Compression bad in 2 cylinders-How long will it last? - PeachParts Mercedes-Benz Forum

gawlbpd · #1 05-08-2003, 10:30 AM

My 83 300SD was abused before I got her. The only thing that was done was that the oil was changed religiously. She doesn't smoke at all, but idles more than a bit rough. The compression test tells it all. 380/380/260/260/340 are the numbers. (I found out that the valves were not adjusted for at least 11 years.)

Do any of you think it will last for a while, or is this compression variance going to fry a bearing? What's the typical failure mode and time period?

This car still gets 27 mpg, and runs like a champ at freeway speeds, but at a stop light, it's a shaker. If I pop it into neutral, it smooths out the idle considerably, and even more so when I bump up the rpm's. It's idling at about 800 rpm's right now.

Can I bump up the idle in an attempt to smooth it out a bit, or does that push other problem areas into the equation?

JimSmith · #2 05-08-2003, 11:01 AM

gawlbpd,

How many miles on the car, and how bad were the valves when you adjusted them? Especially on the two bad cylinders? Does the car use oil? Was your compression check done with the engine hot or cold? Did you do anything to try to isolate the problem in the two low cylinders to valve/head leakage or cylinder to piston interface leakage?

A little more data will go a long way. Good luck, Jim

gawlbpd · #3 05-08-2003, 11:11 AM

I've got 172K on it. The valves were "tight all the way around, but not bad," according to the mech. I wasn't smart enough at the time to ask more questions. I had them done shortly after I got the car and I had done some research. (I couldn't do them then, but I could now.)

The car does not use any oil. Since it idles relatively well when cold, but now wel when warm, I had the mech do the compression check done with the engine hot. As far as isolating the problem, I did not have them do one of those leak-down tests.

JimSmith · #4 05-08-2003, 11:41 AM

gawlbpd,

I am sure others will pipe up with their opinions, but this does not sound bad enough for me to dig through a bunch of stuff that is not broken to get to something that is likely pretty minor. 260 psi is not a sign there is a big problem with metal part tolerances. The biggest issue is probably the variation from cylinder to cylinder. It is not clear to me that this will result in anything but rougher running on these cars. Someone else with more experience may offer a more fact based assessment on this though.

I would not expect, from the "uses no oil" statement, that the problem lies with the cylinder to piston interface, but if this bothers you I would go ahead and have the leak down test (or, at my local dealership, the shop does wet tests on Diesels) to get an idea of where the problem lies. If you know where the problem is, it may be a little clearer what the consequences, if any, of continued operation might be.

Good luck and I hope this helps, or at least stimulates some other input. Jim

gawlbpd · #5 05-08-2003, 11:51 AM

Thanks Jim,

This gives me a pretty good idea which way I want to go with this car/project. The body on it is absolutely perfect, but the paint is shot, as is the drivers seat. I'm going to keep it going since it has such relatively low mileage, and I'll nickel and dime it back into shape.

I paid virtually nothing for it, but I don't have the skills, yet, to make the repairs. Although I have done the belts, hoses, instrument panel lighting, and door locks. I appreciate folks like you and Thomas Pindelski(?) (Thomaspin) with all the information.

I've had a Dodge (Cummins) pickup for 6 years and just started in on the Mercedes thing in February of this year. I have picked up a ton of knowledge and plan to continue learning and repairing as I go along.

Greg

dmorrison · #6 05-08-2003, 11:57 AM

Real world experience.
My wifes 300SDL had 17 atm on the #1 cylinder. I decided to get a new car, 99 E300. We decided to keep the 300SDL for the kids to drive. That was 4 years ago and 25,000 miles. the car just keeps running. I can't kill it.
I'm sure that the car will run. Will you ruin anything, Its hard to say. Motor mounts and shocks will get a workout.
This all depends on what you want out of the car. The SDL has held on for 4 years with only oil, filter and $50 worth of repairs.
When we bought the car we discovered an injector noise that wound up being the #1 cylinder low compression. Mercedes replaced the #1 piston, $2200. And we drove the car form 93-99.

Dave

gawlbpd · #7 05-08-2003, 12:07 PM

Yes, I expect to replace the motor mounts on a somewhat regular basis. That was the very first thing I had to replace as both were history. Let me tell you about how one of these 5 cylinders can rock-and-roll when both are shot. WOW!

What's "ATM"?

#8 05-08-2003, 12:13 PM

You may find that with regular and frequent oil changes the compression improves. IF the rings are varnished that may free them a little.
But don't bet the Farm on it.

jbaj007 · #9 05-08-2003, 12:24 PM

It could be as simple as "stuck", carboned, rings in those two cylinders, due to a lot of low speed driving over the last coupla' years. There are many here who subscribe to the "Italian Tune-up" school of thought when dealing with this exact problem.

Some on the mbz.org diesel mailing list believe a compression test on our diesels should only be done when the engine is hot, valves adjusted and followed with a leak-down test, if it is to be of any value. FWIW. How does it start when the weather is chilly?

A good cold idle with a poor hot idle is sometimes more related to injector nozzles being dirty/worn.

I've never personally seen, nor had a reliable close first-hand report ,of the lower end of a 617.951 going south under anything resembling normal usage, so spinning a bearing wouldn't be high on my list of worries.

engatwork · #10 05-08-2003, 12:28 PM

It will run a long time like this. If you don't want to spend the $ for the rebuild the determining factor will be when you can't keep oil in it.

leathermang · #11 05-08-2003, 12:40 PM

Before I got to this forum I was the same way about trying to anticipate and fix stuff... these penny pinchers have turned me around.... Do what would be considered a tuneup.. valves setting, change out ALL the fluids, run injector cleaner through it, be sure the setting for making the idle as smooth as possible are tweaked... and just keep on driving it till it dies....
Your mileage figure is great... my 240 supposedly has a low reading cylinder and I get 28 mpg out on the road ( but I am using one qt oil per 50 miles )... so something is wrong but it just sails along whereever I want to go..
So yours really does not sound bad at all...
Drive and enjoy great workmanship and design... and start reading all the threads concerning the items you want to address first...
That is my opinion....

gawlbpd · #12 05-08-2003, 01:28 PM

Yeah, no doubt about it. I'm driving it 'til it dies. It starts instantaneously, and it only leaks a little bit of oil. It's a keeper.

rwthomas1 · #13 05-08-2003, 02:51 PM

Its possible the valves/seats were a little carboned up from being too tight and may seat in after a while. As mentioned, the rings may be a little sticky too. I would run her for a few thousand miles and then check the valves again. You may find them tight again. Try some diesel purge to smooth the idle. Maybe try adjusting the rack damper bolt. Some have had great luck with it. If it starts in the cold, doesn't use any oil and everything else is up to snuff just drive the snot out of it and quit worrying. RT

Fisherman · #14 05-09-2003, 10:04 AM

You know the rough idle thing got me to thinking about the infamous rack dampener. I am unsure if your year had it but you nmight do a search on this and see what comes up.
I'm not certain that this would make any differnece since the roughness seems to increase as the engine heats up but this is a cheap fix for soem on fixing rough idle...

Maybe someone with more knowledge than I will jump in here and save me from giving you bad advice.

leathermang · #15 05-09-2003, 10:16 AM

And you can plug your car info into these sites to see if some tech service bulletins have been issued.. I know for some cars they have been related to rough idle...

http://www.nhtsa.dot.gov/cars/

http://www.alldata.com/recalls/index.html

This gives you the title of the tsp or safety recall... then you can get the actual full document from MB or get a subscription to Alldata ( about $15 per car per year )...

Or someone on the forum might look up and email the info if you can quote the tsb # you are interested in...