Compression Test

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emnick

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Hi All,
I want to have my injectors checked and cleaned, while they are out I would want to do a compression test. I have a decent (Snap-on) compression tester which is caliberated up to 250psi, I need to know (1) if this is to low for a Diesel Yanmar 3jH series engine (2) can I buy an adaptor to fit the injector thread as I have a conventional spark plug hose connection on gauge (3) Recommendation for injector cleaning people.

I want to do the test as when I was bleeding the injectors today there was a noticable less drop in revs when pipes cracked open on two of the three cylinders (middle cylinder caused more of a rev drop than 1&3).

Engine had run for about 5 mins prior to opening each pipe. There is also a slight 'diesel knock' from the one that caused most rev drop (hence clean the injectors).

Thanks in advance for ANY help.
 
tester

You need a gauge that will read up to 1000psi for a diesel engine.
Its more than likely that reconditioning your injectors will even out the rev drop you tested for- but a good injector shop will test them for you before stripping down if necessary.

There is an excellent diesel injection pump specialist in Stornoway if that any help?
 
A normal diesel have a compression pressure of 25-30bars or 360-430psi.
when you are cranking it cold!

Teoretical compression pressure is pressure in Bottom dead senter mutiplyed
by compression ratio^kappa air wich is 1.39 for a diesel.

If your starting pressure is one bar and compression ration 16 you vil get

1*16^1.39= 47bar. Normaly you get less. The filling is more like 0.95 and when you crank the engine the engine is cold and you leek a lot of heat to the cold surfaces! Small leakage is making a mess also because the low speed! You need to measure during ideling to get it rigt! That is not easy!

Buy a gauge that is ment for diesel engines or take a leekage test!
 
Last edited:
ulyden said:
A normal diesel have a compression pressure of 25-30bars or 360-430psi.
when you are cranking it cold!

Teoretical compression pressure is pressure in Bottom dead senter mutiplyed
by compression ratio^kappa air wich is 1.39 for a diesel.

If your starting pressure is one bar and compression ration 16 you vil get

1*16^1.39= 47bar. Normaly you get less. The filling is more like 0.95 and when you crank the engine the engine is cold an you leek a lot of heat to the cold surfaces! Small leakage is making a mess also because the low speed! You need to measure during ideling to get it rigt! That is not easy!

Buy a gauge that is ment for diesel engines or take a leekage test!
Click to expand...
My engine manual gives the cold cranking pressure figures for the cylinders. This mean that you don't need to check compressions with the engine idling. (It would also be a bit difficult if you had a single cylinder engine!) The figures take into account what you call 'leakage'. The only problem that I have is that my diesel engine compression tester hasn't got the right fitting for my injector threads, so I never get round to checking our compressions... Mind you if the engine starts well from cold, its a pretty good indication that there's nothing much wrong.
 
When I needed to do a compression test on my engine I bought a diesel compression tester from eBay, it had obviously been hardly used and after I had used it, I resold it on eBay - from memory I made a small profit!
 
emnick said:
Always starts first time from cold, now smoke either.
Click to expand...

That being the case, I would waste neither time nor money going further than getting the injectors sorted at this stage. If compressions were down, cold starting would not be so easy, and there could be white smoke. You will almost certainly find all is well once the injectors are done.

If you really are worried then forget compression testing - it will only tell you which cylinder is giving problems. The Leakdown test suggested by Ulyden will tell you where to look if there is a problem, i.e ring/bore damage, valves not seating, or a gasket problem. The test pressurises the cylinder, and measures the rate at which the pressure falls away far more accurately than compression testing.
 
I bought a compression tester for diesels, it came with a couple of injector bodies and adaptors to plug in instead of glow plugs. Unfortunately they did not fit my Volvo so had a local engineer make up a suitable replacement.
The compression between cylinders did show a significant difference so am in the middle of a complete rebuild, will be interesting to see what the new figures will be.
As mentioned already, have the injectors checked, diesel bug or wear and tear will take their toll. My engine was suffering from partially blocked injectors dribbling rather than spraying a nice jet of fuel so have now drained and cleaned the tank just to make sure it does not happen so soon. If you get hold of an injector puller it makes it simple and doesn't risk damaging the injectors

Good luck
 
Do you have a problem or are you looking for one? What I mean is do you have an engine problem normally? One that might be related to wear and lack of compression?

If not, dont bother - you will only give yourself something to worry about.
 
Problem

I started looking due to what I considered a diesel knock, when I released the high pressure pipes to see what cylinder was making the knock I released number 1 and very little rev difference, number 2 stalled the engine (was ticking over 750-800 rpm) number 3 showed very little difference in revs.
When number 2 was released the knock went prior to engine stalling. I concluded , Cylinders 1&3 were doing little but engine was ticking over too good for that or number 2 was doing too much., Hence I thought a compression test was the way forward.
What do youthink????
Thanks for replies
 
From what you have described it seems that the injector pump may not be perfectly balanced between the cylinders. The one receiving most fuel is doing the work at tickover.

Alternatively you have a dirty injector which is not spraying as well as it might if it were cleaned up.

Start with getting the injectors tested and cleaned, and then have the pump tested if the knock persists and is worryingly loud..
 
I'd still get the injectors checked first, they repair shops normally check free of charge and let you know right away what the status is. Do the easy stuff first!
 
emnick said:
I started looking due to what I considered a diesel knock, when I released the high pressure pipes to see what cylinder was making the knock I released number 1 and very little rev difference, number 2 stalled the engine (was ticking over 750-800 rpm) number 3 showed very little difference in revs.
When number 2 was released the knock went prior to engine stalling. I concluded , Cylinders 1&3 were doing little but engine was ticking over too good for that or number 2 was doing too much., Hence I thought a compression test was the way forward.
What do youthink????
Thanks for replies
Click to expand...

Could be that injectors 1 & 3 are not spraying properly at tickover, so No2 is working hard to keep engine going, hence the apparent knock. You will only know by having the injectors checked/re-set. Do that first before you waste time and money looking for faults that are not there.

OR try switching injectors around and see if that makes a difference.
 
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