Mercruiser 5.0L EFI , low cylinder compression on one cylinder after overheat.

Ive learnt a lot about my boat on this and other forums.. On the weekend I had an overheat.. My friend alerted me to say she thought she heard a slight ticking noise and I stopped the boat.. Had some smoking from the exhaust riser rubber elbows.. I think I have buggered by new impeller running with a kinked hose for a minute or so when flushing the other day.. I didnt notice the guage temperature at the overheat but when I looked again it was around 170 where it should be. The mainfolds were hot.. I think I had the water issue the time before as I had a new vapour lock issue which should have triggered by interest but I was going to check temp when I stopped the boat.. Didnt get that far.. The boat cooled and I started it eventually though think I had the vapour lock issue and motored slowly to the ramp. It started again when home.

When I got the boat I got a compression test done it was 125-140 done by the guy who sold it to me last year.(Dealer). I think it also hadnt been run for a while which may have accounted for lower compression.

I did a compression test yesterday and got 150-160 on all but one cylinder with 90 on that cylinder , number 2 on the port side with some water in the cylinder.. The oil on the dipstick looks ok and was good when I changed it the other week pre overheat issue.. Plugs looked dry when I changed them a month or so ago..

The boat is starting though may be running a little rougher than before..

After reading here and elsewhere I think I have overheated one of the valves in the cylinder with low compression.. Thats was the slight ticking noise was that my friend alerted me to.. Apart from replacing the impeller and making sure of good cooling(and the overheat alarm not working) could the valve reseat and improve compression.. Yes its salt water cooled..

I have a mate that will look at it for me. Whats the best way to work out why I have low compression without pulling the head off.. Its hearing towards the end of the boating season and my mate/me will have more time to pull the head off over winter.

Thanks in advance..

Time for a dissassembly

remove intake and distributor, leave efi components attachedjust disconnect fuel lines and wire connectors and label and photograph everything.

remove exhaust manifolds

remove heads send them out to be checked warpage, cracks and bad or burnt valve and have reconditioned.

purchase a new head bolt kit, marine head gasket kit, intake manifold gasket kit and exhaust manifold gasket kit.

after tear down, clean all passages with compressed air,

retap all head bolt holes with proper tap and clean with compressed air,

Reassemble.

change oil and filter with new and cheap oil,

run and adjust as needed on a un kinked hose.

after running for 1/2 hour change oil and filter to normal oil and go enjoy

I went thru this last year myself on a friends boat cuased by over heating.
a 350 carbe motor but motor parts are the same

Parts purchased from a napa parts store but Numbers may be cross referenced

Intake gasket set 17-320 order qty of 1

Head gasket one each, 17-0302 order qty of 2

Head Bolt Kit One kit contains bolts for both heads, ES-72856 order Qty of 1

Exhaust Gasket set. fel pro FPG 17420 Qty 2 or if two come as a set order only 1 set



see link below

http://www.marineengine.com/boat-fo...50-mercruiser-inboard-ski-boat&highlight=head

I have read elsewhere that its best do do some diagnostics , pressure leak test before I have the head off? . Is it likely to be just a head gasket with only one cylinder wth low compression? I thought that was more common with two adjacent cylinders.. Could the water in the cylinder(s) be when the engine cooled water ingressed via the manifolds and crappy flappers?

You over heated.. Is this a salt water use boat?
The combination of water and low compression
Tells me bad head gasket.

The common issue you refere to is only if the
Two center cylinder head gasket went bad.

If 1, 7, 2, 8 cyl go then it can be only one.

If you want to do a leakdown test that is ok also.

There are many ways to check for water leaks

You can do a smoke test on the water system
if you want....
A 60 psi difference in one cylinder says to me either a
Blown head gasket or bad valve....either way it has to
Come apart.

Also this is the internet and i am not there to diagnose
Your issue personally..it is only advice.

Thanks for the advise and I really appreciate it.. I just hope its only the head gasket but think the slight noise heard before shutdown is my main worrry that its something worse. I dont hear it now on startup but havent put it under any load, its only on the muffs.

Thanks again. You guys on these forums are so good. I dont know what we did beforethe internet.

Check for a tulip-ed Valve before you pull it all apart. On the suspect cylinder you can loosen the rocker arms and see if one of the valves is higher than the rest.