Compression blowby past prechamber/injector - PeachParts Mercedes-Benz Forum

engrx2 · #1 12-28-2002, 02:49 PM

I removed injectors to test them from my 87 300D, but after I installed them back I'm getting compression blow by between the pre-chambers and the injector on cylinder #6. I can feel a very distinct puff of air over injector when the engine is running at idle. I sprayed some diesel in that area and I can clearly see the air leaking from that area.

I replaced the heat shield and also switched the injectors but it did not make any difference.

Any suggestions?

leathermang · #2 12-28-2002, 04:09 PM

Did you make sure ( like brass brush ) the threads were clean ?... was the mating surface absolutely clean ? Did you use a torque wrench on it.. and how many ft/lbs did you use ?

engrx2 · #3 12-28-2002, 05:08 PM

Yes I torque the injectors to 55 ft-lbs.

I removed the injector cleaned and reinstalled it with a new heat shield. Initially it looked like the problem was fixed but after a while it started to leak again.

leathermang · #4 12-28-2002, 05:25 PM

Sorry, that is all I know to ask on that deal...
I have often wondered why they did not put oring seals in the face of things like this... I have seen copper orings used for high pressure sealing like this...and you only have to put the cut in one side of the combination.. it would be pretty easy to chuck up that injector and cut a groove...but don't take my word on that...

#5 12-28-2002, 06:49 PM

Many diesel cars use a steel crush washer near the injector nozzle and a copper washer at the base of the injector threads. I find it hard to believe that Mercedes-Benz doesn't have this also.

per eriksson · #6 12-29-2002, 02:35 PM

Is the steel crush washer in place and oriented the right way?

They must be replaced each time the injector is removed.

JimSmith · #7 12-29-2002, 11:16 PM

I believe a long post on this subject exists from a few months ago, and while I cannot confirm this right now since I am not home, the seal on the injector casing to the prechamber occurs just below the threads. The seal is a metal to metal type of seal (the injector body has a conical seating surface that gets driven into a shoulder of a counterbore in the prechamber), and if the injector is not going in far enough you get the problem. One reason for this could be having two heat shields installed. There is an old style that looks like a top hat in cross section, with the top portion of the hat going into the hole at the bottom of the prechamber, and rim visible when you look into the prechamber with the injector removed. In the last case I seem to recall the new style washer type of heat shield was installed over the old style, causing a stack up of metal that prevented the sealing surfaces on the injector body outside diameter below the threads and the prechamber bore from contacting. The old heat shields are not loose and can be difficult to remove. I have used a tool from Sears with a tapered threaded end on a Phillips head type screwdriver that you drive into the hole, then pull out. I hope this helps, and good luck, Jim

gsxr · #8 12-30-2002, 11:05 AM

First, triple-check the injector seal ("heat shield") installation. The curved side should be facing down, the flat/stepped side goes against the injector. Here's a photo of the shield:

They can be re-used, you don't HAVE to replace them every time. I've done this and had zero leaks. However it doesn't hurt to change them. Make sure if buying new you don't get the older OM617 style, which have a larger center hole and do not seat properly on the injector face (even though WorldPac and some others claim they interchange.)

If that is all correct, check the prechamber seat area. If that is also clean, it is possible - but VERY unlikely - that the leak is between the prechamber and cylinder head mating surface. This normally only happens when prechambers are removed or replaced. Newer engines with the splined p/c lock rings can have the rings crack and leak, but your 1987 won't have these (they use the "pin wrench" to remove the rings.)

Good luck,

a1jatt · #9 04-15-2005, 05:20 PM

Quote:

Originally Posted by JimSmith

I believe a long post on this subject exists from a few months ago, and while I cannot confirm this right now since I am not home, the seal on the injector casing to the prechamber occurs just below the threads. The seal is a metal to metal type of seal (the injector body has a conical seating surface that gets driven into a shoulder of a counterbore in the prechamber), and if the injector is not going in far enough you get the problem. One reason for this could be having two heat shields installed. There is an old style that looks like a top hat in cross section, with the top portion of the hat going into the hole at the bottom of the prechamber, and rim visible when you look into the prechamber with the injector removed. In the last case I seem to recall the new style washer type of heat shield was installed over the old style, causing a stack up of metal that prevented the sealing surfaces on the injector body outside diameter below the threads and the prechamber bore from contacting. The old heat shields are not loose and can be difficult to remove. I have used a tool from Sears with a tapered threaded end on a Phillips head type screwdriver that you drive into the hole, then pull out. I hope this helps, and good luck, Jim

do you think i have the same problem?? i took the shield out and the there was this very smooth oval surface on which this shield rested. i was expecting a hole as big as the injector itself (a threaded part on the PM itself), because many people said they used an old injector housing to pull the injector out.
mine is 89 2.5 (i think 602 motor)

t walgamuth · #10 04-15-2005, 11:22 PM

i seem to be having the same problem with my '90 350sdl. i had a head gasket replaced and valve job.. after getting it back after a few days i noticed the #6 injector leaking. it was pfft pffting around it and leaking fuel and missfiring. they took out the injector and re torqued it but it soon leaked again. so i ordered new injector seals (6). they havent come in yet so i'll have to see about it. i am thinking the guy who assembled it lost or inverted the injector seal... after seeing the dave post above.

i also have another problem. the diesel knock at idle is so loud that it sounds as if something is in the process of breaking up. the car runs fine otherwise and has oodles of power. it also smokes more than i think it should and has used half quart of oil after 1000 miles. it has 142,000 miles. when they disassembled it the intake was jammed with oil gum and was about 85% blocked off at some intakes. can it be the turbo oil seals?

gsxr · #11 04-18-2005, 09:11 PM

Quote:

Originally Posted by t walgamuth

i also have another problem. the diesel knock at idle is so loud that it sounds as if something is in the process of breaking up. the car runs fine otherwise and has oodles of power. it also smokes more than i think it should and has used half quart of oil after 1000 miles. it has 142,000 miles. when they disassembled it the intake was jammed with oil gum and was about 85% blocked off at some intakes. can it be the turbo oil seals?

Hate to be the bearer of bad news, but...

http://mbz.org/info/complaints/350SD/

t walgamuth · #12 04-21-2005, 07:15 PM

in an earlier comment above i noted leaking aroung the injector and loud diesel rattle. it turns out there were no seals at the bottom of the injectors. new seals installed presto... no leaks and very acceptable diesel rattle. it looks as if the mercedes dealer in indy or the specialty mercedes shop that checked out the car for the previous owner may have left them out. not a pleasant thought.