Poor fuel economy 1987 300TDT (W124) w/ OM603? - PeachParts Mercedes-Benz Forum

swogee · #1 03-10-2008, 05:02 PM

I have a 1987 300TDT that has only been getting about 19-20MPG in combined cycle (city 50%/highway 50%) and about 25 MPG on purely freeway driving. I'm trying to determine the cause of the poor fuel economy.

I have already checked the odometer and it is functioning correctly, and it doesn't slip.

I have also verified that none of the hoses/seals are leaking. The rubber hoses between the tank and hard lines and between the the hard lines and the fuel/filter and lift pump have also been all replaced. Most recently the return lines between the injectors have been replaced. The delivery valve seals appear to be good as there is no fuel leaking on the top of the injector pump.

The air filter was replaced a couple years ago, and since then I have only put about 5K miles on the car. The fuel economy did not improve after it was replaced.

I have narrowed the cause of the poor fuel economy to one of two things. The ALDA adjustment is off. I replaced the ALDA because the seal was bad. The tamper proof can is gone from the top of the unit. I am speculating that perhaps the ALDA is adjusted too rich. I was thinking of trying to turn the adjustment screw CW some to lean it out some. The other possible cause of the poor fuel economy may be worn fuel injectors. The 300TDT has 190K miles on it and the injectors are original. I was thinking of have the injectors rebuilt and tested by the local Bosch service center.

Does anyone have any other thoughts or ideas as to what may be causing the poor fuel economy?

Thanks in advance.

-Steve

#2 03-10-2008, 05:04 PM

Quote:

Originally Posted by swogee

I was thinking of trying to turn the adjustment screw CCW some to lean it out some.

CCW enriches the setting.

swogee · #3 03-10-2008, 05:11 PM

Thanks for the correction on the ALDA adjustment. Its been a while since I last adjusted it.

-Steve

dieseldiehard · #4 03-10-2008, 06:50 PM

First point to consider - Does the car still have the Nasty Trap Oxidizer on it? (I sure hope not!) very bad for good mileage

Second thought, sometimes the Catalytic device (MB replaced the NTO with the Cat thing on the exhaust line under the car) and if it gets plugged, usually from frequent short drives without getting warmed up good, that will reduce the mileage and performance very significantly.

Smell the exhaust (not right up close, just take a whiff) and if it smells of fuel (raw fuel - a nose with experience will help!) then its running rich for some reason and you need to look at the injection system, including the ALDA. Usually if the ALDA is cranked out too much the exhaust blows dark grey smoke on heavy acceleration.

Lastly maybe the timing chain is stretched? a timing check will verify if that's the case. I know it made a huge improvement in mileage when I replaced mine.
You ought to get plenty of acceleration and 25 MPG minimum in the '87 turbodiesel. 29 MPG is about the best you will probably ever see (a littel less maybe due the the wagon's extra weight) if you can't get 24 MPG you have a problem, 19 or 20 MPG is a real problem worth finding and fixing

babymog · #5 03-10-2008, 08:51 PM

The sedan will get mid-20s around town, 30-34 on the highway depending on how you drive.

If it is an ALDA problem, it wouldn't affect highway driving, primarily off-boost.

Timing is a strong possibility, can affect power and mileage, also usually makes it start hard.

Injectors usually start to affect idle quality as they age, if it idles smoothly and without smoke (warm) it can still be injector nozzles but less likely.

Smoke, smoke smoke. Is there smoke? Accelerating? Idling? normal driving? If so, what color? If you aren't generating smoke, many things are less likely and even eliminated.

What about power? Does it get up and go or is it sluggish? How about the top-end of the tachometer, does it pull hard to the shift-points/max. RPM or seem to run out of steam.

How about tire wear? A dragging brake is significant drag, and significantly mis-aligned wheels can drop fuel mileage significantly.

Last is the load. If you carry a lot of weight around, and keep the roof rack crossbars on or a bike rack on the roof, you will lose some fuel mileage. The roof-rack on my quattro takes 15mph from my top-speed and you can really feel it, my wagon's cross-bars make a difference also, especially if in the wrong places.

Brian Carlton · #6 03-10-2008, 10:06 PM

Quote:

Originally Posted by swogee

I have narrowed the cause of the poor fuel economy to one of two things. The ALDA adjustment is off. I replaced the ALDA because the seal was bad. The tamper proof can is gone from the top of the unit. I am speculating that perhaps the ALDA is adjusted too rich. I was thinking of trying to turn the adjustment screw CW some to lean it out some. The other possible cause of the poor fuel economy may be worn fuel injectors. The 300TDT has 190K miles on it and the injectors are original. I was thinking of have the injectors rebuilt and tested by the local Bosch service center.

