Poor Fuel Economy - PeachParts Mercedes-Benz Forum

sublettd · #1 08-15-2002, 08:05 PM

Spent the last month in Las Vegas and as usual was browsing the For Sale ads. Came across an '84 300D Turbo which, to make a long story short, I wound up buying. Reason it was being sold was this 79 year old retired oral surgeon and his wife wanted to buy an SUV to drive when they go skiing!!! I LOVE THAT ATTITUDE!

Drove it to Houston and dropped it off to my son. He was going to buy his mother's 300D Turbo, but I didn't really want to make my 500SL an everyday driver. Thus, the search for another.

The car is white, no rust, original paint still in superb condition, a few small 'door dings', excellent interior, 192,000 miles and worth every bit of the $3,500.00 paid for it. You can't hear the engine in it while driving 90MPH out in West Texas. Literally runs like a champ!!!

Only problem, which probably really isn't a problem, is the mileage I got--22 MPG. Ran a couple bottles of STP diesel treatment through it along the way, but that didn't seem to help any. My 300D (5-spd) gets about 28 MPG and the 300 Turbo gets about 25MPG. This one seemed to run about 400 RPM higher at 75 MPH than the wife's, but it doesn't seem this should be enough to cause the decrease in mileage. I suppose the injectors might need to be rebuilt, but wonder if there are some other things we might try before going to that expense.

Any help will be appreciated.

Also, for those living in the Houston area, my son has an '80 300D that he's looking to get rid of. It's has a little rust, and a poorly shifting transmission that may be vacuum related, but is very driveable. He's had to put a couple glow plug relays in it and can't seem to find out what causes them to keep going bad. TX Bill was going to take a look at it, but I believe he got sidetracked with some of his other projects.

Sorta apologize for the long post, but most everyone likes to brag when they come across a good deal. I'm no exception!

Cheers!

psfred · #2 08-15-2002, 08:13 PM

Don:
Don:

Congrats on the good deal -- better than the $50 I paid for a 280 SE 4.5 last month, since yours runs and mine doesn't!

Check for fuel leaks, specially the return lines, and check for correct tire size -- if the odometer is "slow" you may be getting the correct milage but the odometer is registering less.

Other common causes are clogged air filter, dragging brakes, and poor alignment. The last is more common that I had thought, seems a lot of shops have trouble getting these right. Along with that, check tie rod ends, too, if they are loose you will have excessive toe-out no matter what you do for alignment.

If it is VERY quiet, you may need a new timing chain -- both injection timing and valve timing will be late if the chain is stretched.

A good trip with good fuel and a full bottle of injector cleaner in the tank can do wonders, too!

leathermang · #3 08-15-2002, 08:19 PM

You won't know what is causing that mileage until you do the normal tuneup stuff....set the valves, check the timing chain stretch, set the injection timing , run diesel purge through the injectors,check the compression,check all the fuel filters,.... if you still have that mileage you might check the injector spray pattern or replace the injectors and be sure the precombustion chambers are open and clean... good luck,,

sublettd · #4 08-15-2002, 08:22 PM

psfred,

Still smiling about the deal and my son seems especially pleased!

Believe my son plans to run a filter full of automatic transmission fluid through the injectors this weekend and that might help. It's what my mechanic does when I take mine in for the 15,000 mile service. He swears by it as far as an injector cleaner.

Didn't think about the air filter, but will definitely check that. Brakes could be part of the problem as the front pads need replacing, but didn't notice them dragging. Judging by tire wear, I'd say the alignment is good.

At idle, it sounds like a diesel, but it's certainly quieter at speed than either of my others are. So, hope there's no requirement for a timing chain in the near future.

Appreciate the suggestions.

sublettd · #5 08-15-2002, 08:23 PM

leathermang,

I'm taking notes...writing fast as I can!!! Thanks for the suggestions.

leathermang · #6 08-15-2002, 08:49 PM

#7 08-15-2002, 10:05 PM

My 300d gets about 22.5 mpg no matter what. Valve adjustment, fuel. purge. etc. Chain is in specs, car runs great, no smoke, tons of power, 115+ top speed, etc. I drive above the "legal" speed limit 99% of the time on the highway. Average speed is 80-90 mph. I guess if I was to drive 70 I would see a big difference in average MPG but the car has a "groove" and runs most quiet and seems happy at about 3800 rpm or 87mph.

sublettd · #8 08-15-2002, 10:15 PM

I was driving 80-85 a big part of the way because, as you say, those diesels tend to settle in at a certain speed that they could be driven all day with no strain. So, I realize the mileage would suffer from the higher speed. However, I drive the other Turbo 80-85 on trips and still get about 25 MPG on it. Just thought this was a bit unusual.

Thanks,

psfred · #9 08-16-2002, 06:32 PM

Well, they are all different. I guess I won't gloat then about getting 31-32 mpg at 80 mph or so in the 87 300D.

