14.5 MPG!!!!!!
 

'79 300SD, great compression, valves adjusted regularly, timing chain within specs. Idle, rack dampner, linkage all correct. No fuel leaks anywhere. Clean air filter. Runs great. Starts easily. BUTTTT....in the last 12 months, the fuel mileage has gone from 25MPG down to 14.5 as of today. It has been a steady decline, 25, 22, 21, 19, 18, 14.5. The car goes really fast. I have used injector cleaner on a regular basis. No smoke, regular oil changes. I drive a combination of city, and highway, nothing different recently.

I suspect that the metering in the fuel pump has failed, or the nozzles of the injectors are worn.

Does anyone have any ideas about what would cause such a progressive loss of fuel economy?

Hey Patsy, er a...Super Turtle,

Are you sure the mileage is dropping off? Have you paid close attention to the odometer to ensure it was registering all miles? The mileage on the 280 dropped to between 6-8 MPG, and that just didn't seem right. I started watching the odometer and soon realized it was only logging about 1/2 the miles. The inside temperature of the car seemed to affect the function - if the interior got too warm from sunshine, or the cluster got too warm from the dash lights being on for extended periods, the odometer would stop turning. It would even stop turning if I had the heater on for too long. The speedo would continue to work normally whenever this happened. Could possibly be the same problem on your end.

There are some DIY fixes, if you want to try, do a search on speedo/odo problems. If you don't want to mess with it, you can farm it out. I opted to farm the job out to Palo Alto Speedo ( http://www.paspeedo.com ), and am pleased with the results. They had the speedo head back to me in one week, rebuilt and recalibrated, new parts installed. I also opted to complete the speedo ratio test, to verify calibration.

Check your EGR valve and intake crossover pipe for blockage. Also an out of alignment condition will cause a gradual loss of fuel economy.

Howdy Mike, Did you know you can't put smilies in a topic? :o I just found out.

I have repaired the odometer, and it is turning normally. I wish it was that, and I appreciate the input. I have a really normal driving schedule, and I just filled up a few days ago, and drove to only a few places, and ended up at a quarter of a tank. I thought maybe the fuel gauge had gone crazy, so I filled up again and was 6 gallons lower. I normally can drive "forever" before the needle even moves. I think my car is drinking too much. My injectors seem loud too.

Do I have an EGR valve??? I don't think they started using an EGR system until 1980.

I will check my switchover valve. That is a good suggestion, perhaps it has failed. I had a fuse blow recently, after starting the car, maybe there is a connection. Would a failed switchover valve cause the injection pump to deliver too much fuel?

I am out of alignment, even though I have had two alignments in the last 2 years. It is so hard to find a good alignment shop for a Mercedes.



Patsy, seventies disco queen, woops, I mean "savin' those seventies beauties".

[Edited by '79 Super Turtle on 03-04-2001 at 10:07 AM]

blown fuse causes mystery thirst
 

Well, I think I know why I went from 18mpg to 14, but I don't know why I am at 18 from 25. I guess maybe I could use some rebuilt injectors, or a little fuel pump rebuild.

The cause of the 14mpg recently was the blown #4 fuse. I drove my brother on some errands, and every time I used my blinker the interior light flashed, and the gauges went dead, all kinds of gremlins occurred, but no noticeable change in driving or power. I noticed the car was really thirsty at the same time. I found the blown fuse which fixed the gremlin problem, and today I realized the overload protection on the transmission is also on fuse #4. So I guess I was having a free for all on fuel from the changeover valve. Thanks for the help Mike and oldsouth.

Brakes????
 

The fuel economy on our 240D had dropped about 5mpg. There wasn't a noticeable change in power/acceleration. "Luckily", before I could begin my investigation under the hood, my wife got a flat tire one evening. When I jacked the car up to change the tire, I could barley turn the front passenger side wheel. I replaced both front calipers .... problem solved.

Thanks Imul, I have a suspicion you may be right. I have noticed a grabbing lately. I will check that too, and thanks for the idea!

Not that this is your problem, but when my thermostat went "bad" and wouldn't let the engine warm up beyond 60 deg C, my mileage dropped similarly, from 19 to 15 mpg.

If your calipers are binding, you should notice some heat buildup on the rotors, I would think. If one is hotter than the other, for sure this is what's going on.

Just wonder if a plugged or pinched off return line would have any effect. Probably not but a brake or two dragging quite a bit would do it as suggested. One good test would be to check each wheel to see if it is warm or hotter than the others after a jaunt. If all are equal temperature they are probably ok. The other thing is you mention car is really fast. That in itself lessons the likelyhood of a brake problem. The simplest thing that can and does radicallly affect milage is a really dirty air cleaner filter. easy to check out as well.

fuel leak perhaps? I had one of those. I think it was the main line coming out of the tank.

'79 300SD is a pre-egr engine.....trust me I have two in my driveway.

You can get a 10-15% drop in fuel economy with winterised diesel versus regular #2.

My last tankfull took a huge dump from the previous...but I think I didn't manage to burp the tank due to a slope at that gas station so what was "FULL" really wasn't full..(if the pass side it not the high side you can easily have a bubble that would displace 3 gallons)..will see how far I get on this one. Found a local station that just dropped to $2.51 a gallon since tuesday...some stations are still at $2.99 a gallon here.

I paid 2.35 a gallon up at the truck stop....in town Shell is 2.85:eek: :eek:

Guys this thread is over four years old.