The ALDA is not the problem. If the ALDA is completely removed and you drive it hard below boost levels..........you'll get smoke and the fuel economy will drop. In all other driving modes, the engine has more air than it can use, and, therefore, adjustment of the ALDA will not yield any fuel economy improvement.

The injectors are a possibility if one or more have a very poor spray pattern.........but, this usually results in poor idle quality. However, I don't see more than a 10% improvement under the best possible circumstances.

What about IP timing and/or camshaft timing? Very poor economy results from late timing on either or both.

What's the 0-60 time and the boost level? This would give us a measure of the performance of the vehicle. If the performance is down but it has sufficient boost, you might have issues with a partially clogged exhaust. Wouldn't be the first time with one of these.............

swogee · #7 03-11-2008, 12:26 AM

Thanks for all of the posts and ideas so far. I'll answer the questions as best as I can.

The trap oxidizer recall campaign has been done on the exhaust system according to the local dealer. The football shaped object is not present under the hood. I would suspect that there is a catalytic converter since the trap oxidizer is gone.

I have not done a timing chain check or an injection timing check. It might be a possibility that the timing is off because of chain stretch. I don't know about the injection timing either.

There is only smoke present when flooring the accelerator. Under normal driving conditions, there is no smoke.

At idle the exhaust doesn't smell excessively rich. There is a very faint odor of diesel fumes only. The engine doesn't smoke at idle.

There is no lack of power. The last time I checked the acceleration time it was 10.8 seconds 0-60 using my G-tech. The engine pulls hard to the shift points.

I had an alignment done shortly after I rebuilt the front end in Summer of 2006. The poor fuel economy issue existed prior to the rebuild. Tire wear is normal. I have also replaced all the flexible brake lines with stainless steel lines, and verified that the brakes don't drag excessively. I did this when I changed out the rotors and pads on four wheels and flushed the brake system.

I don't carry around much stuff in my 300TDT. I might have 10lbs worth of stuff in the back. There are no roof racks on the 300TDT, and the crossbars aren't installed right now.

The boost level was approximately 12 PSI the last time I measured it. I believe it is pretty much right on the mark according to what I read.

So based on the replies I have gotten so far, it looks like the poor fuel economy may be due to the the IP timing being off, or the camshaft timing being thrown off by a stretched timing chain. A distant third possibility might be injectors. It sounds like the ALDA is most likely not the culprit.

Any other thoughts or ideas?

It looks like its time to get the FSM out and figure out how to check the IP timing and measure chain stretch. What special tools required to check the IP timing?

Thanks again for all the replies.

-Steve

Bio300TDTdriver · #8 03-11-2008, 02:38 AM

I know you did the brakes, but what about an emergency/parking brake hanging up? We get 23/27 on ours and that is on biodiesel.

swogee · #9 03-11-2008, 11:00 AM

I changed out the parking brake shoes and readjusted the parking brakes at the same time that I changed the rotors and pads. The parking brakes were adjusted per the FSM. (The previous parking brake shoes were cracked, and I suspect that the P.O. must have driven around on occasion with the parking brake on.) Thanks for the idea though. It looks like my fuel mileage is definitely off.

-Steve

Hatterasguy · #10 03-11-2008, 01:38 PM

With a 0-60 time that good I'm having a hard time beleiving anything is out of tune. Yet the mileage is off, so something is up.

Just for the heck of it I would probably time it, and have the injectors tested. Although I don't think either are going to be really bad.

At 190k miles you may have one or two so so injectors, and the timing is probably off 1-2 degrees.

Are the brakes dragging? I assume you are running pretty stock wheels with the proper pressure?

babymog · #11 03-11-2008, 05:12 PM

Fuel leak?

swogee · #12 03-12-2008, 12:30 PM

Thanks for the replies. I am running the stock tires and wheels (195/65R15). I also run the max allowable pressure of 44 psi in the rear and 35 psi in the front. I have already checked that there is not excessive brake drag. (I did that when I serviced the brakes on four wheels.) I have also verified that there are no fuel leaks in any of the hoses or seals.

In summary it looks like the main causes are most likely 1) camshaft timing off due to possible chain stretch, 2) injection timing is off, and 3) worn fuel injectors.

Any additional thoughts or ideas are welcome.

Thanks,

-Steve

swogee · #13 03-18-2008, 05:18 PM

Another question: Currently the EGR is disconnected so that it never opens. The tee vacuum connector has been replaced with a "F" vacuum connector. Would the bypassed EGR have any affect on the fuel economy?

-Steve