Mercruiser 470 low compression on cylinder #4

[Max-Falcon]

I had water in oil of my Merc 470. After some diagnosis and tests decided to change the head gasket and service the head. Machine shop resurfaced the head, and said no crack or other issues found. Put the head back on with a new marine head gasket (put Sierra gasket not Merc!). Had a little issue installing the pushrods where I bent a few but I got new pushrods that where very close in length but slightly shorter (around 1/32" shorter). before I finish the reassembly and put the exhaust manifold back on, I did a compressing test. I got 150 psi on cylinder 1, 2 and 3 and 100 psi on cylinder 4. All valves seemed to work. I put some oil in cylinder 4 and did compression test again and I got 150 psi. The manual talks about compression test very briefly. It says if I get good compression with oil in the cylinder, then piston rings are the cause! But something tells me there should be more to this! What else could be the reason for my low compression test?

 

[Scott Danforth]

Did you properly measure the lenght of pushrod needed as outlined in the factory manual?

 

[QBhoy]

Sounds like a valve issue. And these engines are normally so reliable as well !

 

[Max-Falcon]

Did you properly measure the lenght of pushrod needed as outlined in the factory manual?

I showed to a mechanic and he measured the new one and old one and said the new one is slightly shorter but within the margin! Like I said to a naked eye it looked like about 1/32” shorter. I didn’t do the valve lash adjustment. Just put all the pushrods in the order I took them out

 

[QBhoy]

I showed to a mechanic and he measured the new one and old one and said the new one is slightly shorter but within the margin! Like I said to a naked eye it looked like about 1/32” shorter. I didn’t do the valve lash adjustment. Just put all the pushrods in the order I took them out

Worth a go at the push rods ? Try rolling it on a flat surface and see if it has a wobble to it. Another habit of these engines is to never really recover after an over heat (because of the usual issues). As you’ll know the block and head will heat up at different time intervals. So often can give the false hope that all is well after a head skim...but then fail to seal again when you add a few factors into it. Oh man. I’d like to like these engines...just for their querks...but I just shudder each time I think back to the ruined family days out with friends.

 

[Max-Falcon]

Sounds like a valve issue. And these engines are normally so reliable as well !

As I said I got 150psi compression when put some oil in the cylinder 4! Manual says if that happens valves r fine and most likely it’s the piston rings! I just want to know if there are other causes beside the possible ring issue

 

[QBhoy]

As I said I got 150psi compression when put some oil in the cylinder 4! Manual says if that happens valves r fine and most likely it’s the piston rings! I just want to know if there are other causes beside the possible ring issue

Good point...but oil can seal and hide all sorts of issues.
disnt want to scare you...but also likely that cylinder is damaged or scored and rings are struggling to seal. As I said...they just don’t recover long term. Forecast is bleak ahead. Mercury even abandoned the engine after only a short interval of installations of these. Merc weren’t known for frequent evolution of engines historically. Tells a story

 

[Max-Falcon]

Worth a go at the push rods ? Try rolling it on a flat surface and see if it has a wobble to it. Another habit of these engines is to never really recover after an over heat (because of the usual issues). As you’ll know the block and head will heat up at different time intervals. So often can give the false hope that all is well after a head skim...but then fail to seal again when you add a few factors into it. Oh man. I’d like to like these engines...just for their querks...but I just shudder each time I think back to the ruined family days out with friends.

I did the pushrod roll test and they were all straight. Head is machined and block surface was tested by a flat edge and it was flat and marine grade head gasket is installed. It’s just lower compression on cylinder 4.

 

[Max-Falcon]

Good point...but oil can seal and hide all sorts of issues.
disnt want to scare you...but also likely that cylinder is damaged or scored and rings are struggling to seal. As I said...they just don’t recover long term. Forecast is bleak ahead. Mercury even abandoned the engine after only a short interval of installations of these. Merc weren’t known for frequent evolution of engines historically. Tells a story

That’s true! These engines don’t seem to have a good reputation. What happens if my only issue is lower compression in 1 cylinder and I just leave it as it is?

 

[QBhoy]

That’s true! These engines don’t seem to have a good reputation. What happens if my only issue is lower compression in 1 cylinder and I just leave it as it is?

I’m not sure to be honest. It is low and out of spec from the rest at 100. But given the history of these engines over the years...it is likely a sign to pay attention to. You sound like you know your stuff...but I would really just keep an eye on that. Check that plug to see what’s happening there too. Only a few things can show a drop in compression. None of them usually good on a normal engine...never mind this. I would still be thinking you have an issue with sealing at valves, head or you have have something else going on. Have you had it running recently? Dare I mention is there any sounds to suggest there is and top or bottom end movement or play ?

 

[nola mike]

So... Your next step should be a leak down test, maybe you can see where you're losing compression. It does sound like rings--if you had a water leak into #4 then you definitely could have damaged the cylinder. you half assed the push rods, so tough to totally rule that out, but didn't sound like your problem. If you run it like that you'll have a poorly running underpowered engine, but it probably won't damage anything.

Guessing you had a leak from your reservoir into #4 cylinder. Sitting water damaged the cylinder over time.

 

[Max-Falcon]

So... Your next step should be a leak down test, maybe you can see where you're losing compression. It does sound like rings--if you had a water leak into #4 then you definitely could have damaged the cylinder. you half assed the push rods, so tough to totally rule that out, but didn't sound like your problem. If you run it like that you'll have a poorly running underpowered engine, but it probably won't damage anything.

Guessing you had a leak from your reservoir into #4 cylinder. Sitting water damaged the cylinder over time.

The water never sat in the cylinder for a long time! Boat was winterized with clean oil. Then I ran it on muffs not knowing the out drive impeller is gone so engine overheated. Then took it to the lake and got milky oil. Right after that (like within days) I began the head service. There was no visible rust or damage on cylinder 4 when head was out! So I’m going to put everything back together and do another compression test on a warm engine.