Hello all,
I have a 1985 300sd with around 215000 miles.

Recently fuel mileage has become extremely poor (about 180 miles to the last tank!), and I have not been able to find any leaks in the system.

The car is very hard keep running after initial start without giving it some pedal right after ignition, at which point the idle smooths out after blowing out a cloud of smoke.

The car also surges on the 2/3 upshift, and sometimes on the 3/4 I think. By surging, I mean the rpms actually increase then decrease quickly several times before the shift actually occurs. Not quite sure at what rpms as the tach is non-functioning. The transmission also has a loud clunk when coming to a stop, or slowing to a very low speed, although this only occurs when the car is in 'D' I have realized. When driving around in '3' there is no clunk. Is this referred to as flaring?

I typically drive pretty short distances during the week and put about 40 highway miles on it every weekend.

I have searched multiple posts but am fairly ignorant when it comes to terminology and parts on Mercedes diesels so please bear with me. Thanks for the help.

By the way, in regards to the mileage problem I am certain the odometer is functioning correctly. Also, a valve adjustment was just done recently. Thanks again

I assume that they checked for chain stretch when they adjusted the valves? How about IP timing?

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

Did the poor performance coincide with the valve adjustment?
It is not too difficult to remove the valve cover and check the clearances even if you are not confident in your ability to do the actual adjustment.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags

The poor performance did not coincide with maintenance. Valve adjustment was over the past summer. I put very few miles on the car, maybe 2000 since then.

The mechanic said the chain was starting toshow a little stretch and would need to be replaced before long, although apparently nothing too bad. As I say I am not familiar with these cars, is a little stretch really OK?

Thanks guys

I think I would pull the injectors and have them checked. You might have one leaking down heavily. Not that expensive to check. If the car has black smoke out the tailpipe at idle might indicate overfuelling. Not a good ideal to just keep driving without checking in my opinion. Also although I do not think it is the problem. Remove the return line from the injection pump and blow some air down the line just to make sure it is open. Remove the fuel cap before doing this though. You have to start somewhere. Will car still run at highway speeds?

I sounds like your bigger problem is the transmission, which may or may not be related to the fuel mileage. Have you had the transmission diagnosed yet? If it is slipping significantly it will not help your fuel mileage. Either way, that is what I would worry about first.

have you checked the fuel filters lately????

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If it ain't broke take it apart and find out why.

1983 300SD, 4 speed
1994 C280
1987 300TD wagon
1996 HD Road King

Ride in Peace Eric Peterson, Harley of Macon

Why I would not drive a lot more is the unknown by me possibility of putting a hole in a piston. The clue for me that you might have an open injector was the cloud of black smoke to clear itself out after starting. Excess fuel is getting in there somehow.
I agree with Craig that you have two problems though. Again I could be wrong about an open injector. I just would not chance it. You can pull and number them yourself and take them for a check. Or take a good one and substitute one cylinder after another until you find the bad one if there. Remember to install new heat shields after the old ones are disturbed. They are not very expensive. These are a do it yourself car in my opinion to make them practical to own.

A lot of good posts.
I would start with the simple things first like:
Check fuel filters
Check air filter
Check glow plugs
Check valve clearances
Check chain stretch
Check injectors

Anything else that you missed? Like does your fuel cap make a sucking sound when you loosen it?
Also it would be nice to know where you are. Go to the USER CP and put your location and vehicle information in your profile. This will allow us to help you better.

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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The hard downshift is almost certainly a vacuum problem. Try turning your climate control off and then come to a stop. If there is no 'clunk', there's a vacuum leak in your climate control system. Obviously, there could be other vacuum leaks, but this is a simple test you can do anytime by just pushing the far right climate control botton.
Is there any other evidence of vacuum problems? Slow shut off? Hard brakes?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I have recently replaced both fuel filters, the smaller clear one still appears to be clear, and the air filter appears to be good as well.

The cloud the car blows out at startup, after hitting the accelerator, is more grayish in color than black, does this still mean fuel leaking into the cylinder? I forgot to mention that the rough idle condition at startup only happens when the car is cold, usually after sitting for more than an hour.

Also, there is a sucking sound sometimes when I unscrew the fuel cap, what does this mean? Also, the car runs great on the interstate just like it always has. Usually run at about 70-75 with no problems at all.

The GP light doesnt indicate any problems ith the GP, its not flashing or anything. Though I understand there could still be problems with them even though the light functions fine, correct?

There are no other signs of vaccum problems, as far as the car shuts off quickly, the doors lock and unlock immediately, and the brakes work fine.

Lastly, are there any good repair manuals that would show me, for instance, how to remove injectors. I am very mechanically inclined, I just like to have an idea of what the process of taking something apart is before I jump in

On a W126, 300SD, the door locks utilize an electric vacuum pump that is in the trunk. They door locks do not rely on the engine vacuum pump as does the W123. For that reason, the operation of the door locks, while comendable is not relevant to finding the problem.

Many people have found a CD version of the Factory Service Manual to be very helpful..Often it is available on eBay.

To those who suggested checking the fuel filters.... Since the complaint was excessive fuel use, why would you suspect the filters? This is not meant as an attack on anyone. I am merely trying to improve my own diagnostic skills. It seems to me that if the complaint were "lack of power" that checking the fuel filters would be the right course of action as clogged fuel filters= less fuel= less power. But if there is excessive fuel use, how can that be related to a filter problem, other than a hole which would be seen as a leak? Please educate me.

Is there a vacuum-related item that would affect both the driveability and the transmission issues? What I mean is, I have seen many posts that stress the importance of the correct vacuum signal for the proper operation of the tranny. If there is some vacuum device that could enrichen the fuel delivery, couldn't it also affect the transmission's shifting? The Alda senses engine manifold pressure, so I think that is out. But what about some EGR devices? Could a vacuum leak in one of these circuts casue both the excessive fuel consumption as well as the erratic shifting ( due to loss of vacuum)?
Just thinking out loud....

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags

If your cloud of black smoke is only when pressing the accelerator on a cold engine, then I would say the injectors might be good. If you constantly get black smoke at idle, especially with a warm engine, then I agree with getting your injectors checked out.....and soon.

From what I have seen in your posts, I believe timing would be more suspect. Timing, from what I've seen and read, has a major effect on fuel economy for diesel engines. In fact, some engines will pour out loads of black smoke with only the slightest timing error. The "little" bit of chain stretch, and how much effect, would depend on how close the timing was set initially.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Thanks for the help guys,
I was looking under the hood for a while to see if there was anything that would catch my eye (not sure what exactly) and noticed that the "U" shaped plastic piece running from the air cleaner to the turbo was loose, loose enough to possibly be pulling in outside air.

I know it is there for a reason, and thus should be tight, but would this be affecting mileage? Also, in trying to tighten it back up I am having problems with one end popping off while trying to get the other on, any trick to this or just trial and error?

SD Blue, there is only a very small amount of gray smoke at idle with the engine warm. As I am not too familiar with diesel engines, are you refering to the IP timing? Also just calculated mileage for about last 3/4 tankfull and it came out to around 15mpg, mostly short trips around town with a little highway.

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1985 300SD
216,000 miles