How to compression test

[BOATKID]

A Farryman single cyl diesel engine in a fischer panda genset, I am presuming the gauge goes over the injector hole, ( I have never actually tested compresion on a diesel so any tips would be welcome - I understand thay I need a higher range than a petrol tester)

ta

 

[nimbusgb]

I understand thay I need a higher range than a petrol tester)

ta

very much so. Using a tester designed for a petrol engine will either a - prove to be impossible since the petrol ones are usually only held in place by hand or b - result in a permanently knackere d guage or c - both of the above.

Petrol engine cylinder pressure at top of stroke 8 - 14 bar or about 200 psi, diesel engine around 40 bar or 600 psi. Given that an injector port has a surface area of about 3/4 of an inch to 1 inch squared you need to hold the thing in with a force of 400 to 600 pounds - not easy The ones designed for the job come with an adaptor tailored to the type of injectors fitted to your engine. That is bolted in place of an injector and the gauge hooked up to it.

 

[JohnTH]

Yes, that's the way to do it.

Diesel gauges actually have to be screwed in to replace the injector.

Why do you suspect lack of compression?

73s de
Johnth

 

[savageseadog]

The tester needs to go in the hole where the injector goes with a properly secured adapter or the heater plug holes. There's no chance of doing a a measurement without it being sealed, screwed in or clamped. You will need to use a diesel compression tester as the pressures are higher than petrol engines. The test is not absolute only comparative between cylinders on the same engine or another one using the same equipment. A leak down test where the cylider is pressurised with compressed air can be more informative.

 

[BOATKID]

Johnth

I am trying a process of elimination, I inherited the genset with the boat but can not get the bloody thing to start

ta

 

[john_morris_uk]

A Farryman single cyl diesel engine in a fischer panda genset, I am presuming the gauge goes over the injector hole, ( I have never actually tested compresion on a diesel so any tips would be welcome - I understand thay I need a higher range than a petrol tester)

ta

You need a dedicated deisel compression tester with the correct connector to screw (or clamp) into the injector hole. The compression pressure is too high to hold a conventional petrol engine compression tester in the hole - and the scale would be wrong anyway.

Borrow or hire one, or pay a diesel mechanic to come and do the test? I can't access your profile to see where you are, but if you are in the west country and if it fits you could borrow my compression tester?

 

[oldharry]

I dont think a compression test is going to tell you very much about a non starting single. As suggested a leakdown test will tell you much more.

The problem is far more likely to be something to do with the fuel system anyway, unless the engine is truly knackered - which a leakdown test would highlight far more clearly than a compressin test.

Give it a whiff of Easy Start - just this once. That will prove once and for all whether or not it will start. I had a Bukh that had not been run for several years. Nothing would induce it to go. We kicked it with a dose of Easy Start, which so scared it that it leapt in to instant life, and started perfectly ever since!

 

[vyv_cox]

I agree with Old Harry. New compression varies enormously on diesels anyway, according to design, and a low value could well be correct. A simple test is to turn the engine over slowly by hand. It should take a good bit of force to get it over TDC on the firing stroke. Then do the same with oil squirted into the bore. If the force increases considerably it is an indication that rings/bores are worn. If it's relatively easy in both cases it's more likely to be a valve problem.

 

[JohnTH]

I agree with Old Harry and Viv_Cox.

Just one more thing, can you hear the injector cracking off when you turn the engine over?

Oh and if you do not have a can of easy start you can try a sniff of gas instead !

73s de

Johnth

 

[BOATKID]

thanks for all replies guys - you have all got me thinking again. I suspected at first a fuel prob but I am getting fuel up to injector, I sent injector away for ultrasonic cleaning ( dont know if they tested for spray pattern tho)

jonth - no I dont think I have heard a crack from injecto
!!!!!!!!!!!!!

 

[NorthUp]

its

more of a creak than a crack, but once you hear it, you'll know!

 

[William_H]

Just to explain aLeak down tester

With a leak down tester, compressed air is introduced into the cylinder via the spark plug hole or injector hole. The engine is clamped so it can't turn. The injector device has 2 gauges with a tiny restriction between the 2. ie one gauge on supply one at the cylinder.
A well sealed engine gives similar pressure on both gauges while a leaky cylinder gives a low reading at the cylinder. The big advantage is that if you have a leak you can hear air escaping out of the air inlet system or exhaust if it is valves and through the crank case vent if it is leaking past the rings.
Everything being static there is no fear or danger from rotating engine and no rush.
You do need to have a calibrated restriction or use the device as a comparison. On small aircraft the results are recorded for each cylinder every 100 hrs of operation so you can see a trend if any.
Is the same device used on diesels?
olewill

 

[DaiB]

I used to have a FP genset. Yours may be similar. The fuel is supplied via an electric pump which you should be able to hear when switching on the control panel. A steady click-click. Find this by following the fuel pipe and check it out. When you stop the engine all you do is stop the fuel feed pump so if this is not working your engine won,t start. Also in this fuel feed line an in line filter may be fitted, if so change it. Use the cold start button combined with the auto decompresser and it may then start.

 

[Billjratt]

We had a faryman single in one of our previous boats.
The sound of the injector creaking is key to the diagnosis.
It is very obvious once you listen if it is being hand cranked (Don't know how noisy your electric start is -if you have one.)
The point is, that if there's no creak, it could be air in the system -fuel pressure is not getting high enough to open the injector needle against the spring pressure.
Could also be a faulty injector pump - not enough oomph.
No creak - no go. finito.

 

[BOATKID]

this is all great food for thought

much appreciated thanks

 

[BOATKID]

DaiB

this all sounds familiar but I didnt know I had a cold start button (remember I have never actually seen this engine run)

 

[DaiB]

DaiB

this all sounds familiar but I didnt know I had a cold start button (remember I have never actually seen this engine run)

Cold start is a button you lift up on the fuel pump to give max fuel during starting.
Look up Faryman website and you can download the manual, Page 46

 

[oldharry]

The problem is far more likely to be something to do with the fuel system anyway,

... and if you are not using the correct cold start procedure then it will never start.

 

[sunquest]

... and if you are not using the correct cold start procedure then it will never start.

Mine always does!
The OP needs to check the inline fuel solenoid near the injector. This is a small black box about 1" square, the cap is secured by a screw. Remove this, pull off the cap and clean the male contacts. additionally on mine I twisted the contacts a fraction to ensure a good contact. Geoff.

 

[oldharry]

Mine always does!

Maybe, but OP's doesnt.