Terrible Mileage with '85 300SD - PeachParts Mercedes-Benz Forum

taxman · #1 03-06-2008, 04:10 PM

Hi folks,

I've got an '85 300SD which I have been getting pretty poor mileage in for sometime. I had the front suspension and transmission rebuilt recently as both were very bad and I thought they may be having adverse effects on the mileage. It is still about the same, 17-18 mpg. When I first acquired the car it was around 22 mpg. It currently has around 225,000, and was around 206,000 when I got it.

I do make quite a few short trips around town, and tend to have a heavy foot, but I make sure to do about 40 minutes of highway driving per week. The valves were adjusted around 8000 miles ago. Also, the ALDA screw has been tampered with as the cover is missing. Sorry for the long post, but I really am not sure where to start in trying to fix my problem.

engatwork · #2 03-06-2008, 04:14 PM

I'd start out by looking for leaks. Has the IP timing been checked lately?

taxman · #3 03-06-2008, 04:43 PM

Pretty sure there are no leaks. The lines from the tank were leaking a couple years back and replaced those, as well as the cloth wrapped lines. I dont ever smell raw diesel or see any puddles, so I'm fairly certain it isnt a leak.

As for the IP timing, I'm not sure how this is done. I am pretty good with DIY stuff, but I can't find a very clear explanation of what I should be doing or how exactly to go about it. Thanks for the help

pawoSD · #4 03-06-2008, 04:55 PM

I'd suspect a fuel leak somewhere.

How does it run? Any excessive black smoke?

taxman · #5 03-06-2008, 06:05 PM

It doesn't smoke at all when warmed up and at operating tempreture. It can be hard to start if it sits for a day or more, and i have to give it some throttle to keep it from stalling immediately after cranking due to a rough idle and what sounds almost like a miss, but after i give it a little throttle it smooths out immediately. During that short process there is a little smoke which smells very "diesely." I have never been able to diagnose the starting condition either, would it be connected with the poor mileage?

barry123400 · #6 03-06-2008, 07:03 PM

I might in your situation do a compression check and have the injectors checked out. That might lead you somewhere if not the actual problem.
Prior to that I would check for any dragging calipers, slipping odometer is very common as well remember so might also be a factor. Plus an extra hard look for any possible fuel leaks.

vstech · #7 03-06-2008, 08:17 PM

the starting scenario you mention sounds like several Glow plugs have failed. start there.
this will not effect starting, but you could be running on 4 cylinders and not know it.
next I would verify the odometer is spot on. drive it on the highway and note the mile markers over at least 20 miles. check the trip odo as well.
the 85's have a specific odometer unlike any other year. so if yours is not original you could be looking at amazing mileage or horrible mileage depending on where the odo came from.
check back.

t walgamuth · #8 03-06-2008, 09:05 PM

I would check the odometer.

Tom W

daw_two · #9 03-06-2008, 10:57 PM

Quote:

Originally Posted by taxman

Pretty sure there are no leaks. The lines from the tank were leaking a couple years back and replaced those, as well as the cloth wrapped lines. I dont ever smell raw diesel or see any puddles, so I'm fairly certain it isnt a leak.

As for the IP timing, I'm not sure how this is done. I am pretty good with DIY stuff, but I can't find a very clear explanation of what I should be doing or how exactly to go about it. Thanks for the help

Hi taxman.
The first thing to do this weekend would be to see if we can meet up. You LIVE in Memphis!!!! WOW!!!! Anyway, maybe I can help. I'm thinking odometer myself.

argus445 · #10 03-06-2008, 11:00 PM

Speaking of anyone else have one that's about 5mph off? every speed trap display says i'm doing about 5mph faster than my speedo is showing. Yet everytime i try and check by the mile marker it reads pretty close to correct. Is this pretty common i've heard there not at all accurate from day 1?

theo3000 · #11 03-06-2008, 11:24 PM

Quote:

Originally Posted by argus445

Speaking of anyone else have one that's about 5mph off? every speed trap display says i'm doing about 5mph faster than my speedo is showing. Yet everytime i try and check by the mile marker it reads pretty close to correct. Is this pretty common i've heard there not at all accurate from day 1?

My speedo is off in the opposite direction (it says I'm going faster than my GPS says), but the odo is correct.

taxman · #12 03-07-2008, 10:43 AM

I forgot to mention earlier that I replaced all the glowplugs in October with new Bosch. It seems to be a pretty clear consensus that the odometer might need some help, I guess I will check highway signs and see if there are and discrepancies. Thanks for the help!

By the way, any thoughts on the cold start issues?

junqueyardjim · #13 03-07-2008, 11:08 AM

I think a lot of of your rather poor mileage figures is due to your driving. I have an 85 300D which is a smaller and lighter vehicle. I run it really just at 45 miles a day. I can save a few miles by taking the by pass around town with a number of fast starts, (if I want to keep up with the traffic) If I do that 4 times a day at 11 miles per run, my mileage drops to 18mpg. However if I drive around the west side, through the gravel pits. ect.,I get only two stop lights, and two long runs, one of 4 miles and one of 5 miles with no stops and a legal speed of 55mph. Running that route, is a mile further, but my mileage goes up to 23 mpg. No doubt about it, if you drive with your foot in it and do a lot of short runs, stop and go, you won't get any mileage out of these vintage Benz Diesels. They weren't built for that type of driving. For that you need a 240D.

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