1985 300D engine rebuild
 

It looks like the engine in my 300D is starting to get pretty tired. A few months ago I pulled the cyl head and had new guides and seals installed. Now it looks like doing that work has brought out the fact that the lower end is starting to get tired. It runs real rough when cold and shakes pretty bad at idle. If you do not give it fuel when cold it shuts off. It does run out good on the highway and you can't tell anything is wrong with it except the heavy smoke on full throttle acceleration. I tested compression on all five cyl this evening and found #5 only got up to around 160 psi. 1-4 were over 200. Also, I put #5 at TDC and put compressed air to it and it pressurizes the crankcase. I am trying to decide whether to do a rebuild as a winter project or just part it out. I figure new sleeves, pistons, and crank bearings would get it back to where it needs to be. I kinda hate the thought of parting it out because it rides/drives pretty doggone good. What do ya'll think?

Don't part it out !! Fix it. I have been working on my 83 Turbo tonight, I had a front end crunch. New fender ,headlight support, Headlight assembly , Hood and the MISC. When I get her back
together she will be fine. I drove my latest 240D rebuild to work today and she purrs like a kitten. I just can't see parting out a fixable MB.

MBJOE :D

Jim,

Not that it should make any difference in your decision making process, but I look forward to reading your account of the process, from the identification of the issues, to matching parts and then getting it running perfectly again. So, I hope you do decide to fix the car, and Good luck, Jim

are you sure the valves are adjusted properly.might be worth pulling valve cover and cking clearances.

I have adjusted the valves at least 4 times LOL. I will get going on it over the next few weeks.

If the other cylinders are so good, I hope what you find is just a minimally broken ring on #5. That way you don't have to do cylinder sleeves or even take out the other pistons. I know you like to do complete overhauls, but this may be a case of minimal parts - but a lot of labor.

It sure would be nice to fix the engine without having to strip it down completely and sending it off for a resleeve. Just drop the crank and pull out the #5 piston. That should tell the story.

Ken300D

Uh... I don't think 'over 200 PSI' is very good for a 617 engine - even if 'over' meant 290 PSI. As I recall the minimum acceptable figure was something like 300PSI. The 240D I recently rebuilt had compression numbers between 420-440PSI, before the rebuild all cylinders were between 280-330 PSI.

I love the 616/617 engines but it seems like quite a few seem to start to drink oil at the 200-300K mark. They still seem to run fine but the cylinders start to taper and the don't start very well when cold. On the other hand I've seen engines with 300K that don't use any oil at all.

When you had the head off did you measure the cylinder taper? I bet you will find it out of spec.

Tim

Yea. there is no doubt that the rear 2 cyl were out of spec. The finish was a "mirror" finish. At least I could still see some cross hatching on the front three cylinders. Funny thing about it is that I do not have to add oil between the 3k mile oil changes. 1-4 cylinders were at around 250-260 psi.

If the car's a keeper then rebuild. 250-260psi is getting low and that much pressure mismatch is a bearing killer. If not a keeper then I agree with Ken300D and sell or re-ring #5 to buy time before selling. Strange that the engine doesn't drink oil. Kind of sounds like either a stuck ring or rings that have worked around to where the openings are slightly aligned, or a bad taper in #5. All provide a path when cold. Plugging it in (if you have a block heater) might buy you some time. Have you tried comparing hot and cold compression on #5?

I'll try the hot/cold compression next weekend sometime. It has got to be the severe taper. When it is hot at idle it seems like I can almost hear piston slap.

These engines certainly have minds of their own. :D My 617 runs and starts beautifully, uses very little oil (I think I've put one quart's worth in over the almost 5K miles I have put on the car; granted I need to change the oil; will do that as soon as I get access to my driveway again), and has good compression (400 psi on #'s 1 through 4 and 375 psi on #5). In spite of this, I've got enough blow-by smoke that I almost didn't buy the car.

She gets the job done and I'm not expecting a catastrophic failure, but there's always that nagging doubt (I'll openly admit to being paranoid)...

Jim,

There's no need for sleeves AND pistons. Do one or the other. The pistons are very highly stressed, so I prefer boring and using oversized pistons. Make sure that the piston crowns are the same configuration as the ones you take out.

Since you already have the head in shape, this would not be too expensive. Just strip it, bore it put in the new pistons and maybe bearings. The bearings in my daughters car and the crankshaft were absolutely perfect, so I carefully marked them and put them back exactly as the were. The engine still holds great oil pressure.

Best of luck,

That reminds me...what's the ideal idle oil pressure?

My oil pressure usually is halfway between 1 and 2 bar at hot idle (maybe a little closer to 1 when it's warmer than usual), and it jumps to the peg at around 1200 RPM and stays there through the rest of the RPM range. At cold idle it's pegged at any RPM. :)

I'm guessing that that's okay; just wanted to double-check. Thanks!

Warden - your oil pressure sounds fine.

Okay, cool...just wanted to double-check while it was still kinda fresh on my mind. :)

My truck idles with only 10 to 15 psi, so the 300D's definitely better. :cool: