Universal Diesel Compression Test - SailNet Community
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Universal Diesel Compression Test

I have a 1982 Universal Diesel 5411 which was showing low compression. I hauled the engine and installed new piston rings and a new head gasket (of course). Additionally, the raw water cooled engine cylinder head was almost rusted through to the cylinder head bolts. I was able to find a fresh water engine for which I paid too much. The donor engine had signs of being run pretty hard and the exhaust valves were just in specs.. Since I don't do long range cruising anymore, I didn't change the exhaust valves.

Today I did a compression test and found the following: #1 cylinder - 210 psi. #2 cylinder - 345 psi.. Both cylinders showed no significant leak down of pressure after 40 seconds. Also, while the engine was in operation last season, there was no unusual vibration either at idle or high power, which one would clearing expect if the compression was that far off. I had an engine with 15% compression differential that vibrated more.

I'm thinking that the number 1 cylinder exhaust valve may not be seating properly, or the head gasket is releasing any pressure over 210 psi.. I'm taking the head to a machine shop and doing the things regarding block and head surfacing, as well as the valves, that I should have done last season. Any thoughts as to why the gross difference in the compression readings? Thanks.
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Re: Universal Diesel Compression Test

Sorry for the delay in getting this post out.
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Re: Universal Diesel Compression Test

Disregard part about leak down test; I didn't do a proper test. The more I think about it, the head gasket must be leaking near the number one cylinder.
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Re: Universal Diesel Compression Test

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Originally Posted by jimd333 View Post
I have a 1982 Universal Diesel 5411 which was showing low compression. I hauled the engine and installed new piston rings and a new head gasket (of course).
I was just wondering (without wanting to insult your abilities), you did deglaze the barrels prior to installing new piston rings, right? Because if you didnít, the high compression on one cylinder would be more a case of good luck than anything else. New rings will not bed into a glazed barrel. The very fact that you undertook what is actually quite a challenging job suggests that you probably know this. Just asking . . .

Also, squirting a little oil into the ďsoftĒ cylinder prior to a compression test will probably reveal whether the loss of compression is due to rings not sealing or a valve that is leaking. The oil temporarily improves the seal on the rings but does nothing to improve a leaky valve.
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Re: Universal Diesel Compression Test

No insult, I'm just a sailor on a limited budget trying to convince the spouse that we can still afford to sail. Yes, I deglazed the cylinder walls. I'll try the squirt of oil; if the head gasket leak is causing the loss of compression, the results should be the same, right?
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I remember doing a lot of band aids...and living on hope.
Maybe just run her until you can do a topend job without making wifey worry.....
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Quote:
Originally Posted by jimd333 View Post
No insult, I'm just a sailor on a limited budget trying to convince the spouse that we can still afford to sail. Yes, I deglazed the cylinder walls. I'll try the squirt of oil; if the head gasket leak is causing the loss of compression, the results should be the same, right?
Correct. Oil in the cylinder will not change the numbers if you have a faulty head gasket or burnt valve.

Another silly question... You reinstalled the pistons back in the same cylinders facing the same way, correct?
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Re: Universal Diesel Compression Test

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Originally Posted by jimd333 View Post
I'll try the squirt of oil; if the head gasket leak is causing the loss of compression, the results should be the same, right?
As another post also confirmed, oil wonít improve readings with a leaking head gasket but if the faces were clean and the head was tightened using a correct sequence and torque settings, itís unlikely a new gasket will leak. With a well-used 2nd-hand cylinder head you probably would have been well-advised to lap the valves in (and fit new valve stem seals).

Good luck, hope you find the problem.
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Re: Universal Diesel Compression Test

Thanks for all the feedback. The cylinders were replaced in proper order, but as Cassidy hit on, I took short cuts with the very well used cylinder head in order to not cut last year's short sailing season in Connecticut.
I even used the same head bolts which I suspect were already used twice. Gonna do it right and turn the whole cylinder head mess over to a machine shop. I should be able to check back with you in a couple of weeks with the verdict.
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