1985 mercruiser 140 blows head gasket

[Shawn168]

My 85 mercruiser 140 has high compression in cyl #3. Compression in 1,2 & 4 are between 140-150psi, in #3 it's 180. It blows the gasket out on the outter wall under the manifold. Has done this 2 times in 2 weeks. Any ideas?

 

[Bondo]

Re: 1985 mercruiser 140 blows head gasket

Ayuh,.... Welcome Aboard,.... You've had the head off a couple of times, What's different 'bout #3,..??

 

[Shawn168]

Re: 1985 mercruiser 140 blows head gasket

Nothing that I can see. Put a straight edge on the block and head and I have about .002-.003 gap. Could it be a valve adjustment?

 

[Bt Doctur]

Re: 1985 mercruiser 140 blows head gasket

Is #3 piston different than the othere 3,dished, flat, etc.

 

[Shawn168]

Re: 1985 mercruiser 140 blows head gasket

No. All 4 are slightly dished, (im thinking for valve clearence) No scaring on the walls, no carbon build up on or around the valves, no water in the cylinders, nothing odd I can see. It starts running great! Then a slight ticking sound, then just gets worse and obvious power loss. The only thing not stock is the ignition conversion and bigger coil.

 

[cr2k]

Re: 1985 mercruiser 140 blows head gasket

Doubt if it's a valve adjustment. This time take the head to a machine shop and have it checked.
Are you sure the block or head is not cracked between one of the adjacent cyls?

 

[Shawn168]

Re: 1985 mercruiser 140 blows head gasket

I can't see any cracks, and before it blew, I did a leak down, and all was good.

 

[Grub54891]

Re: 1985 mercruiser 140 blows head gasket

Stuck rings? Causing to much compression?

 

[Bt Doctur]

Re: 1985 mercruiser 140 blows head gasket

Compression is not "caused" Its a result of air pulled into a closed cylinder and compressed.Numbers you get are based on cylinder volume ,length of stroke, shape of combustion area.the air pulled in is compressed and the psi numbers are read.

 

[Bt Doctur]

Re: 1985 mercruiser 140 blows head gasket

Remember what happens during the compression stroke of the four-stroke cycle: Both the intake and exhaust valves are closed so no air can escape, and the piston moves upward from bottom dead center (BDC) to top dead center (TDC) so that the air/fuel mixture in the cylinder is compressed into the combustion chamber. Compression ratio is the relationship of cylinder volume (or displacement) with the piston at BDC to cylinder volume with the piston at TDC. If the volume of the cylinder with the piston at BDC is 10 times greater than the volume of the combustion area with the piston at TDC, then 10 units of volume get squeezed into 1 unit of space, and the compression ratio is 10.0:1.

Read more: How To Calculate Compression Ratio- Car Craft Magazine

 

[Chris_94_Sprint_TX]

Re: 1985 mercruiser 140 blows head gasket

Are you sure you got the torque sequence right and got even torque when you put the head back on? I have seen same problems on car motors when the head was removed and re-installed and the mechanic did not follow the proper torque sequence and also when the mechanic torqued the head down his wrench setting got disturbed and some of the bolts weren't torqued right.

Chris

 

[Shawn168]

Re: 1985 mercruiser 140 blows head gasket

I got the sequence from a manual, and I'm pretty sure I had the torque correct. This just driving me nuts!!

 

[Shawn168]

Re: 1985 mercruiser 140 blows head gasket

I understand how the compression is created, but I don't understand why it's so high in the one cylinder. I'm thinking I wouldn't blow the gasket out if the pressure wasn't so high.

 

[Shawn168]

High compression in #3

High compression in #3

85 mercruiser mcm140, a395548, has high compression in cyl #3. Compression in 1,2 & 4 are between 140-150psi, in #3 it's 180. It blows the gasket out on the outter wall under the manifold, the thinnest part, All pistons are the same, no scaring on the cyl walls, valves look good, doesn't burn oil, over all runs great. The only mod is an ignition conversion and 1.5ohm 40,000v coil. Passes leak test, I replaced the head gasket today for the 3rd time, followed the correct torque sequence and torque settings. I am truly stumped.

 

[Shawn168]

High compression in #3

High compression in #3

85 mercruiser mcm140, a395548, has high compression in cyl #3. Compression in 1,2 & 4 are between 140-150psi, in #3 it's 180. It blows the gasket out on the outter wall under the manifold, the thinnest part, All pistons are the same, no scaring on the cyl walls, valves look good, doesn't burn oil, over all runs great. The only mod is an ignition conversion and 1.5ohm 40,000v coil. Passes leak test, I replaced the head gasket today for the 3rd time, followed the correct torque sequence and torque settings. I am truly stumped.

 

[achris]

Re: High compression in #3

Re: High compression in #3

Duplicate post... Please don't ask the same question in 2 threads

http://forums.iboats.com/mercruiser...-mercruiser-140-blows-head-gasket-622875.html

 

[achris]

Re: 1985 mercruiser 140 blows head gasket

Check the piston heights when they are at top of stroke... You may have a stretched con-rod.

Chris....

 

[Shawn168]

Re: High compression in #3

Re: High compression in #3

That does make sense. And I'll check, but wouldn't that give a really odd noise? Especially at mid-high rpm's??

 

[cr2k]

Re: 1985 mercruiser 140 blows head gasket

That would do it if there were enough clearance so as not to tag the head with the piston.

 

[achris]

Re: 1985 mercruiser 140 blows head gasket

Only needs a tenth or so of an inch. Won't tag the head, but enough to decrease the combustion volume (thus increasing the compression). About the only other thing I can think of is carbon deposits in the head/on the piston, but I would think he'd notice that when he had the head off.... One more possibility is one wrong piston in the engine, but I see that as so highly unlikely as to be not worth suggesting....

Chris....