How do you run a compression test on a diesel? - PeachParts Mercedes-Benz Forum

meek · #1 03-13-2004, 12:56 PM

How do you run a compression test on a diesel? Going to buy one if it passes a compression test. What numbers should be good for a 300D 2.5T?

#2 03-13-2004, 01:05 PM

The same as a gas engine except you remove the injectors instead of the spark plugs. You will need a diesel compression gauge and adapter for your particular engine. The diesel tester goes to well over 300PSI.

There are some good diesel compression tester kits available. I can't remember where I got mine, it might have been from Baum tools on the internet.

Hope this helps,

stevebfl · #3 03-13-2004, 01:27 PM

We quit doing compression tests for prepurchase inspections many years ago unless specifically asked for.

Making a good judgement on diesel compression is real hard. MB has numbers that are a specification. Something like 320-340psi or so. Almost all gauges I have used have read closer to 400psi on good motors.

Compression gauge readings are used for comparative judgements. Like this year to last year or this cylinder to the next. Absolute readings are based upon a single techs knowledge of his gauge in that motor over many tests. No tech can get everything with a brand new gauge even if he has tested that motor group multiple times with other equipment.

The worse problem for diesels is that they probably have relatively the same compression cyl to cyl till they fail. I have seen huge oil burners with good even compression.

The best judgement of a diesels condition is to know how much oil it is using and how much blow-by it currently has.... another very experience laden judgement call.

haasman · #4 03-13-2004, 01:59 PM

What about the old quick test: Remove a valve cover vent tube and when hot and running, put a thumb over the tube and see how long it takes to push your finger off the tube?

The idea being blow-by- the longer is takes the less blow-by and the better the condition of the engine.

dmorrison · #5 03-13-2004, 02:08 PM

Gentlemen
If you could, just for a knowledge base, explain the step by step proceedures for the compression check.
I have done compression checks on gas engines. As well as a leak down check. I have both guages.
Now for the diesel compression check. Of course the guage range must be higher. So a different guage to buy.
My questions would be.

1. Test through glow plug holes or through the injectors holes? Benefits of either?
2 What about the injector lines. Should these not be removed ( if testing thru the glow plugs) to prevent the injector from affecting the pressure reading. And from the engine to prevent it from starting in a warm climate?
3. How do you prevent the injector pump from spraying fluid everywhere ( if the lines are removed)?
4. The pressure readings are of course subjective.

I guess what I'm asking here is for a blow by blow description of the procedure.

Dave

PS
Well!!! If I would take my own advise and refer to the SM job 01-010 compression test proceedure.

http://skinnerbox.steaky.org/123_DISK2/program/123-cd-index.html

stevebfl · #6 03-13-2004, 02:42 PM

1. Test through glow plug holes or through the injectors holes? Benefits of either?

It doesn't matter as long as the adapter is designed for the application. I used to go through the glow hole (20 years ago). My first attempt to do it through the injector hole was to get a nozzle holder and remove the nozzle and weld a fitting on the chamber. I found that I got terrible readings. The reason was I had significantly changed the combustion chamber size by leaving the whole nozzle holder empty. The device that screws in the nozzle holder hole for compression readings lets no volume inside.

2 What about the injector lines. Should these not be removed ( if testing thru the glow plugs) to prevent the injector from affecting the pressure reading. And from the engine to prevent it from starting in a warm climate?

See answer to #3

3. How do you prevent the injector pump from spraying fluid everywhere ( if the lines are removed)?

For all the automatic vacuum controlled systems, just pull a vacuum on the fuel shut-off diaphram.

4. The pressure readings are of course subjective.

Of course!

kerry · #7 03-13-2004, 05:37 PM

Or have someone hold down the stop lever while cranking.