info on rear main seals - PeachParts Mercedes-Benz Forum

JHZR2 · #1 07-22-2002, 12:15 PM

Hi,

Im still trying to learn more about diagnosing problems, and some of the things I see point me to a head gasket problem, as seen in my other thread. However, I have been researching, and understand that on these all iron engines such as in my 83 300D, the head gaskets dont usually go bad. However, I understand that rear main seals can and do go bad, causing a lot of oil leakage onto the ground or even into transmissions.

-A major point of leakage to the ground is dripping off of what Id guess is the transmission. The best way for me to describe it is as follows: From the fromt of the car thgere are three things that drop down far below everything else. The engine oil pan, a round housing that I would assume is the connection between the engine and the transmission, and then another pan a little farther back that Id assume is the trans oil pan. The middle, round section always has oil dripping from it.

Having researched it a little, I understand that the rear main seals go somehow between the back of the engine and the transmission, and a bad one can leak oil into the trans or onto the ground, Id assume dripping down the round piece on its way. If this is so, perhaps my main leakage is actually a rear main seal.

Id liek to know a few things:

-what the rear main seal actually seals/does
-signs of a bad one
-difficulty to replace
-cost to replace

Any help is most appreciated.
Thanks so much for all the help Ive gotten for my various questions.

JMH

leathermang · #2 07-22-2002, 12:48 PM

The rear main seal allows the oil pump to supply oil to the rear main crankshaft bearing without shooting the oil out the rear of the engine. It seals between the radius in the block for the crank on top, the radius in the oil pan on the bottom and to the inside the rotating surface of the crankshaft.

You may be able to take off an inspection plate or a place provided for draining the torque converter to look up into that space to see if you can determine where the oil is coming from.

But first you should attempt to catch some of the fluid dripping from beneath your car on a clean piece of paper... you may be able to tell by the color and texture whether it is trans fluid or engine oil. Trans fluid typically has a red tint to it... and Diesel oil is typically filthy black looking.

I don't know about your model for sure.. but in the past when this was discussed concerning MB diesels I believe it was determined that the rear seal is designed with a locating pin in the upper radius which 'locates' the seal . So the old fashioned capability on some of engines of 'rolling' out the old seal and 'rolling' in a new one from underneath is not an option. The crank must actually be lowered some from block in order to put the upper half of the seal in ...and it may even be a round seal instead of upper and lower halves.

If it had been the rollable type then you could have taken just the oil pan off to replace it. If it had just been the round type you could have just taken the transmission off to reach it from the rear. But with the crank needing to be lowered I suspect that taking out the engine is the most dependable way to address it. I am sure someone will speak up if they have managed to address it while still in the car. But some people on here can set the valves with just straight wrenches and would not dream of owning the spring retainer tool... so some of these things are relative to a person's stamina and other things....of course , if I was going to lower the crank I would certainly look at putting new crank and rod bearings in just as a matter of philosophy.... Greg

kerry · #3 07-22-2002, 01:00 PM

Do a search on oil leaks or rear mains and you will come up with posts indicating that some people have replaced just the lower half of the rear main seal. This can be done without lowering the crank and at least in some cases it solves the problem. There's at least a 50% chance it can solve the problem. I believe it can be replaced by just dropping the oil pan.

#4 07-22-2002, 03:46 PM

I stuck a used 300D turbo in my 81 this spring and after getting it up and running I soon found large puddles of fresh engine oil under the car. The diagnosis was not hard. It was the rear seal. I just climbed under there and wiped the fresh oil away and then drove it a few minutes. Went back under it a the oil was just pouring out again. It will get into the front of the trans where the torque converter is, but that is no worse than going on the floor.
I don't know about changing the lower half of the rear seal. I figured the reason the seal failed was because this engine had sat out and the seal dried up. So I just yanked the engine again and replaced the entire seal.
To do that one is a big job. I recall the shop estimating quide rated it somewhere around 30 hrs. I replaced the front and rear seal, trans seal, and all the bolts to the crank. They are stretch bolts and most, but not all, of the recommendations are to just replace them once the torque is broken.
I was able to remove the timing chain tightener and that put enough slack there to lift the rear of the crank up to get the seal out and the new one in. There are pins holding both halves of the seal. top and bottom in place so forget about trying to slide them out. Also the seal comes as a length. You will need to cut it to fit top and bottom. Be sure to cut it the thickness of a putty knife long so that it will squash together both halves when reassembled.
Just my .03 worth. Have fun. This is a big one.

leathermang · #5 07-22-2002, 05:05 PM

Old Deis, wow ! what a horror story. By telling it you may have saved forum members thousands of dollars and hours... when you already have an engine out ready to install... there are some things which can be "easily" accessed right there and then compared to later on.....great insurance ... and peace of mind....

psfred · #6 07-22-2002, 08:48 PM

Check to make sure you don't have a leaking valve cover gasket before you panic -- if it is leaking at the rear it will run down the back of the block and off the bell housing, mimicing a rear seal leak.

Otherwise, try some engine restorer (Kmart or Walmart) before you replace the rear seal if it is indeed leaking -- the restorer contains a seal swelling agent that can permanently fix the problem.

Peter