Compression Check:

[bjperry]

As those who have been following my postings know I'm trying to revive a 1978 v-6 150. I've done all of the things outlined in the FAQ revival post (Rebuilt carbs, rebuilt fuel pump, replaced poppet assembly, replaced thermostats, Completely rebuilt water pump, replaced water tube). I picked up a compression tester over the weekend at harbor freight and checked my compression yesterday with the engine cold at cranking speed. On 4 of my cylinders I'm at about 70 psi. On 2 of them I'm right around 100. I have the service manual but can't find the specs on what my pressure should be. Also, will it change once the engine has warmed up? If so should I wait until after I've let it idle for awhile and come up to operating temperature before testing again? If my pressure is really shot what's the most likely culprit? When testing yesterday I sprayed a little oil in one of the cylinders to see if that would seal the rings and up my pressure a little. I couldn't tell that it did. Does that point to a possible blown head gasket instead?

Any thoughts will be appreciated.

 

[WillyBWright]

Re: Compression Check:

I'd say either pull the heads and have a looksee before you spend anymore money on it, or keep on truckin and check the compression once it's been run a bit.

Those readings sound low, but it all depends on the gauge and the number of compresion strokes. I do 7-9. It should be pretty close to top pressure by the 5th. If you're getting your readings with a rope, those readings would be about what I'd expect. Cranking with a battery, they should all be over 100 ... depending on the gauge.

 

[bjperry]

Re: Compression Check:

These readings were obtained using the electric start; but again, with the engine cold and not having been run in a long time. I'm thinking I'll let it warm up and test it again. This may improve things or I may just need to take it to the lake and see if I can really rev it up to see if it blows anything out. I would be all for pulling the heads, but don't know how easily that will be if the bolts haven't been out in a while. One bolt was broken off when I got the thing and I'm working on getting that extracted now.

 

[steelespike]

Re: Compression Check:

Sometimes a decarb treatment will help improve compression.I wouldn't pull anything apart until you give it a try.

 

[timmathis]

Re: Compression Check:

Give a good dose of seafoam and deep creep.
A good decarb like spike said.May help losten rings and boost youe compression. Then re check it they all should be with-en 10% to be in tip top shape.
Tim

 

[bjperry]

Re: Compression Check:

Well,
Thanks to all who gave advise. I had it running for a little while today at idle just to let it warm up. I had water coming out around one of the head bolts. So I figured I might as well just tear into it and see what things looked like. After breaking off about 20 of the 26 cylinder cover bolts and about 5 of the actual head bolts (in addition to the one that was broken off when I got the motor) I pulled the heads and received some bad news. One of the pistons, I think it's was number 4, was melted on each side such that I could see the top ring and ring stop. The cylinder walls all look good but in the water jacket area there is much decay of the aluminum. I don't know if this motor was used in salt water but I suspect that it was and was never flushed. I noticed the same thing earlier when I removed the thermostat covers and the poppet assembly. Much pitting and wear like it had been pumping sand water through the engine for a long time.

So now I'm at a crossroads. I'm leery having the head bored and replacing the pistons and rings because I fear that the first day I take it to the lake I'll throw a rod and all of my work will have been for nothing. Still, everything but the powerhead seem to be in good condition. So maybe a rebuilt powerhead is what I should be seeking. I'm going to post the question and see what others think. Also, it looked like the engine had only been running on 2 of the four cylinders. Not surprising it was the two that actually had decent compression when I ran the first test.

Back to the drawing board.

 

[ddennis]

Re: Compression Check:

Since there is that much corrosion in the jackets, my vote would be for a new powerhead. Some of the pros may disagree with me and have better ideas...

 

[timmathis]

Re: Compression Check:

Get in touch with Clams.
Tim