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  #1  
Old 07-29-2003, 03:37 AM
Cazzzidy
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Compression Test (the saga continues...)

Hey guys,

I am the guy with the smoking 220D.

I was told by my mechanic that I most likely had a bad injector, because the car was squirting unburned diesel smoke. He thought that it might be stuck open, thus injecting on all cycles.

His theory was wrong though, because I systematically replaced every injector and found no improvement.

Thanks to helpful members of this forum, I began to think about the motor assembly itself. I was idling the car, and began touching the injectors. Sure enough, as predicted by a distant uncle, one of the injectors wasn't heating up. The #4 cylinder doesnt produce a complete combustion as far as I can tell, because the injector never heats up and the line doesn't "hit" like the other combusting cylinder's diesel lines. Damnit.

This means one of two things: either #4 has lost compression, for one reason or another, or the injector pump is not providing the correct fuel pressures to that injector at the appropriate time.

Before diving in to the injector pump, I am going to investigate the cylinder and head by running a compression test. I need help though ... what adapter size should I get for a diesel compression tester? Lots of companies offer the compression tester for $90 or so, and various adaptors for extra monet. What # adpator works with my injector size? Any good model compression tester I should look for?

After I test compression, I will repair as needed. I suspect a bad valve. Could it be that the valve is just waaayyy far out of adjustment?

Any advice on the pump? Do these fail regurally? Any common issues/fixes?

Thanks for any more help guys!

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  #2  
Old 07-29-2003, 10:38 AM
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Join Date: May 2002
Location: Hayward, CA
Posts: 45
You should always check the valve adjustment first, as the engine can't run smoothly without consistant compression. The pump can also cause shake at idle. The part you should be interested in is inside (under) the fitting where the injector line connects to the pump. It's the cone shapped thing next to the spring. (If your checking the pump timming, the service manual outlines removing this part from #1 to connect a pipe) This part holds residual pressure on the fuel line between times of injection. Sometimes that valve/seat gets dirty or damaged and allows the pressure to leak back. Sometimes one can clean it and correct the shake. Whatever you do in this area, don't let any dirt (anything but clean fuel) get inside.
If #4 injector line doesn't have the pulse feel of the others, this valve could be worth looking at.
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  #3  
Old 07-29-2003, 12:37 PM
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Join Date: Jan 2000
Location: PA
Posts: 5,440
Cazzzidy,

I would first reverse injectors 3 and 4 and see it the problem follows the potentially bad injector. This will only eliminate a bad injector and if there is no change, the problem is the comprssion of the engine or the IP.

Usually if its the IP, its a defeincy of fuel, not too much that would cause black smoke.

P E H
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  #4  
Old 07-29-2003, 02:55 PM
Cazzzidy
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Thanks!

Mickey,

That is a great tip. I will check it out today.

Haiges,

That is what I have done. So far I have only narrowed down that the injectors are NOT bad. You and I think the same buddy!

Anyway, I would still like to know the adaptor size I need to use for a compression tester? Does any one know the rated size for our injectors?
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  #5  
Old 07-29-2003, 09:55 PM
Cazzzidy
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Okay guys,

I just borrowed a compression tester from my local mechanic (who has had a lot of business from me ). I ran the test on all my cylinders, paying special attention to number 4... and got the following readings (averaged)

Cyl #1 = 300
Cyl #2 = 330
Cyl #3 = 320
Cyl #4 = 300

That figures to about 10% maximum difference between cylinders ... which is just barely within spec. I think its pretty good for a 31 year old motor, personally.

So now that I know my compression is good, I have ruled out the possibility of damaged piston, burnt exhaust valve or cracked head gasket. I can start to investigate the fuel pressure and timing for #4. Tonight I will pull the valve cover and check the timing chain. .. but first I might swap in my front W123 brakes.

More to come! And as always, any advice is welcomed!
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  #6  
Old 07-30-2003, 12:17 AM
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Join Date: Jan 2000
Location: PA
Posts: 5,440
Cazzzidy,

You can't check the timing on cylinder #4. There are no timing marks to check #4. If timing is correct for cylinder #1, it will be OK for all.

P E H
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  #7  
Old 07-30-2003, 11:45 PM
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Join Date: Mar 2002
Location: northeastern PA
Posts: 437
Hey Cazzzidy,
Could you please describe this method a little more in depth? Sound like what I need to do!


"Thanks to helpful members of this forum, I began to think about the motor assembly itself. I was idling the car, and began touching the injectors. Sure enough, as predicted by a distant uncle, one of the injectors wasn't heating up. The #4 cylinder doesnt produce a complete combustion as far as I can tell, because the injector never heats up and the line doesn't "hit" like the other combusting cylinder's diesel lines. Damnit."

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Paul
2004 E500 4matic; 72,000mi
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