Mercruiser 7.4 LX MPI EFI problem

I was getting zero pressure in the #7 cylinder when I was testing all the cylinders. It had 150 according to my notes before the rebuild. I know the cylinder walls and the piston looked free of damage when it was apart so its not likely the piston or rings that failed. I loosened the rocker arms to make sure both valves were still closed and the cylinder still would hold zero pressure. Unless the piston failed after assembly which I would expect more noise and symptoms if it had. The only thing I can think of is I somehow screwed up the head gasket and the pressure is leaking out the gasket. Either way I'll only know for sure once I tear the whole thing down again. Also either way this boat is becoming exhausting both physically and financially with just trying to get it to run. I really don't want to be doing this myself but I still can't find a single mechanic willing to work on it for me. All the ones I trust here either are too busy or now say they refuse to work on gasoline engines and will only work on diesel.
The correct use of a good leak down tester will tell you where the leak/loss of compression is going.
 
The correct use of a good leak down tester will tell you where the leak/loss of compression is going.

I have no idea where the air was going. It did not seem to be going through either valve and with both rocker arms loose it still held zero pressure. It also didn’t seem to be going into the block. When I swapped out to my compression gauge it also gave me a zero reading for pressure and with both rocker arms tight I could see both valves opening and closing. I’m going to have to pull the head back off to figure it out is all I know. I’m thinking I somehow kinked or otherwise screwed up the corner of the head gasket and there just is no seal at all in that cylinder because of the defective gasket.
 
Are you sure your risers weren't leaking to begin with? Maybe you hydro locked it and blew a hole in the piston? Try scoping it.
 
Well, got the head back off but no sign of any damage to the cylinder or the head gasket. Going to take it back to the machine shop and get it vacuum tested. Hopefully it’s a defective valve otherwise I have no idea what the issue is.
 

Attachments

  • 6E55C4C3-C3BC-4209-89A4-6B1BE1C4F638.jpeg
    6E55C4C3-C3BC-4209-89A4-6B1BE1C4F638.jpeg
    251.5 KB · Views: 102
  • 6F071BA0-65D2-4576-AD81-2382FEDE19C8.jpeg
    6F071BA0-65D2-4576-AD81-2382FEDE19C8.jpeg
    241.6 KB · Views: 99
I would check the heads for straightness and the block decks to ensure they are flat. Looking at the pics, it looks like the head gasket has uneven torque applied. Did you torque the heads with head bolts oiled or Perfect Sealed and in a 3 step torque sequence (20-50-85(92)?
 
@Bill Curtis I did the 3 step as per the manual with the perfect seal. I also used a tap and die on both the threads on the bolt and the holes to clean them prior. Since I haven’t pulled the block I can’t get it on a machine to test for straightness. However, I did put a metal straight edge across it and found no obvious deformation. The block never overheated to my knowledge so warping is unlikely. Also prior to pulling the heads the first time cylinder pressure in 7 was perfect. #5 cylinder was the one that seized its valves. I’m also using a craftsman mechanical torque wrench which has served me well for several years and has been used to build two other engines with no issue so it’s not likely torquing improperly. I also used the felpro gaskets that your not supposed to use the copper spray with. I may try a different gasket with the spray when I get the head back from the machine shop.
 
Well, seems as the installation was 100% by the book. The pistons / bores / vlaves look perfect in those pics.

I am at a loss as to anything that may be causing loss of compression, since you checked everything else.

A shot in the dark: A bad lifter / wrong length pushrod maybe holding a valve open?
 
Last edited:
I agree that something is off on the head gasket. Unless I am mistaken, I would expect the metal ring at each cylinder to remain perfectly round after torqueing.
To my eye, in your 2nd photo, it may be the angle but it appears the intake valve in the upper cylinder is not fully seated.

Edit:
I take back the head gasket comment. i see they are not "perfectly round"
 
Last edited:
I believe the manual says to use no sealant (no one I know has ever used RTV) on the gasket. Are the Mecruiser OEM gaskets self sealing S/S shim gaskets?

The automotive metal and / or non sealing gaskets work well with spray on aviators permatex.
 
rtv is clearly visible in the pics. hung valve would be the only cause for 0 compression seeing there is no hole in the piston.
 
Well, got the head back off but no sign of any damage to the cylinder or the head gasket. Going to take it back to the machine shop and get it vacuum tested. Hopefully it’s a defective valve otherwise I have no idea what the issue is.
rtv is clearly visible in the pics. hung valve would be the only cause for 0 compression seeing there is no hole in the piston.
I think the orange that you are seeing is nasty rust and I also see it dripping from the valve in the second pic in the top right hand corner.
 
@Shaps the head gaskets are the same shape as when they were new. Not sure why the manufacturer didn’t make them perfectly round around the cylinders. Same gasket is on the other side of the engine and I’m having zero issues with it.

@scoflaw the only RTV used was a dab of black RTV in the corners of the intake manifold gaskets where the four parts of those gaskets meet. I know not to use any on a head gasket.

@techmitch yes the nasty orange is rust. My engine is raw water cooled and operates in the Pacific Ocean and the boat lives in a slip. I wasn’t quite able to drain it all out when I pulled the head so it managed to drip all over a little bit when I pulled everything apart. There are zero signs prior to pulling it back apart of water or oil intrusion into the cylinders and zero signs of water intrusion into the oil pan so I know that part of the gasket was sealing properly.
 
Last edited:
@techmitch I wasn’t terribly aggressive with the cleaning but I used a rag wet with brake cleaner followed up by another wet with some PB blaster oil to clean it before reassembly. There was no thick caking of carbon there really to begin with.
 
@techmitch I wasn’t terribly aggressive with the cleaning but I used a rag wet with brake cleaner followed up by another wet with some PB blaster oil to clean it before reassembly. There was no thick caking of carbon there really to begin with.
Did the tops of all pistons look the same when originally disassembled?
 
Pretty much. The engine had about 680ish hours total from new 26 years ago. The previous owner was deferring a lot of maintenance and was not using the boat hardly at all. Which as we all know is definitely not a good way to keep a boat. I’m now trying to catch up on all the deferred maintenance and get it running right again. If I can’t solve this issue soon I may just have to resign myself to a repowering of the boat.
 

Forum statistics

Threads
112,950
Messages
1,422,861
Members
60,932
Latest member
juliediane
Back
Top