Shade Tree Compression Test

I had someone (used car salesman) recently tell me that it was possible to get an idea of how strong (compression) an engine was by covering the breather hole on the top of the camshaft cover & timing how long it took for the engine to shut down. 8-15sec.= good. 15-20+ sec.= not so good.

Truth or Tale?

[rickjordan]

Are you saying to place your hand over the hole that the breather hose comes off of?

yes.

[rickjordan]

Have you tried this?

[Mercedes Man]

Here's my opinion:

I've put an electric engine fan on my 84 300SD. When the fan is running (I know it is running because I installed an indicator light in the dash), the engine takes to longer to shut down than when the fan is not running. The fan is hooked up to the sunroof circuit, so it should not interfer with the ignition system.

With my experience, I'm very skeptical that this is a valid compression test since the shutdown time can be affected by a mere fan.

What would be the interaction between an electric fan and the the vacuum activated fuel shut-off valve? If there is a draw of electrical power that is dedicated to the fan when it is running and thus retards the vacuum pump from activating the fuel shut-off valve, then this does not impact the compression.

The only acid test for compression that I'm familiar with is opening the oil cap on the valve cover while the motor is running and seeing if blowby is visible.

[P.E.Haiges]

I wouldn't believe anything a used car salesman said.

P E H

[be459]

Originally posted by Mercedes Man
Here's my opinion:

I've put an electric engine fan on my 84 300SD. When the fan is running (I know it is running because I installed an indicator light in the dash), the engine takes to longer to shut down than when the fan is not running. .......by a mere fan.

This is very interesting. The only thing I can think of, assuming that the fan is conneted to a temperature switch, is that the engine is very hot when the fan is on. When you have a minor vacuum leak in the system causing minor fuel delivery to the engine after key off, the engine may diesel a little longer than a not-so-hot engine. This is just a guess.

David

[Mercedes Man]

Guys,

The 300SD isn't the only car that did this me. I bought a used 1991 Pontiac Grand Am which appeared to overheat at the used car dealer's lot. I told him that the fan wasn't coming on in time. He wired the fan directly to the ignition, and the fan always ran when the engine was on. Well, the engine ran about an extra second after the ignition was turned off.

I don't know what the cause of overheating was, but I removed the jumper wire, and the engine turned off normally. The car never overheated again.

By the way, I'm using an aftermarket mechanical thermostat to control the fan on the 300SD. Even when the fan was set to come on at a lower temperature, if the fan was on, the engine would take longer to shut down.

If the engine does in fact take longer to shutdown when the fan is disengaged, it is likely just due to the electrical load on the alternator (less load=less drag on engine). The "ignition system" on your diesel is just a vacuum diaphragm attached to a lever inside the injection pump--apply vacuum and the engine shuts off.

-Joe

[Mercedes Man]

Yes, the electric engine fan puts more load on the alternator, and the engine should shut down immediately, but it doesn't. Ever turn off the engine with the air conditioner running? The blower and a/c fan are running. My 300SD shuts down immediately as it should with the a/c running, and so does every car I've seen (unless a gasoline car is dieseling ). Maybe the compressor load causes the engine to shut down fast.

[kweimer]

The only way to really check compression is with a compression gauge---if you are a long term diesel "nut", as I am, a compression gauge is well worth the cost. Any other methods of checking compression, IMHO, is a "snake oil fix".
Ken
300D 1976 (3)
240D 1975 & 76
220D 1968-1973
200D 1966

[P.E.Haiges]

Kweimer:

I have almost as many old MB Diesels as you have but I don't drive them anymore. I got spoiled by a Turbo Diesel and bought 3 more and now thats all I drive. The increase in power is fantastic. I can go up hills 50+ MPH in high gear that I only went 35 MPH in 3rd with the non turbos.

I was near Vail CO and my 300SD was passing everything on the road because of the high altitude. I was doing 60 MPH in 3rd at 4000 RPM. Even the V8 cars couldn't keep up with me. What a high it was.

If you want to stay happy with those old Diesels, don't get a turbo and get spoiled like I did.

I agree that the only valid compression test is done by using a gage.

P E H

[kweimer]

The only thing that keeps me away from any diesel after 1976 is the heavy use of vacuum actuators---some of which appear to be unreachable without pulling the dash. And then, there is the battery tray---bad enough to have to lift that big heavy battery (I'm a little guy) over the fender on the 76 and earlier models---but that extra 90 degree twist to get the battery into the tray next to the firewall on later models seems to be a great way to end up in bed a couple weeks with a painful backache.

[P.E.Haiges]

The battery on the W116 300SD is along side the fender similar to the older modelsand you can get one as early as as a '78.

P E H