md21722 - Brian

I am sorry for my previous post - upon re-reading it I don't like the "tone" of the message. I was trying to make a "wise-acre" remark and I think the tone was a little more serious than I was trying to be.
Sometimes its hard to get the tone right - I'm still kind of new at this game...

My appologies. That said I think its important to differentiate between opinions and factual information. The engine book is quite clear that its OK to measure compression from the glow plug threads or the injector threads (prechambers) It is your opinion that the injector is the better place to go because of the reduced risk of stripping the glow plug. I'll take the risk of stripping the glow plug because, in my opinion, the risk of stripping out the plug is small in comparision to the work required to remove the injectors ( removing lines, removing injectors, replacing injector heat shields ect, ect) Its a judgement call but either way is acceptable to the factory.

I have measured compression cold and then warmed up the engine and re-measured the compression. As I recall there was no significant difference between the hot and cold numbers. The hot numbers were slightly higher (5-8 PSI) and I would attribute this to the faster cranking speed of the warmed up engine. I do not know if some types of engine failures would cause false compression readings if a cold compression check was done. Its best to do it by the book when possible.

Thanks - Tim

Nib,

I would have to say Leathermang did a good job with the instructions. As to the question of hot or cold, Mercedes says to take compression readings hot which has always confused me. In the official MB diesel engine diagnostic book there is a list of procedures to diagnose an engine problem. One of the steps to diagnose an engine that won't start is to do a compression check. How in the H am I supposed to do a hot engine compression check, if I can't get the engine started?

Tim said he only noticed a 5-8 PSI difference. My question, where did you get a guage that lets you discern 5 pounds? You must be interpolating.

Last comment. If you buy a used compression tester or borrow one, be sure to be positive the schraeder valve in the tester is for DIESEL engines, because valves for gas won,t work. Your readings will be low. The little seal on the valve must be made of VITON rubber and not buna n or nitrile.

Peter

Thanks for the support guys...

Here are some of the other considerations that went into my "glow plugs as the preferable route" thinking....

1. Anyone that buys a 200k plus old diesel , after changing the fluids and filters ought to consider taking the glowplugs out to inspect and clean them anyway.... They may never have been taken out in the life of the car.... They are more likely to come out cleanly now than 100 k or more down the road... and your car is starting and running now... why wait till you are having to add the glow plugs into the equation for why your car won't start ? So take them out, clean the threads, use the carbon reamer if you can find one... and put some antisieze on the threads before reinstalling... that is one simple thing to cut down the possibility of breaking a glow plug later in your life....

2. Someone new to diesels may or may not have three tools which I think are important to pleasant diesel living.... a torque wrench, the special "line" wrench which fits( the steel fuel line nuts) to the torque wrench..and the 27 mm deep socket ....

3. The glow plugs can be taken out with normal tools... a 1/4inch universal and a socket and regular little wrenches....There are posts about glowplugs breaking.... but I do not think percentage wise this is a problem....One or two broken glow plugs will probably not disable your car at this time... but messed up injectors can easily do that...

4. If the previous owner or their mechanic set the injection pump by leaving the steel lines in place and forcing the pump to position... then when you take the fuel line loose you may feel you have been screwed... because all of a sudden the item which you simply unscrewed is not in a configuration which will allow it to just be bolted back on.... The steel lines are not fun to readjust to fit the moved injection pump.....

In other words I was figuring the least likely problems method of checking the compression.... given we are talking about someone new to thier car and MB Diesels...

Leathermang,

Your last post makes some good, strong arguments for removal of the glow plugs for testing compression on a 617 engine.

Thank you Brian, I try to convey conservative attitudes towards mechanicing because even before getting in MB's I have always worked on old cars (and sometimes cheap like VW).... where I could count on the worst happening at sometime during a project.... I know that some people lead charmed wrenching lives and I wish that I was one of them....:D

disable IP- who knows how?
 

Fisherman speaks of the injector pump having to either wire it open or not - this could effect my compression no.s

Does anyone know how I do check my IP- I am idiotchild right now to this Mercedes- first one.

And if I check compression thru the Glow plugs- how do I disable the fuel supply to the cylinders?

This is why I did it thru the injectors the first time

please and thanks

peace:

Pretty simple I think but I could be wrong so wait for the second opinion here.

The disabling of the fuel comes naturally when you disconnect the fuel lines. I would do this even if I were checking through the glow plugs cause you can get them a lot easier without the tubing inthe way. I just lay a rag over the IP while turning then to stop any shooting diesel.