Peter

sublettd · #10 08-16-2002, 07:41 PM

Peter,

I did that well in my 5-spd--before I added the A/C when I brought it back to the states. The addition knocked me down to about 28 MPG. Would be tickled with that mileage in the turbo, but doubt I'll ever achieve it.

The one I'd really like to improve is the 500SL. About 14 around town and 16-17 on the road. Make up for it in "style" though. ha!

gsxr · #11 08-16-2002, 10:33 PM

Hiya Don,

Congrats on the cool new car! Now:

1- Do all the tune-up stuff & checks that Peter and everyone else mentioned above (filters, brakes, valve adjust, etc.)

2- I wouldn't use ATF in the fuel at all. It can leave ash deposits. It is an old trucker's tale, not good for Mercedes. Either use Lubra-Moly Diesel Purge (straight out of can), and/or one quart of dino 10W-40 oil in a full tank of fuel. And drive the car HARD, that does wonders for cleaning out the combustion chambers. Synthetic oil could help too.

3- Peter is right, I can get 30-32mpg at 75-85mph on my '87 also, but ONLY on good fuel. The stuff out of the pump in CA will drop the mileage by 5%-15%. Now that I'm in Idaho, my mileage has been higher on all 3 cars. The fuel in your area could be part of the problem, I don't know how to tell though...

4- My 1984 has the same problem yours does, terrible MPG. It gets about 19-21 in town and 22-24 on the freeway, regardless of terrain, speed, etc. I've tried almost everything to no avail. Had the injectors pop tested, they were fine. Power is decent but not fantastic (0-60 in 15 seconds at 13psi boost, at sea level). Others have had similar results. The only things I have not done are a compression test, and checked chain stretch. If both of those check out, I'm very heavily leaning toward the injection pump. The IP is the heart of a diesel engine and they do not stay in spec indefinitely. After 150-200kmi they can get tired. My one '87 that gets 30-32mpg only attained that MPG figure after I had the pump calibrated at a Bosch diesel shop. It looks like a real PITA to R&R the pump on the OM617, so I may never mess with it. I'm much more tempted to sell it and find a nice 190D 2.5L auto for the wife...

Good luck, and keep is posted on what happens!

Regards,

sublettd · #12 08-16-2002, 11:00 PM

Dave,

I appreciate your, and all the other, thoughtful responses to the query. Just for clarification, my son in Houston has the car. In fact, he drove it to Baton Rouge on Wednesday to do a job and is heading back home now. He's really impressed with it, and having driven it from Las Vegas I understand how he could be.

He was to have put it in the shop today for a 30,000 mile service, but was unable due to being delayed getting back to the house. So, it will go in the shop on Monday or Tuesday instead. FYI, I've sent him a link to this thread and will be talking to him over the weekend about some of the things suggested. Will get him to post his mechanics comments about the timing chain, injection pump, his thoughts, etc.

Understand what you are saying about running ATF through the injectors. One alternative is Red Line Diesel treatment, which I use regularly in my other diesels. It works very well and I wish Red Line made an injector treatment for gasoline engines. However, I have bought some Lubro Moly injector and valve treatment that I'm using in the SL.

Evidently MB made some design improvements when they switched models, which accounts for the improved mileage over the 123 model. They look a little more aerodynamic and might have had some weight pared off. I expect there were some engine improvements too.

Below is a link to an article on the http://www.mbz.org which someone posted on another thread a few days ago. About half way down the page is a link to timing the injection pump. (For some reason I was unable to get to that particular page today, though I did yesterday.) Doesn't sound too difficult if an individual has some mechanical skills. I tend to be somewhat mechanically challenged myself at times, but I understand what they are talking about. Also, that website's home page links to a LOT of good articles about MB diesel maintenance. I printed out several of them to attempt during my son's visit over Labor Day. He's the genius in the family!!!

http://business.baylor.edu/Richard_Easley/autofaqs/wgate.htm

Thanks again,

leathermang · #13 08-16-2002, 11:32 PM

Here is the direct link to the injection pump timing instructions:
http://business.baylor.edu/Richard_Easley/autofaqs/iptiming.htm

#14 08-17-2002, 03:53 AM

My 300cd tended to get about 26 mpg, but in the winter months it seemed to drop to about 20 mpg. I am hoping to get improved mileage once i get the car back from the shop.

#15 08-17-2002, 10:47 AM

where I buy fuel does make a little difference. +-2 mpg. On the road in the SE I use Flying J's or T&A (ever wonder what that stands for?)

Anyway both of these big truck stop places give me lower MPG than when I buy from a local Exxon down the street.
As for adding tranny fluid, or 10w40 motor oil I stay away from both. When they burn they create ash. In place of either of these I use good ol 2cycle oil. It is made to burn, it does reduce smoke to where it is almost nonexistant even at highway speed. Adding oil is a cheap way to increase cetane level of any diesel fuel. I add about 1/2 bottle to every fillup.