# How to do compression check?



## sailak (Apr 15, 2007)

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 (Oct 22, 2007)

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 (Oct 30, 2011)

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 – 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 – remember you are looking for consistency rather than the actual number, so a little inaccuracy isn’t 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 – everything looks good ? – take a close look at the exhaust, heat exchanger, belts, hoses, etc. How well does it start, idle and run? - Make sure they don’t 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 (that’s a tech term). No leaks before or after you run it. Any strange noises? Trans shift smoothly. Take it out and run it hard – 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 - it’s worth what you paid for it. Good Luck.


----------



## davidpm (Oct 22, 2007)

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 (Oct 30, 2011)

davidpm said:


> 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 (Oct 30, 2011)

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 (Aug 21, 2003)

Hello,

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. 

Barry


----------



## IronSpinnaker (Mar 28, 2011)

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 (Sep 10, 2007)

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 (Jul 31, 2008)

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.


----------



## miatapaul (Dec 15, 2006)

I don't think it is necessary, but it would not hurt to have oil tested by Blackburn. An oil test will tell you more about the condition of the engine than anything else and they are cheap.


----------



## IronSpinnaker (Mar 28, 2011)

miatapaul said:


> I don't think it is necessary, but it would not hurt to have oil tested by Blackburn. An oil test will tell you more about the condition of the engine than anything else and they are cheap.


They actually only tell you anything if done on a schedule with the data trended on a graph. All engines have wear metal present in the oil. The presents of wear metal means the engine works. The oil analysis only shows you the rate of deterioration if you have multiple points of reference over time.(like at scheduled oil changes) 10, 20, or 30 % increase in wear metal in a three months scheduled oil change is normal.... 100% increase in the same amount of time is bad.


----------



## sailak (Apr 15, 2007)

I think we do demand Formula performance from out engines in the sense that we expect our engines to perform up to spec. 

Oil test without a trend won't do much --- maybe if they found metal with part numbers it would indicate bad things but single test won't tell you much.

I've eyeballed the engine. It has 3800 hours and has been in service since 1995 so it isn't showroom clean, but looks fine. Oil on the dipstick looks and smells okay. Coolant in heat exchanger looks fine, no evidence of oil (found that on my 3GM30 which led to the head change). Filters are new appearing so he's probably doing regular changes. One little sea-water leak at a hose fitting. Area under engine clean of stains, oil, etc. Overall the engine looks okay --- not as nice as my engine, but okay.

Seems concensus is a compression check isn't needed. Sure hope you guys know what you're talking about. 

Will start it at the survey and see how it does, then run it hard on the seatrial.

Thanks for the responses.


----------



## IronSpinnaker (Mar 28, 2011)

3800 hrs is like the equivalent of 200 K miles. Even if treated well that is more than half of life expectancy for a Yanmar.

I would use the hrs alone as a negotiating point.


----------



## sailak (Apr 15, 2007)

IronSpinnaker said:


> I would use the hrs alone as a negotiating point.


Already done.


----------



## BarryL (Aug 21, 2003)

Hey,

You should ask your surveyor how he will evaluate the engine. When I had my boat surveyed (in 2006) he said he would check all fluids (engine oil, transmission oil, coolant), check all hoses (fuel and coolant), perform a visual evaluation, and then test it during the sea trial. The sea trial included running it at wide open throttle (WOT) for a minute to make sure it didn't overheat (it didn't) or smoke (it did - due to being overpropped), and make rated rpm (it didn't, again because of the prop). He also performed an emergency stop to test the engine mounts. 

The glow plugs on my engine didn't work well (bad wiring) so the engine was hard to start. It had 3600 hours at the time of the survey. I wasn't concerned because the engine ran fine, and I only put about 50 hours a year on it. It now has about 3900 hours and has worked very well. 

Here is the surveyor's report on the engine:

ENGINE OPERATION
The engine started easily after pre-heat and ran without smoke. The volume of cooling
water was good. There was no oily residue on the water.
Due to the tachometer problems it was not possible to see if the engine ran up to the
manufacturer's specification of 3200 RPM. At wide open throttle the boat speed,
measured by GPS, was 6.7 kts, below the theoretical hull speed of 7.3 kts.
Engine temperature at WOT was 180°F. The temperature differential between the raw
water and the exhaust was 20º at cruise and 30° at WOT. No hot cylinders were noted.
The alternator was charging at 14.35V.
The engine ran without significant vibration. The emergency stop test did not disturb the
engine mounts.


==

Barry


----------



## IronSpinnaker (Mar 28, 2011)

BarryL said:


> The g*low plugs on my engine didn't work well* (bad wiring) so the engine was *hard to start. *
> 
> Barry


Just so nobody is confused there are different types of diesel engines. The engine we are talking about in this thread is direct ignition and has no glow plugs.

Many modern small diesel engines are what is called pre-chambered diesel engines and they have glow plugs that pre-heat the fuel/air mixture in a small chamber prior to delivering it to the cylinders. They operate at lower compression and usually will not start if the glowplugs are not functioning.


----------



## dsmauney (Feb 29, 2000)

Iron

So if I understand what you are saying, a cold start with a diesel with pre-chambers ( universal M25xp) includes using the glow plugs?

Thanks
DaveM


----------



## BarryL (Aug 21, 2003)

dsmauney said:


> Iron
> 
> So if I understand what you are saying, a cold start with a diesel with pre-chambers ( universal M25xp) includes using the glow plugs?
> 
> ...


Hi Dave,

I have an Universal M25. It has glow plugs and I always use them if the engine is cold. If the air temp is cold, like 50 degrees F, I energize the glow plugs for 1 minute. It the air temp is hot, like 80, 10 seconds of glow plug is enough.

The M25 glow plugs are designed to heat the combustion chamber. No fuel is injected at this time. The glow plug is just a heating element that is used to raise the temperature of the combustion chamber before the engine is running. It's pretty easy to remove a glow plug and test it - just supply 12V and it should glow red pretty quickly. Make sure to use at least 10 gauge wire because it draws a good amount of current.

Last but not least, many boats with M25 engines are wired so that the starter doesn't get power unless the glow plug is powered too. So you need to press both the glow plug button and the start button to start the engine.

Barry


----------



## dsmauney (Feb 29, 2000)

Thanks Barry.

What I am asking Iron is the following. When he offers the opinion that a responsive start on a cold engine is a sign of good compression does that include using glow plugs on engines, like the M25xp, that have them. Response from anyone that has experience with diesels would be appreciated. 

Goodwinds
DaveM


----------



## miatapaul (Dec 15, 2006)

IronSpinnaker said:


> They actually only tell you anything if done on a schedule with the data trended on a graph. All engines have wear metal present in the oil. The presents of wear metal means the engine works. The oil analysis only shows you the rate of deterioration if you have multiple points of reference over time.(like at scheduled oil changes) 10, 20, or 30 % increase in wear metal in a three months scheduled oil change is normal.... 100% increase in the same amount of time is bad.


They can tell you if something is out of wrack. Blackstone has a pre-purchase test kit (includes a pump to get oil out) and will give results same day they receive the sample. I agree it is not as good as a trend, but it could find coolant from a small leak in the head gasket, the most common issue. It will show very high levels of metals if there is a bad bearing. It can also indicate if the oil has not been changed in a long time. There is a lot of info in there. I have no interest in Blackstone, but have used there services in the past and found it to be useful information.


----------



## BarryL (Aug 21, 2003)

dsmauney said:


> Thanks Barry.
> 
> What I am asking Iron is the following. When he offers the opinion that a responsive start on a cold engine is a sign of good compression does that include using glow plugs on engines, like the M25xp, that have them. Response from anyone that has experience with diesels would be appreciated.
> 
> ...


Yes. If an engine has glow plugs, they should be used for a cold start test.

Barry


----------



## Geoff54 (Oct 30, 2011)

dsmauney said:


> Thanks Barry.
> 
> What I am asking Iron is the following. When he offers the opinion that a responsive start on a cold engine is a sign of good compression does that include using glow plugs on engines, like the M25xp, that have them. Response from anyone that has experience with diesels would be appreciated.
> 
> ...


Yes - if you have glow plugs, use them. An engine with poor compression will usually be harder to start and often idle unevenly, although both conditions can be caused by other things.


----------



## IronSpinnaker (Mar 28, 2011)

Yes; use them. A Pre-chamber Diesel is designed with lower compression than a Direct Ignition Diesel, which is why they have glow plugs.

Ignition takes three things, fuel air and heat. In a Direct ignition diesel the compression of the fuel and air causes the heat for ignition. In a Pre-Chamber diesel glow plugs are used to gain the heat at a lower Compression. If you have glow plugs your engine was designed to be started with their use. It will start without them usually, but it will be much harder and that is normal.

You can see blowby if you remove the oil cap, you would have exhaust coming out the fill spout. That would be a very good indication that engine has significant wear.


----------



## dsmauney (Feb 29, 2000)

Thanks Iron. Neat idea on oil fill cap. Appreciate other input. 

Goodwinds
DaveM


----------