As to the IP wired open I believe the way to discern if yours is one that needs to be wried open is if it has a connection to a intake manifold throttle valve. Actually the wrong word here since there is no such thing as a throttle valve on a diesel but my point being that if it has a restrictive device in the intake manifold that is controlled by the throttle position then you should wire it open.

Again please note i am speculating here as I only do what the manual tells me for my cars and I know on the 240 it says wire it open. it has a manifold connection to a butterfly valve. Actually just makes sense since if there is a restricitive device in the intake then your engine will not be sucking all the air it can get which could reduce or change your numebrs on a compression check. Then again I may be WAY off base here.

Hopefully someone more knowledgable in your year and make will help me out here...;)

The old 240D engines (pre 1977 or so, not really sure) had pnuematic governors. This engine has a restrictive flap in the intake manifild just like a gas engine, for this version of the 240D engine you should wire the flap wide open to remove the restriction caused by the metal plate. This is also true for gasoline compression checks - this is not a diesel specfic issue.

If you remove the high pressure linjection lines the default condition during cranking is that the injection pump WILL continue to deliver fuel. A vacuum applied to the shut off line connected to to the rear of the injection pump will stop fuel flow. If you have a mighty vac I would recommend applying a vaccum to the line while you are cranking the engine for your compression test. If you don't have a means of applying a vaccum make sure you have some paper towels handy becuase things will get messy. This is another reason why I prefer glow plug testing...

Tim

Good point Bill.
 

My concern with doing the check through the glow plug holes was simply any fuel entering the system via the injectors would get into my hose and possibly guage. I guess though that if you sufficiently wired the throttle at the shutoff point that this would elminiate that possibility as well as applying vacuum to the shutoff valve on the IP.

I actually find that removing the injectors is as esy as acessing the GPs simpl;y because of the tubing. Also gives you a chance to look at the prechambers to see how they are looking but I might be fooling myself to think that I will see anything significant through the injector hole...;)

All he asked about was how to do a compression test..... comparing what you can 'see' in the two holes is not a fair comparison....My answer was directed towards 1. competant results... 2. Fewest tool purchases required 3. least work 4. least potential problems...

Removing the steel fuel lines is easy if they have not previously been rounded off on the corners or warped by someone using vise grips on them.... and if they are not in TENSION from a half assed injection pump setting procedure ..... but for new guy without the proper tools the safest way is the glowplug route and wiring closed the injection pump handle....

I also think taking the injectors and the precombustion chambers out is great policy... too few people do it... but that is a different issue than his question about doing a compression test...particularly since he admitted to being new to this game...

Bill

Yes I bet you could wire the manual stop switch in the off position - I never thought of that approach. Lots simplier than pumping the mightvac like crazy as you crank the engine.

Thanks much

Tim

Leak down test perhaps
 

TxBill- are you speaking of the lever on the fuel linkage that says " STOP"
I thought about that but did not know- I pulled the injector lines Because it was the only way I could be sure that fuel would not get into the tester.

Next time I will go thru the glow plugs just cause I want to take those out to see how the GP's look

I did see lots of oil and when I used the adapter for the injectors it came out everytime WAY oily. I know I am burning oil but how long before I gotta pull her apart

Now - remember I am new to the OE's but not at all timid with the wrenches. So I ask cause I really wanna learn ( from other mistakes I may avoid )

What is the feel of a leak down test- The way I see it this tests condition of the cylinders/ walls/ rings - please correct me cause I really came from the school- " lets open her up and take a look shall we !!!" And I am about to look for a leak down tester
I have a toyota diesel ( Hino engine - subsid of Toyota Mo Co) I am gonna check next

Please and thanks

Peace:

You already have the compression guage... you can make a leak down tester cheap compared to buying it...all you need is a way to put air from an air compressor into the cylinder and a valve to let it out....you put it in to a certain pressure and measure the time it takes to leak down to certain values...just add some 'plumbing ' to your compression guage...

Tim,

You should only need to draw a vacuum once with the Mityvac. If you have to keep trying to hold the main rack back because you keep loosing vacuum you have a torn shut off diaphragm and need to replace it. Your symptoms would be a car that doesn't shut off with the key or is slow to shut off.

Peter

#2 dead -0- reading bad head or valve or what??
 

:confused: ok so compression test done engine cold and I get NO compression from #2 engine seams to run great when hot what does/ could this mean??:confused: