction System.

 

[achris]

.. with no more than 30% off from the highest cylinder, so obviously it falls short.

The spec is no more than a 10% variation between any cylinders.

Wet test on both the suspect cylinders made no difference in PSI. We had time to hook up a compressor to one of the suspect cylinders. Heard air escaping out the carb.

Good indicator that there's a leaking intake valve. Having sat for 4 years, I'd suspect that/those valves were open the whole time, and the seats are rusted.

Question is mainly - can we run it?

I wouldn't, likely just burn up the rest of the valve seat.

Any fixes we can try that do not involve pulling the head?

Nope, none.

I don't think my buddy wants to invest a great deal of money or effort into the boat, it needs a good amount of TLC all-around.

May I politely suggest if he doesn't want to spend time and money maintaining, then maybe boating isn't for him...

Chris......

 

[jamesm113]

Welcome to iboats

When doing the Leakdown test(Compressed into Cylinder) you had verification that the Valves were Closed? A bit of something is likely keeping the valve(s) from Seating, and thereby Leaking. You can try a few Applications of PowerTune, or Engine Cleaner. Myself I would pull the Valve Cover and spray the Heck out of the Valve Stem sticking out of the Valve Guide, and also to verify if the Valves are Closed when using the Air Compressor.
Running it with a Leaking Intake Valve is great way to have all kinds of Combustion in the Induction System.

Yes, we found TDC on the cylinder. Actually ran it twice - once on TDC of exhaust, then on TDC of compression stroke - results were the same.

 

[tpenfield]

It would be good to know the % leakage on the 2 'bad' cylinders, which would require a cylinder leak testing gauge.

You could try a combustion chamber cleaning with Mercury PowerTune to see if it cleans up the valve seats at all.

See if you can get an inspection camera (borescope) in the intake to get a look at the intake valves. You could remove the intake manifold to get a good look.

Any issues on the exhaust side of the engine?

 

[itsathepete]

Here's my 2 cents, take it for what it works. If it runs, and you don't want to invest a lot of time and money in it, run it. Do what you need to do to make it as safe and reliable as you need to ie tune up including ignition, charging, cooling and fuel and exhaust systems. Change the oil and oil filter as well as fuel filter. A lot of times, engines that have sat and have buildup on the valves or stuck rings will come back around after running them for a while. I would run it several times in the driveway up to operating temp and put some rpms on it and let it cool completely down. Probably over a couple days. Then recheck compression. You might find it has improved. If you take it on the water, be safe and only go where you can get back easy and preferably have a friend in another boat in case you break down (and you will,lol). I've had a lot of fun and learned a lot from junky old boats over the years and as long as you use common sense and be safe, you can too

 

[Lou C]

The causes of valve seat & stem corrosion:
Not fogging the engine for long term storage & being stored outside in a damp climate
Not maintaining the exhaust system to prevent water from leaking back into cylinders
Leaky head gaskets that let water in a cyl (I had this one due to a previous overheat)

It is possible that it could clear up but if it doesn’t you’re likely to start burning valves because if the valve doesn’t make good contact with the valve seat keep in mind that is how 4 stroke valves are cooled. Same thing when an engine with mechanical valve lifters is run with too little valve lash (clearance) learned this in my days of running & repairing air cooled Volkswagens. These were known for the exhaust valve that was in the # 3 cyl under the oil cooler burning due to it getting less cooling air.
To do a valve job on a Chevy small block is not a bad job. Remanufactured cyl heads are widely available. I bought a set for my old 4.3 for about $550, that and a set of Fel Pro marine head gaskets, & a set of ARP cyl head bolts….

 

[jamesm113]

Thanks for all the responses everyone.

We only had time to check one of the cylinders with compressed air, so not sure if the other cylinder has intake or exhaust valve issues.

Let's say this is something that would resolve itself by just running the motor in the driveway - how long should we run the motor to give it a chance to clean up the seats but before seriously risking roasting valves?

If we end up roasting valves, will will we cause any more damage beyond the roasted valve(s)?

As far as rebuilding it - I've done a few valve jobs on cars/bikes. How bad is it to get the head out? Should we do both sides or just the bad side?

Would taking the head to a machine shop for a valve job be OK, or should we get reman'd head(s)?

 

[itsathepete]

4 years isn't terribly long for an engine to sit. With the compression numbers you have, the valves are at least mostly sealed. There could be stuck rings as well. I personally would run it for 20 or 30 minutes at a time, varying rpms (don't get too carried away) with a few quick revs to build pressure in the cylinders. Let it cool and repeat. If it doesn't improve you can pull the heads. Easy lo lap the valves and replace seals. Getting it apart isn't bad. I don't think you'd need to replace head bolts on that vintage motor

 

[Lou C]

Those questions are hard to answer, you can try to run it paying close attention to ignition timing & fuel mixture (not too advanced, not too lean) and contrary to what many say I run at least 89 if not 93 octane fuel. The other thing that helps is to make sure the engine can reach the specified max rpm under wide open throttle. I have mine set up to just hit 4900-5000 rpm. This puts much less strain on the engine pulling the boat up on plane. It may limit top speed but to me that’s irrelevant. Engine life is the main priority…

 

[Lou C]

Valve job on the small block Chevrolet is about the easiest ever. You can have your heads redone if they are other wise in good shape. If there is excessive corrosion or cracks in the exhaust valve seat areas (common after bad overheating) then buying a reman set makes sense. That’s what I did with my 4.3 V6.

 

[Rick Stephens]

I don't think my buddy wants to invest a great deal of money or effort into the boat, it needs a good amount of TLC all-around.

Needs to find a different hobby. Boats are not for him .

 

[nola mike]

1. Go to wrecker yard
2. Find one of the zillions of 5.7l engine there.
3. Grab a head for $20
4. Get a new head gasket for $50
Spend ½ day swapping. Go boating.

 

[jimmbo]

Not all heads are the Same, some work better than others in a Marine Application.

 

[nola mike]

Not all heads are the Same, some work better than others in a Marine Application.

You buy one that matches your other head. Or you splurge for a pair.

 

[jimmbo]

You buy one that matches your other head. Or you splurge for a pair.

And buy two that aren't as good

 

[jamesm113]

Those questions are hard to answer, you can try to run it paying close attention to ignition timing & fuel mixture (not too advanced, not too lean) and contrary to what many say I run at least 89 if not 93 octane fuel. The other thing that helps is to make sure the engine can reach the specified max rpm under wide open throttle. I have mine set up to just hit 4900-5000 rpm. This puts much less strain on the engine pulling the boat up on plane. It may limit top speed but to me that’s irrelevant. Engine life is the main priority…

What do you mean by paying close attention to ignition timing and fuel mixture? Checking plugs to check for lean/rich conditions? Using a timing light to check timing?

 

[Lou C]

Yes & yes

 

[Lou C]

And also if you have an advance timing light check the total timing advance against Merc's spec.

 

[nola mike]

And buy two that aren't as good

As good as what? The ones that he has with bad valves?