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sailak 07-31-2012 08:58 PM

How to do compression check?
Survey time fast approaching and I'm having a hard time finding a mechanice to look at the engine.

I trust me to be able to check general condition, etc but would like a compression check for some insight on what in insides are like.

The engine is a Yanmar 4JH2E. I've helped to compression checks on my aircraft engine, just never messed with a diesel (I did replace the cylinder head on my 3GM30 a couple years ago so have some experience turning wrenchs).

Is a compression check difficult? Are the required tools expensive?

davidpm 08-01-2012 12:05 AM

Re: How to do compression check?
Not difficult and have not done it myself so beware of following.
The diesel has high compression so the tester you need is not the kind with a rubber end. It is made for diesels and has a hose that is screwed into the injector hole.
The test is done by removing all the injectors.
Screwing the compression tester into each injector at a time and turning over the engine.
A wet compression test is when you squirt a little oil into each cylender just before the test.
If wet is better than dry you have bad rings.
Actual numbers can vary a lot between engines but not between cylinders of the same engine.
Do not be tempted to remove one injector at a time, test and replace. Remove all, test all, replace all.
The engine could possibly start causing death and destruction the first way.

Geoff54 08-01-2012 12:05 AM

Re: How to do compression check?
If you have done a compression check on an aircraft, you know the basics of how to do it on any IC engine. There are two differences with a diesel Ė you remove the glow plugs instead of the spark plugs and, because of the very high compression, you need a diesel specific compression tester.

Ever since I found a tiny crack which closed up when the engine was hot, I like to test cold, replace the plugs and warm up the engine, and then test hot. Unless the engine is completely shot, having numbers that are all close to each other is more important than the actual numbers.

Pro. testers are expensive but (this will probably upset someone) a cheap (Harbor Freight ??) tester should suffice for your purposes Ė remember you are looking for consistency rather than the actual number, so a little inaccuracy isnít so important. You just need to make sure that it comes with, or you get an adaptor that fits your engine. If you can borrow or rent a professional tester, so much the better.

Having said that there are things that are as/more important that a comp. check. First, a good visual check Ė everything looks good ? Ė take a close look at the exhaust, heat exchanger, belts, hoses, etc. How well does it start, idle and run? - Make sure they donít warm it up before you get there. Any ominous exhaust smoke? Oil is clean and not at all milky. Coolant clean and no yucky crap (thatís a tech term). No leaks before or after you run it. Any strange noises? Trans shift smoothly. Take it out and run it hard Ė does it overheat?

If you are confident that it passes all those tests, you are probably fine. If you are at all unsure, get a professional to look at it before you commit.

This is my opinion - itís worth what you paid for it. Good Luck.

davidpm 08-01-2012 12:12 AM

Re: How to do compression check?
it is unlikely someone is going to let a non-professional take apart their engine.
Also when you replace the fuel injectors some require a crush washer be replaced because the existing one will leak if reused. Buy them in advance.

I have had problems getting the hi-pressure line connected so it would not leak on one engine. So you are taking some responsibility taking the engine apart which is probably why the pros are not keen on this job.

Be sure to shut off the raw water so you don't pump the cylinders full of water and ruin the engine.

Geoff54 08-01-2012 12:17 AM

Re: How to do compression check?

Originally Posted by davidpm (Post 903812)
A wet compression test is when you squirt a little oil into each cylender just before the test.
If wet is better than dry you have bad rings.

Please DON'T do this on a diesel. It's fine on a gasoline engine but not on a diesel because the oil can ignite (compression ignition, don't forget). It's dangerous and can also destroy the tester.

Geoff54 08-01-2012 12:29 AM

Re: How to do compression check?
Oooops - just noticed it's a Yanmar which I don't think use glow plugs (all Yanmar?) - Too much time spent with crappy old diesels - So yes, you would have to pull the injectors themselves. Not sure about the adapter you need.

And, either way, make sure the stop is activated so the injectors aren't working.

BarryL 08-01-2012 11:45 AM

Re: How to do compression check?

If I am selling my boat and you want to have it surveyed and tested (non-destructive) that is fine.

If you want to start removing things that can be broken (like fuel injectors), I want YOU to put up escrow money in case YOU (or your surveyor) damage something.


IronSpinnaker 08-01-2012 12:06 PM

Re: How to do compression check?
There is no need to compression test a diesel just for the sake of doing it. Hit it with a laser thermometer to make sure it is cold, and start it... if it starts right up from a cold start there is nothing wrong with the compression.

If it cranks a while get it tested.

PorFin 08-01-2012 12:25 PM

Re: How to do compression check?
Two separate questions: "How do I...?" and "Should I...?"

I happen to agree with Iron that it's probably not necessary. If she fires up from a cold start, runs smoothly at idle, and performs well during the sea trial then the engine's gonna be servicable. Remember we're talking about something that's there to get you away from and back to the dock.

If you are looking at cruising, then the parameters change a bit as you're probably going to be asking for more out of your engine. But still, we aren't looking for or expecting Formula 1 kind of performance here.

captbillc 08-01-2012 12:47 PM

Re: How to do compression check?
i worked for cummins diesel for 30 years. we never did a compression test on any engine. if the rings are bad you get a lot of blow by to the crankcase. to find a leaking valve we would sometimes remove the intake manifold & start the engine. on an engine without a water cooled exhaust manifold we could remove that & start the engine to check for leaking valves. then if the valves were leaking we already had the manifolds off & could remove the rockers & heads.

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