1982 Mercruiser 470 - low compression

[Blueghost924]

As a couple of folks know, I've been trying to get my 1982 Mercruiser 470 up and running. A couple of months ago, I performed a compression test on all 4 cylinders and came up with numbers greater than 100 psi. Tonight I was thinking that I may have not performed the test exactly as outlined in the bulletin (I don't think I have all 4 plugs out or the throttle may not have been wide open).

I still can't get it started, so tonight I decided to re-perform the compression check. I made sure that all plugs were out, the coil was grounded and the throttle plate and choke plates were wide open with the spark arrester removed. Keep in mind this was a compression test on a cold engine (just like it was being started for the first time that day). I also had my SeaDoo fogging oil spray can on hand to see if that would make a difference after the first cold run. Here's what I got:

First run without using fogging oil:

#1 - 30 psi
#2 - 90 psi
#3 - 100 psi
#4 - 95 psi

Second run with fogging oil:

#1 - 33 psi
#2 - 95 psi
#3 - 100 psi
#4 - 95 psi

This is not good, and I'm surprised that #1 is that low (compared to what I thought I got a couple of months ago). My initial thought is there's a "remote" chance my compression tester is off, but it's only 8 months old, and is in good shape. Maybe I'm in fantasy land. Just for kicks, I may rent another compression tester from the local auto shop (like AutoZone) just out of curiosity. But it looks like I'm done for. I've looked at overhaul kits (including all the gaskets), and they're not cheap.

 

[stonyloam]

Needs a head gasket, not so sure you need an overhaul. Try putting in a little motor oil, the fogging oil may be too thin to make much difference. Those numbers are not necessarily bad for a cold engine that has been sitting for some time. Were it me, I would pull the head and have it checked out and try a new OEM Mercruiser head gasket. Might just do it.

 

[iand464]

The accuracy of the compression tester is pretty much irrelevant here. If it reads 30 on the first hole and 100 on 3 there is something wrong. I think i was over 150 in every hole on a good running 488 (same thing). A good test is every hole within 10% Get the coolant pressurizer kit from az check the head gasket. Or just pull the head.

 

[bfjou812]

It could be a valve stuck open on #1 cylinder. A lot of times when a head gasket is bad it will show low readings on the adjacent cylinder. I'd pull the valve cover off and check the valves to make sure they are all opening and closing as they should.

 

[Bondo]

Ayuh,.... Do an air pressure test, turn each cylinder Tdc, 'n pump air into it to see where the air is goin', Out the intake,..?? the exhaust,..?? Into the base,..??
If it's a bad valve, you'll know,... if it's a holed piston, you'll know,...

 

[Blueghost924]

I did a coolant compression test pressing up the system to 17 psi and holding for 7-8 minutes. No noticeable drop in the needle was noticed. I looked all around and didn't see any coolant leaks. I did look at the oil on the oil dipstick and it had a normal appearance and didn't look to have coolant mixed in.

My next step is to pull the valve cover and see if the valves for #1 are opening/closing properly.

Hey Bondo - I'm not quite sure where to connect the air pressure hose to for an air test.

 

[Bondo]

Ayuh,.... Through the spark plug hole,.... I use the end of my compression tester,...

 

[Blueghost924]

Working.

 

[Blueghost924]

I took the valve cover off this afternoon. A ha! Found the issue! The pushrod has slipped from under the rocker arm. In the meantime, I've placed the valve cover back on (no bolts), and wanted to know advice or comments?? I know the simple answer "seems" to be just re-position the pushrod back in place. I am wondering if there's other pre-cautions or other sage advice. Is this an indicator of another issue or maybe future problems?

 

[Eric 3.7LX Alpha 1]

That's a sign of a possible bent push rod there..... there isn't enough clearance for it to just slip out. Unless the rocker nut loosened up?

Remove it and roll it on a straight edge like a piece of glass or mirror...

If you do have one, I'd remove them all, and check them all keeping them in there proper order (don't mix them up) before you order any.

Your definitely not going to get any air moving into or out of that cylinder like that!! let us know what you find!!

 

[Blueghost924]

Will do.

 

[Blueghost924]

Well, I removed the push rod and rolled it on a flat surface. It didn't appear visually to be bent, but I'll check again tomorrow morning. I had completely forgot that the old 470 was still in the boat, so I do have my own little parts salvage (including push rods). Yup - definitely have to make sure I remember the order and placement of the rods. In reading the service manual, it looks like the rocker arm and nut should be removed for push rod re-assembly. I'm wondering if I can cheat a little by getting my engine lift (in the garage) and lift up on the rocker arm lift enough to slide the push rod back into position.

 

[stonyloam]

No need to do all that. Just remove the nut and rocker then torque it back down. Turn the engine so the valve is closed (TDC compression for #1) torque bqck down to 20 ft. lb. Don't worry about valve lash, that is set by pushrod length. Retorque after you get it running.

 

[Blueghost924]

Well, I did re-position the push rod back into place and torque the rocker arm bolt back down to 20 ft-lbs. Started turning the motor over by hand, and it got a little difficult when approaching the 0 mark. I put the valve cover back in place, hooked up the battery and re-performed the #1 compression test. About a half second into cranking, I heard a pop. Took off the valve cover, and the push rod has come back out again. Looks like I do have a significant valve issue, and I'm gonna have to take it apart to see what the problem is (and how bad). If it's significant, I'm gonna have to weigh my options with this motor (and how much time I want to put into it).

 

[Blueghost924]

And it looks like the rocker arm is leaning to the left slightly out of position.

 

[stonyloam]

Sounds like a stuck valve. You could pull the rocker back off and see if the valve will move.

 

[MikDee]

Maybe you have a broken inner valve spring?

 

[Blueghost924]

I think it's a stuck valve, but right now...disassembly is in progress...

 

[Blueghost924]

I hate to re-open this, but I wanted to bring a little closer to it. I've been busy with other things and such, but 2 things have happened:

1. I found a guy somewhat local that had a 470 that was turn key ready to start up, and it had the alternator conversion. Decent price and compression over 130 psi in all 4 cylinders. I now have a 470 that starts and runs.

2. I did manage to squeeze in a little time and removed the intake manifold off my old 470 (and plan on selling it for parts). Here's a couple of pictures on the intake of #1 cylinder. Is this rust and sediment a little excessive? I know that when the intake came off, a fair amount of "silt" fell onto the garage floor.