Very Low Compessions on Mercruiser 5.0 MPI Engine. with Exhaust Manifold, Thermostat Housing Removed.

[Robertse]

Dear All, I have a Mercruiser 5.0 MPI Engine. I did a compression test with the left hand Exhaust Manifold and Riser removed, Sea Water Pump removed, Alternator removed, Thermostat housing removed. I removed these items because they was rusted or sezied and need replacing or unsezied or removal of rust. The Engine has not been started for over 4 years and some items have developed corrosion. (See the attached Photo of the intake manifold under the thermostat housing.

On the left hand side (Port Side) of the engine the results was Cylinder 1=100, Cylinder 2 =0, Cylinder 3 = 0 and Cylinder 4 = 170, this is a very low/bad result I have not tested the right side as yet. Does it make a difference that the hand Exhaust Manifold, Riser, Sea Water Pump, Alternator, Thermostat housing are all removed. As the results are bad, should I just pull of the head and inspect the head gasket and pistons, wall, values etc? Thank you in advance. Rob

 

[04fxdwgi25]

2 adjacent cyl's with zip compression would point to a blown head gasket.
Those bolt on parts have nothing to do with compression.

 

[Robertse]

2 adjacent cyl's with zip compression would point to a blown head gasket.
Those bolt on parts have nothing to do with compression.

Thanks, Ok I wiil pull the head off and inspect and replace the Gasket.

 

[Scott06]

Thanks, Ok I wiil pull the head off and inspect and replace the Gasket.

Would also look for signs of water intrusion in the exhaust as that could hang up valves . Might be worth doing a leak down test before pulling head off to pinpoint where u are loosing compression

doing a cold compression test would typically expect slightly lower numbers as when engine is up to operating temp rings seal better, but it wouldn’t make a zero into 150 psi.

 

[QBhoy]

I take a guess at there being some sort of sub surface event or similar, at some point. Which might be reason enough to guess further towards the valves being less than ideal at sealing.
Somewhere between there and a head gasket, as suggested above, will be about right.
Hopefully the Mpi injection gear and trickery is all ok.
Best of luck.

 

[Lou C]

Is that pic of the intake manifold with the thermostat housing removed? If so there’s a LOT of rust in the cooling passage. Did the engine have a bad overheat at some point in time? If so that could explain your compression test results. Ideally you want to do a compression test on an engine that has been run to normal operating temps and with the ignition disabled, the throttle wide open and the spark plugs all removed. That will give the most accurate results.
My old 4.3 V6 usually is between 160-170 lbs.
If I were you I would:
Remove both cyl heads & bring to a machine shop & get them checked out, for cracks & valve condition
Check that intake it may be close to rotting thru which will put water right into the oil
Clean the engine block sealing surfaces thoroughly and use a thread chaser to clean out the cyl head bolt holes
Buy a set of new head bolts
Be sure to use marine cyl head gaskets

That’s how I did mine & it’s still running great 7 years after the repairs.

 

[Scott06]

I take a guess at there being some sort of sub surface event or similar, at some point. Which might be reason enough to guess further towards the valves being less than ideal at sealing.
Somewhere between there and a head gasket, as suggested above, will be about right.
Hopefully the Mpi injection gear and trickery is all ok.
Best of luck.

Yeah that much corrosion would also think it was a U boat , maybe why it got parked. If that is the case will need more than a head gasket

 

[Lou C]

In fact when I blew up that picture of the intake it looks like it is ALREADY rotted through right under the thermostat.
I looked at your earlier posts, that was a 2 yr old Merc reconditioned engine? and you had water in the sump? I think the source of that, could be a rotted out intake manifold....and also you said it got up to 200* temp, but that alone most likely did not blow the head gasket, unless it was run at that temp for a while. Something here does not add up. That engine looks like it was left with salt water in it, for a long time.

 

[nola mike]

I'd pull the valve covers and turn the engine, see if the valves are moving. Could for sure be rusted open.

 

[Scott Danforth]

I would pull the motor and tear it down. My guess is you need a new bobtail

If the motor was submerged, the steering cable, wiring harness, and rotating electrics will need replacement as well