Poor Fuel economy --> please help troubleshoot - Page 3 - PeachParts Mercedes-Benz Forum

Orv · #31 04-26-2011, 06:35 PM

When I suspect a sticking brake issue I stop the car after a long run at speed, preferably where I haven't been using the brakes much, then go around and feel how warm each wheel hub is. The fronts may be hotter than the rears, but they should be roughly the same left to right. If one is much hotter than that brake is sticking or you have a very bad wheel bearing on that wheel. Be careful, if one of them is sticking badly the hub can be very hot indeed.

I will say I once had a very bad dragging rear caliper on a Volvo, and fixing it didn't change fuel economy more than 1-2 mpg. But you may be dying a death of a thousand cuts here.

kerry · #32 04-26-2011, 06:51 PM

Given that mileage drop, I'd say it's likely that someone is drafting him and splicing into the fuel return line, sucking up a few gallons as they go down the road, kind of like an airplane refueling in mid-air.

Orv · #33 04-26-2011, 08:31 PM

Actually...can we rule out someone stealing fuel from his tank at night?

vstech · #34 04-26-2011, 08:33 PM

My 82 SD used to get 19mpg and ALL that was wrong was a sticking front caliper.

barry123400 · #35 04-26-2011, 10:07 PM

As the timing chain wears the injection pump timing becomes retarded. Easy enough to have a look for really excessive chain stretch. Many articles in the archives.

It would really have to be stretched though and near destruction to make this much of a contribution to the decline. . Also a set of really sick and tired fuel injectors might be present. Something is really wrong or a combination of things to pull you this far down in fuel milage.

You are basically right that a 123 type car with this low milage should not have serious issues. Look for any unexplained major gaps in the service lterature and documentation you have.

The seats at your reported milage should be like new and well suported springwise unless beat by the southern sun and more milage than reported.

Orv · #36 04-26-2011, 10:10 PM

The Volvo may have been a bad example because it got 24 mpg no matter what I did to it. I did a couple hundred miles towing a trailer and it still got 24 mpg.

yemington · #37 04-26-2011, 11:15 PM

Quote:

Originally Posted by kerry

Given that mileage drop, I'd say it's likely that someone is drafting him and splicing into the fuel return line, sucking up a few gallons as they go down the road, kind of like an airplane refueling in mid-air.

LOL.

I subscribe to the death by a thousand cuts hypothesis at this point. But, the drafting theory comes in a close second. This may take a while but I'm going to do one thing at a time, run a tank of gas through, and see what (if any) difference each repair makes. I'm really surprised that I can't find a good estimate for what each of the repairs will do for fuel consumption. Even more surprised there isn't an active modding forum for trying to get more and more mileage out of these cars. Am I missing something?

I have about 70 miles on a tank that I filled right after changing the oil and putting in a new (STP) air cleaner. This weekend I'll hopefully have a new fuel filter to put in. Follow on updates will come after the tank after the new brake lines and the tank after the injection pump timing and so on as I get to timing chain stretch and fuel injectors.

All in all I spent a lot less than I would have on a 10 year old Jetta so even at 15-17mpg I can afford to drive it for a while and still have less invested than getting 45mpg in a Jetta. Plus, I have a peach of an old Daimler Benz to drive around.

barry123400 · #38 04-26-2011, 11:59 PM

Well since you may have missed it a restricted fuel system makes the engine less efficient so it burns more fuel. The reverse of what common sense would seem to indicate superficially.

Lower base fuel pressure in the injection pump because of a restriction like a loaded up fuel filter does make for unequal element filling. This can account for one to three miles per gallon as far as I can tell depending on the severity of the restriction.

Your reported fuel milage drop is a little too much for this alone though. As already stated it may be a combination of things.

From you general description you are down at least five miles per gallon I suspect.

Skippy · #39 04-27-2011, 01:29 AM

Quote:

Originally Posted by yemington

This may take a while but I'm going to do one thing at a time, run a tank of gas through, and see what (if any) difference each repair makes. I'm really surprised that I can't find a good estimate for what each of the repairs will do for fuel consumption. Even more surprised there isn't an active modding forum for trying to get more and more mileage out of these cars. Am I missing something?

Gas?

If you put gas in your diesel car, you will have very big problems very soon.

Skippy · #40 05-30-2011, 05:19 PM

31 MPG in a W123 (on B20!):

Ok, actually it's 30.51, but if you round up, it's thirty one. 478.8 miles on 15.692 gallons. This was in my '83 300D with manual conversion. This tank was mostly highway driving, including several trips to Reno, a trip to NAS Fallon, and a trip to Silver Springs. I was burning B20 from Bently Biofuels.

Just thought I'd share.

barry123400 · #41 05-30-2011, 08:19 PM

After a drive park the car and rev the engine to a little less than half for a minute. Move ahead and look for a fuel leak on the ground.

Are the return fuel lines on the injectors dry? No these cars do not get ebay type advertised milage. Still you should be seeing at least twenty miles per gallon with heavy combined driving.

It could be as you suspect a combination of things. If so it sounds heavy on coincidence. Try to get a zero to sixty timed run in and post it.

An inoperative or partially operating turbo might cause this yet I suspect you would be complaining about the power and some smoke. For sure something is wrong.

warmblood58 · #42 05-31-2011, 05:37 PM

Quote:

Originally Posted by DieselPaul

Look at the air filter, diesels run well with clogged air filters but get bad fuel economy.

Also, are you running any kind of additive? Not a bad idea to run some sort of cleaner though there.

I agree -noticed my mileage was slipping and my air cleaner was soaked with oil (other issue which I repaired) HUGE difference -sometimes the simple things work, check your air filter

layback40 · #43 05-31-2011, 10:19 PM

No one has mentioned the possibility of needing a much stronger throttle return spring!!
Lighter throttle use is the single biggest factor in improving fuel mileage!!

t walgamuth · #44 06-01-2011, 01:03 AM

So how many gallons is it taking on a fill up? Are you using the same station every time to fill? Maybe their gallon measure is off.

Really this is not adding up. The only real possibility for that lousy of mileage is sticking brakes or a mis measure somewhere. If the tires were hugely oversize it would throw off the measurement too. (You're not driving with both doors open are you?)

I was going to suggest you have a 369 diff from a 240d in the car but the rpm at highway speeds sounds ok.

charmalu · #45 06-01-2011, 01:18 PM

As mentioned above, you should have a 3:07 Differential. I didn`t read a confirmation of what you do have.

The Diff is stamped on the lower left corner of the case, facing the rear. scrape off the dirt and grease. the last 3 numbers will be your gear ratio.

A manual conversion as Skippy mentioned he has, will increase the MPG by a good 3 mpg.

Charlie