Compression test on '78 240D (first-timer) - PeachParts Mercedes-Benz Forum

BodhiBenz1987 · #1 05-06-2012, 01:15 AM

I'd like to do a compression test on my 240D sometime in the next few days, which I have never done before (on either car). I've always kind of wondered where the 240's engine stands, since it has loads of blowby and idles rough on and off but starts fairly easily even in the cold. I'm getting discouraged with this car, again, and maybe some compression numbers would help me one way or the other.
I have the FSM and have done a fair bit of forum-searching, but just want to double-check a couple things.
1-I have a remote starting device my dad gave me a while ago. This is going to sound silly, but I'm not really sure how to use it. In the FSM it says to attach to terminals 30 and 50 ... does it matter which leads goes where? At that point do I just press the button and hold it as the engine cranks?
2-Since I only have two hands I can't hold down the stop switch, so was going to try to wire it down. How tightly does it need to be wired? I.e., is it going to try to pop up and start or is slight resistance enough to keep it from doing so?
3-The shutoff line is the one going into the very back of the pump, right? How much vaccuum do I need to put on it to keep it from starting, and will applying vaccuum once be enough?
I could figure some of this out by trial and error but I'd rather not have it start accidentally while I'm trying to take a compression reading.

vstech · #2 05-06-2012, 01:58 AM

if you pull all the gp's or injectors, one, it'll spin faster without draining the battery or straining the starter as much, and two, it'll NEVER start!
you should remove the injector lines from the injectors, but leave them on the IP, and wire up the stop linkage. not the stop lever.

funola · #3 05-06-2012, 02:02 AM

Take all glow plugs out before doing compression. No way will engine start and run on one cyl.

remote goes to starter solenoid and + term of batt

jam a piece of wood cut to the right length between stop lever and IP

vstech · #4 05-06-2012, 02:05 AM

Hmm. I'm not sure you want to just hook to ANY terminal on the starter.
I'd hook to the battery side, and the small wire on the starter. both are on the starter itself. the large wire on the starter and the small wire on the starter.

DeliveryValve · #5 05-06-2012, 03:42 AM

Quote:

Originally Posted by vstech

Hmm. I'm not sure you want to just hook to ANY terminal on the starter.
I'd hook to the battery side, and the small wire on the starter. both are on the starter itself. the large wire on the starter and the small wire on the starter.

I always use the junction box near the battery.

See pic courtesy of diesel911.

Connect the two leads on your remote starter on the outer two screws. Once you press down on that button the engine will turn over.

.

funola · #6 05-06-2012, 11:55 AM

240D does not have a junction box as I recall. There's a 50/50 chance she'll get the correct terminal. If it doesn't spin, go to the other terminal. The big terminal is + 12, if you connect the remote starter across the small and big terminal of the starter solenoid, there's no chance of shorting (if wrong batt terminal connection was made).

As I recall, the 240D has a lot more room than the 300D for glow plug compression testing. Removing the injector lines is not really necessary unless you have huge hands. #4 will be the hardest one with the IP in the way.

You can wire the stop lever shut but use a heavy duty bungee on the wire and hook it on the hood hinge. Wire alone has too much slop for complete fuel shut off.

I'd suggest doing both a cold and hot tests. More work but will provide more data. The cold test will be good practice for getting the glow plugs out fast for the hot test.

Have fun!

BodhiBenz1987 · #7 05-06-2012, 04:05 PM

Thanks guys. I think I've got a pretty clear idea of the process now. Hopefully I'll be able to try it tomorrow or Tuesday.

BodhiBenz1987 · #8 05-13-2012, 11:27 PM

Funola, I should have listened to your advice on doing a cold test first as practice on getting the glow plugs out! It took me so long to get them out (and I broke a vaccuum line the process) that the engine was not at all hot when I was done. Since it's all apart now, might as well wait until tomorrow and do a cold one, plug it all back in, warm it up, and then do a hot one. I don't think it will take me so long the next time.

I did get the remote starter to work, but I'm having trouble wiring the stop lever in the right position. First I discovered it didn't even shut off the engine.

