Incorrect, my dear Watson.

The compression in any engine is a ratio of the initial volume of the cylinder to the final volume of the cylinder.

If the final volume of the cylinder is larger due to the tiny adaptor for the injector hole (instead of the honking injector), the compression will read lower than the actual compression when the injector is installed. Whether the adaptor for the injector hole mimics the size of the actual injector is unknown to me.

The same situation applies for the glow plug. If the adaptor is smaller than the actual glow plug, the volume in the cylinder is larger..........however, the glow plug error is likely much smaller than the injector error.

The GP takes up volume in the PC and as such a missing GP would make for lower compression right? The injector plug is virtually the same as an inector, I think you are thinking along the lines of hydraulics and not thermodynamics.

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You're right. The compression will be lower if the adapters a smaller. But how much error are we talking here. Doesn't seem like it would be more than a couple of PSI at the very most. More likely less than one. I don't know that that is enough to worry about.

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A missing GP with a fitting would make for lower compression. If the injector plug is identical to the injector itself........you'll get a perfect result.

Well, considering the volume of the cylinder at the top of the compression stroke is about 28 cc., it doesn't take much of an error to skew the results. An error of 1 cc would be about 11 psi or so. It might not be enough to worry about.

Brian is right. It's a matter of compression ratio between top dead center and bottom dead center of the piston. If the adapter you use in any holes changes the volume, the absolute compression value will change correspondingly.

I tool these readings using the HF compression tester on a Peugeot TD engine.

187,900 miles, Harbor Freight compression tester via glow plug holes:

(1) 460 psi
(2) 460 psi
(3) 460 psi
(4) 460 psi

205,000 miles Harbor Freight compression tester via injector holes:


(1) 430 psi
(2) 430 psi
(3) 430 psi
(4) 430 psi

The Peugeot TD engine happens to use the same glow plug and injector adapters as the Mercedes 617 engine. I took some pics of the adapters and will post them later. The injector adapter definitely add more"volume" and I believe accounts for the 30 psi lower reading. One cab argue that it is due to wear but I don't think so. I guess I can take another reading via the glow plug holes.

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Pic below shows the glow plug adapter next to the glow plug. The increase in volume when taking a compression reading is from the glow plug tip that is missing with the adapter.




Pic below shows the injector adapter next to an injector. Note the large hole at the adapter which fills the entire length of the adapter. This volume is much greater than a glow plug tip.

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I just bought the compression tester from HF.. item # 93644 and it looks nothing like what they have online.

It has like 6 silver fittings for glow plugs, two large fittings for the injectors, 7 adapters to mate to the air line, a hollow bolt with threads, the guage itself and the crappy manual...

which and which for the om617

I dont have a glow plug out to compare

It's one of those seven adapters. You'll have to use the 90 degree additional adapter to get the hose on with some glow plugs. I think I found that I could use the 90 degree adapter with all the glow plugs if I remember correctly. The one pictured earlier in the thread is the glow plug adapter.

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ah.. well.. I know the glowplugs for the om602 are the same exact ones for the 617... so the adapter should of course be the same

thanks

http://cgi.ebay.com/ebaymotors/DIESEL-GAS-COMPRESSION-TESTER-W-ADAPTER_W0QQcmdZViewItemQQcategoryZ43989QQihZ009QQitemZ190203607838QQrdZ1QQsspagenameZWDVW

exact same tester I bought for $25 at HF

I've searched through and can't seem to find a step-by-step on how to do a compression check on a 240d.

I know it's fairly straight-forward, I purchased the HF tester and plan on using the GP hole.

My question is, do I need to remove a vacuum line to keep the motor from starting while cranking? if so, which one? how about shutting off fuel?

Thanks!
Kat

You may want to pull and hold vacuum on the shut off actuator, which doesn't allow fuel to the engine.
Or hold the STOP lever down, but that takes another hand.

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Forgive me for asking, but wouldn't both the initial and final volume of the combustion chamber have changed by the same amount in either case? The ratio shouldn't change, or am I missing something?

I'm going to pick up one of those Harbor Freight kits sometime when they go on sale, then I will wait till I need new glow plugs, and will test it then.

I know my compression is good because it still fires right off even below 15F....and it runs perfectly smooth and strong when idling etc...

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>>Forgive me for asking...

Yes, and ..... no

However, you raise an interesting point.

One way of writing the equation for compression ratio is

CR = (S/C) + 1

where S is the swept volume and C the clearance.

Adding a small volume,v, in the prechamber, whether glow plug or injector will change the equation to read

CR = ( (S+v) / (C+v) ) +1

However, as in these diesel engines S is about 20 times larger than C, adding v makes a much greater difference to the denominator, and the compression ratio does reduce significantly.

So, say if S = 600cc, and C=28cc, you get a compression ratio of about 22.4:1

if v=7cc, S becomes 607cc, and C, 35cc (you can see that the change in C is much more significant than the change in S) the compression ratio is reduced to about 18.3:1