Compression Test - PeachParts Mercedes-Benz Forum

1983/300CD · #1 06-11-2006, 08:58 PM

Basic instructions were included with this US GENERAL diesel compression tester. I still have a few questions about it. The instructions are as follows:

1) Assemble tester and make sure the battery is fully charged.

2) Prepare the engine for compression testing by starting the engine and allowing it to warm up to its normal operating temperature. Then, switch off the engine.

3) Once the engine has been turned off, disconnect the fuel shutdown solenoid (refer to the engine maintenance manual for location).

4) Remove all of the injectors.

5) Using the proper adapter, connect the compression tester to the first cylinder of the engine. Then, with the fuel pedal depressed completely, crank the engine for eight revolutions.

6) After cranking the engine, record the reading on the pressure gauge on the compression tester.

7) Once the maximum reading has been recorded, release the gauge pressure on the compression tester by depressing the air release button.

8) Remove the compression tester from the first cylinder. Then, repeat steps 5, 6, 7 for the remaining cylinders.

9) NOTE: Good engine cylinder compression will be indicated with a high initial reading, and a progressive buildup to the final maximum reading. Poor engine cylinder compression will be indicated with a low initial reading and a much slower buildup to the final maximum reading. The compression readings for all of the engine cylinders should not vary by more than 10%.

10) Reconnect the fuel injection solenoid and reinstall all injectors.

Okay,
Step #3 refers to the "fuel shutdown solenoid". Step #10 refers to the "fuel injection solenoid". I guess this is the same thing. My question is as to where the fuel injection solenoid is. I really don't want fuel squirting all over the garage.

leathermang · #2 06-11-2006, 09:02 PM

Wait until someone confirms this.. but I think on our machines it means to wire the manual shutdown lever to the shutdown position...

#3 06-11-2006, 09:06 PM

It is the small round thing on the back of the injection pump with a vacuum line attached to it.

You should take off all the injection lines to do the test. This will prevent the engine from running and will make the whole process faster. Put a towel or some rags over the ports where the injection lines were on the injection pump. While you crank the engine, it will make a fuel mess without something over those ports.

Panzermann · #4 06-11-2006, 09:23 PM

I did the test with this unit and didn't mess with the "fuel shutdown solenoid." Just put a rag over the nozzles on the fuel injection pump and it will catch any fuel that sprays out. It's not a major problem.

1983/300CD · #5 06-12-2006, 08:12 AM

The shut off switch with the vacuum line attached in the rear of the IP will stop the engine when vacuum is applied to it, but it won't stop fuel delivery when you remove the vacuum line. So okay, a rag over the top seems to be the way to go. Thanks.

Or maybe hook up the shutoff valve directly to the booster line?

rgnprof · #6 06-12-2006, 12:55 PM

Can't you also check the compression by removing the glow plugs - provided you have the correct adapter?

ryan

leathermang · #7 06-12-2006, 01:04 PM

Yes you can... and that has been discussed in the archives... and I posted a picture of the FSM showing the glowplug holes being used for that...

rgnprof · #8 06-12-2006, 01:11 PM

Yea, I guess I shouldn't have phrased it as a question. I was really wondering why you would ever fool with removing the injectors to test the compression - it seems much easier to pull the glow plugs. Am I missing something? I guess it probably depends on which engine...

I'm getting ready to do this on my '78 300D.

Ryan

1983/300CD · #9 06-12-2006, 01:25 PM

The kit has an adapter for the glow plug holes... for a later model Mercedes. I need to go through the injector holes. If you get the same kit as I got ($20 harbor freight) you'll probably end up doing the same thing.

dannym · #10 06-12-2006, 01:48 PM

It all depends on which kit you have and what engine your testing.

Some specify glow plugs some specify injectors.

danny

rhodes2010 · #11 06-12-2006, 01:56 PM

I took a glow plug to a local machine shop and an oversized brass connector
for my compression tester.

He turned the brass connector from the compression guage to fit my glowplug size.

Now I can do compression off the GP holes and leave the injectors in.

Panzermann · #12 06-13-2006, 04:14 PM

I found that the effort to remove the glow plugs was just abotu equivalent to that to remove the injectors, other than needing to buy .75 cent a piece heat seals when you replace the injectors.