New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

[ahickman]

Just brought home a 1978 Baja with a 470 Mercruiser 4 cylinder engine that has been sitting for a year or so. Bought it and the trailer for $325 and now know why. All four cylinders read about 50 psi. Put a little oil in the cylinders and they came up to 75 psi. Since they are all even, I am thinking that the head gasket or valves are damaged. The previous owner said they ran it hot and just gave up trying to get it running well. Any one have suggestions on what to look at next?


Thanks,

Aaron

 

[achris]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

Once a 470 runs hot it usually takes out the head gasket. But, I would only expect that to drop compression on 1 or 2 cylinders. Completely down like that indicates a ring problem/worn cylinder bores. You are going to need to take the head off at the very least, have a good look at the cylinder walls....

Chris.......

 

[Don S]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

You could also have a defective gauge. Put some oil in the cylinders and crank it for a while. Then if you have spark, put some plug in it and squirt some gas in the carb and see if it will fire off. Run it for 5 to 10 seconds a couple of times and then do another compression test. Rusty valves after not being from for years will also give you a false reading.

 

[ahickman]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

I am hoping for a bad HG or rusty valves. Don't want to have to tear down the block. Can either of you suggest a good source for gasket sets and parts that won't break my budget for the 470? Going to try and tear down the head today and see what is in there.

 

[tpenfield]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

It may be useful to do a cylinder leak test before you start puling it apart. That will tell you where you are getting leakage on each cylinder, etc. Your 'wet' versus 'dry' compression tests suggests rings, but it is always nice to have more evidence before proceeding

 

[ahickman]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

Never done a leak test. Can you give me a quick tutorial? Special tools involved?

 

[TilliamWe]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

1978, 470 Mercruiser for $325 and now know why. ran it hot Any one have suggestions on what to look at next?

It was $325 because it's a "78 470 Mercruiser that has been run hot." You paid exactly $325 too much for it.

What to look for next? A different boat that will actually be useable, without hundreds of dollars and weeks of repair. Sorry.

 

[tpenfield]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

Never done a leak test. Can you give me a quick tutorial? Special tools involved?

You need a leak tester and a source of compressed air. Here is a link that explains it . . . you can also Google some other procedures.

How to do Cylinder Leak Down Testing- Car Craft Magazine

I have one of these and it works well.



Here is a link to a thread that I posted a while ago about doing a leak test on my 454 after doing a valve job. . . .

http://forums.iboats.com/mercruiser...mercruiser-454-test-run-leak-test-553966.html

BTW - you will get an ear full about the Mercruiser 470 engine as to its reputation, etc . . . but see what you can do with it.

 

[Slip Away]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

It was $325 because it's a "78 470 Mercruiser that has been run hot." You paid exactly $325 too much for it.

What to look for next? A different boat that will actually be useable, without hundreds of dollars and weeks of repair. Sorry.

One of the few times I will agree with you Billy B. Just reading "470 4 cylinder" explains it all.

 

[Don S]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

You don't need to do a full blown leak down test, just the compression tester hose and some compressed air.
Bring each cylinder up to exactly TDC on the compression stroke, take the radiator cap off the expansion tank, then put compressed air to that cylinder. now use your ears to hear where the air is escaping.
If you hear it coming out of the carb, and intake valve is leaking.
Sound from the exhaust, it's an exhaust valve leaking.
Out of the expansion tank, the head gasket or something leaking from the cylinders to the cooling system,
If the sound is coming mostly from the valve cover cap (take it off too) then you have ring/cylinder/piston problems.

 

[ahickman]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

Well, I have not given up yet. I did purchase a leak down guage from Harbor Freight and am going to get down to the garage Thursday evening hopefully to give it a whirl. I have my fingers crossed that the issue is somewhere along the head and not in the rings or block. Thanks for all the helpful pointers.

 

[stonyloam]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

I suspect a lot of your problem is from the engine sitting for a long time, and are on the right track. I would not tear into it until you at least try to get it running on muffs for a while to loosen things up a little. Having said that you have a bigger problem that needs to be fixed ASAP, and that is the overheating. No matter what the problem is if you do not fix that you are doomed. Being a 78 you have the smaller 3" heat exchanger which is adequate IF the rest of your cooling system is in TIP TOP shape. That means you need to pull the end caps off the exchanger (at least the front one) and make sure that it is clear of debris and is reasonably clean, and pull the aft hose off of the exchanger (that goes up to the exhaust elbow) and stick a hose in there to make sure the water flows freely out of the exhaust. Most importantly you need to replace the IMPELLER AND HOUSING in the outdrive. The vast majority of heating issues result from a worn impeller. Go with the OEM Mercruiser parts, not the cheap aftermarket stuff. Do that, refill the outdrive with good oil, replace, the ignition points slap on the muffs and give it a. If it runs and stays cool you may have a real bargain there. Oh yeah ignore the naysayers, if you can get running the 470 is not so bad:facepalm: Good luck.

 

[ahickman]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

Well.....I pressurized each cylinder and found that air bled solely from the valve cover. Any further suggestions before I pull the head to inspect the cylinder walls?

 

[Don S]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

Time to pull the head. Post pictures when you get the head off.

PS: Be sure to keep track of which pushrod goes where. They are different lengths. Adjusted like Ford rockers.

 

[BonairII]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

All cyls being equally bad makes me think.....bad compression gauge.

 

[Don S]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

All cyls being equally bad makes me think.....bad compression gauge.

Look at post #3 at the beginning of this thread. No reply so guess he doesn't consider it a possibility. He wants to take it apart.

 

[tinkerguy70]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

Look at post #3 at the beginning of this thread. No reply so guess he doesn't consider it a possibility. He wants to take it apart.

He also missed the oil down the spark plug holes in that post. That alone would probably get it running.
In the past, it's worked for me.

 

[ahickman]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

Alright guys. I am going down to the shop to do what you advise. Honestly would prefer to not have tear it down.

 

[ricohman]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

Well.....I pressurized each cylinder and found that air bled solely from the valve cover. Any further suggestions before I pull the head to inspect the cylinder walls?

I am going to assume this was done TDC with valves closed?
Just asking because I have seen guys pump air into each cylinder without realizing they needed a procedure to follow.

 

[Don S]

Re: New purchase - low compression 50 psi on all 4 cylinders of 1978 470 engine

I am going to assume this was done TDC with valves closed?
Just asking because I have seen guys pump air into each cylinder without realizing they needed a procedure to follow.

That's a problem with forums, just because someone says they did a test, doesn't mean they did it right. Doing a test wrong will also give you wrong results.

Look how another thread went from bad to good with just a retest.
http://forums.iboats.com/mercruiser...u-think-these-compression-results-591605.html