I recently purchased a 92 w140 300sd at an auction. I was expecting the worst and Yes i know these arent the most dependable mercedes diesels.having said that, When i got the car there were a bunch of issues. the first major issue was the vacuum lines had been redone with cheap tubing and cheap connectors and on top of that almost all the vacuum lines were hooked up incorrectly! the second issue was the injectors, 2 of them were completely seized up,2 of them were peeing into the cylinders and not building pressure and the other two had bad spray patterns.
I replaced the vacuum lines and rerouted the lines back to the proper locations.
then I rebuilt all the injectors with Monark brand nozzles and balanced them all with a pop tester.I purged the fuel injection system with lubro moly and then replaced the filters before rebiulding the injectors.

The engine ran much better and smoother after the repairs but I was still getting a lot of smoke , mostly at acceleration from a light and after idling for a while and a little injector nailing from one of the cylinders.

I loosened the injector lines one at a time and found that the 3rd injector from the crank pully only has 200psi while all the others are at 400 or higher.

Im wondering if this could be an issue with the head, valves or headgasket?
i hope it isnt the rings!

Does anybody have any ideas or tests that I can perform to determine the cause of the low compression?

If i had a peeing injector a craked piston or burned.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

I am puzzled by the above statement. It seems you are stating the cylinder compression, but how did you get the number from loosening injector lines? There's got to be something I am missing. Can you elaborate ?

Search this forum for "rod bender".
http://www.peachparts.com/shopforum/diesel-discussion/217243-rod-bender-true-stories-how-broke-why.html
http://www.peachparts.com/shopforum/diesel-discussion/205236-3-5-liter-rod-bender.html
http://www.peachparts.com/shopforum/diesel-discussion/295946-sd-rodbender-facts.html
http://www.peachparts.com/shopforum/diesel-discussion/293946-rod-bender-question.html
http://www.peachparts.com/shopforum/diesel-discussion/167207-3-5-bent-rod.html

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Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO

Is the low compression cylinder one of the ones that had a bad injector? I'd also suspect major damage from the failed injector

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

It does sound like it could be a typical bent rod situation or at least that is going to require serious consideration. The issues on these engines are not usually easy. There are exceptions I suppose but not enough of them.

X2

It sounds like a bent rod, the results are what you would expect for a bent rod, these motors are known for rod bending.
Guess what you have?

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

You can try a leakdown test but it'll only point to having to pull the head. There's a slim chance you have a bad cam lobe or lifter or some such issue you can resolve without pulling the head. Pull the valve cover and have a look, then pull the cam (mind the sequence!) and push down on each lifter.

Since no one asked, where did you get the Monarks?

The sad reality of these engines is that even if the low compression is most likely a head gasket problem, you'll find that it has bent rods when you pull the head. But even with bent rods they are very dependable.

Orangevale? I'm going that way this weekend. Maybe it'll be white over gray with a for sale sign when I go by

Sixto
87 300D

I'm assuming by 200lbs you are talking compression in cylinder 3? If so, Sixto is right. Pull the head, and I think you will find that #3 does not rise above the bore- meaning a bent rod. I just went through this, beating my own head against the wall untill I pulled the cylinder head.

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Chief

1991 350SD

I'm not selling it and have began to take it apart.
The car has 264,000 miles , doesn't lose coolant or burn oil. The smoke is not blue and doesn't smell like oil at all. It smells like a very rich fuel mixture.
Im not saying its not a bent rod but I think it would have happened by now and everywhere I've read states that most of the " rod bending" issues happen before 100k miles or so.
I am looking for a used motor or long block ,if anybody has one for sale please don't be shy.

I didn't mention how I determined the compression in cylinder 3, I removed all the glow plugs and tested each cylinder.
Before doing that I loosened each injector line one at a time to find the nailing cylinder/injector and it was the 3rd one from the crank pulley.

sounds like you are on the right track. The general consensus is to redo it as a 3.0 liter for best reliability.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.