I finally figured out (after a lot of reading) it was just because I had the idle cable adjusted too tight. Now it shuts off w/ the lever, but the bungie doesn't hold it up. I'll have to keep experimenting.

kerry · #9 05-19-2012, 10:55 AM

Just disconnect the fuel line to the lift pump. It can't pump fuel if it's not being fed fuel.

funola · #10 05-19-2012, 11:05 AM

The rectangular piece is the jamb. Notice the slot? It's for a positive fit. Use Oak or similar hardwood. Aluminum or steel will do also but take longer to make than wood. Oh and do not disconnect fuel line as suggested . Not only will you still spray lot's of fuel out the holes from the fuel that's in the IP. You will then have to do more work to bleed the air out.

Jammed in place. Make it fit tight.

kerry · #11 05-19-2012, 11:09 AM

Quote:

Originally Posted by funola

Oh and do not disconnect fuel line as suggested . Not only will you still spray lot's of fuel out the holes from the fuel that's in the IP. You will then have to do more work to bleed the air out.

Have you confirmed that via experiment? The engine will shut down if running and the fuel line is disconnected, suggesting quite strongly that the IP cannot pump fuel if new fuel is not entering it.

BodhiBenz1987 · #12 05-19-2012, 01:53 PM

Hmmm, that's kind of similar to what I did (I can snap a picture in a bit), though my linkage is a little different. The first attempt I had it up as far as it would go against the idle cable stop (which I know was enought to stop the engine before, but maybe it was not stopping the fuel entirely). After that try, I took the cable off and put the stop linkage all the way up, so the lever sat against the valve cover. I'm not sure if it was still delivering fuel at that point, but the first try would have been enough to lubricate the cylinder (and comes out the GP holes) on subsequent tries I guess.
I didn't think unplugging the fuel line into the IP would help because the fuel left in the IP would still enter the cylinders, wouldn't it? Even though it's not much, it would do the same thing that happened last night, and provide a little lubrication and give high readings.
I'm going to put it all back together and run it a bit to get out any fuel that may have sat in the cylinders overnight. Then I guess I will experiment. What happens if it fires on the cylinder you're testing?

I know it won't start because the other cylinders can't fire, but wouldn't that be pretty damaging to at least the tester?

funola · #13 05-19-2012, 05:35 PM

Quote:

Originally Posted by kerry

Have you confirmed that via experiment? The engine will shut down if running and the fuel line is disconnected, suggesting quite strongly that the IP cannot pump fuel if new fuel is not entering it.

no I have not. have you?
we know that every time the fuel system is opened, air gets in. what you are suggesting does not seem like a good idea.

kerry · #14 05-19-2012, 07:36 PM

Quote:

Originally Posted by funola

no I have not. have you?
we know that every time the fuel system is opened, air gets in. what you are suggesting does not seem like a good idea.

No, I haven't but there was a post on here a couple of years ago that definitely proved that if you disconnect the fuel line, the engine shuts down promptly and doesn't use up the fuel remaining in the filter/IP etc. It was in regards to the options available when the engine runs away after a faulty shut off valve install. That's why I suspect that that the IP won't pump fuel if there's no fuel arriving at the inlet because the lift pump isn't pumping fuel to the IP.

It would probably allow air to get sucked in which would need to be cleared out with the primer pump.

BodhiBenz1987 · #15 05-23-2012, 03:51 PM

Well, I did determine both the shutoff valve and the shutoff lever easily stop the engine. But I decided to try doing it through the injectors anyway, since like whunter said, I could also inspect the injector nozzles. Everything went surprsingly smoothly until I actually went to do the test. As soon as the guage reaches 250 psi, the connector pops off. There's only one quick connector in the kit that fits the injector adapter, so I can't try another one to see if it's just a bum connector. That would be my guess. Naturally I got the cheap Harbor Freight tester. It worked fine with the glow plug adapter (it popped off once on that round but I assumed I had not seated it right, and it was fine every other time). Frustrating.

Also my injectors came out covered in oil, especially Nos. 1 and 2. Not an encouraging day.