ALDA adjustment of 1985 300D-T
 

Most ALDA adjustment questions involving turning the ALDA "up" to improve engine responsiveness. Mine involves turning the ALDA "down" to improve fuel economy.

My '85 300D-T (W123, OM617, California version w/trap cat and all, 231000 miles) came to me with the ALDA seal removed. I suspect that the PO had adjusted the screw out to try to improve engine response and power. With the factory-installed trap catalyst, the car was definitely handicapped. In addition to relatively poor acceleration (I don't have any numbers), fuel economy was a little low, about 20-21 around town and 24 on the road. This is expected, many others have described similar situations here.

The EGR system has been in a box for the last couple of years and the intake system is fairly clean. The banjo, tubing, and boost protection solenoid are all clean and working correctly. The air filter is clean and the valve clearances are correct. When I had a boost gauge on the car (since removed), boost peaked at about 13 psi. As a final attempt to improve things, I had a "test pipe" built to replace the trap cat, which now lives in the box with the EGR plumbing.

Performance with the test pipe is remarkably better. The turbo spools up quicker and the car definitely has more power (again I have no numbers). However, fuel economy is no better, possibly even worse. Part of this is the fault of my right foot, because the car is so much more fun to drive and I have a hard time "taking it easy." Again, I lack good data because I haven't been driving the car very much. I'm not noticing any smoking and didn't have any before the trap cat was removed. My wife claims she can smell unburned diesel in the car's exhaust.

As an experiment, I have started turning the ALDA "down," that is, I loosen the lock nut, turn the screw 180 degrees clockwise (half a turn) and tighten the locknut again. I have done this twice and haven't noticed any difference in performance. I'm going to continue turning down ALDA until I see a result in performance, then go back up a little. It's also possible that ALDA isn't even related to my situation -- advice would be appreciated.

Finally, does anyone know whether the maker of the ALDA adjusted them and put the seal on at the ALDA factory or was each one custome adjusted on the engine by Mercedes and then sealed? I ask this because I've got a junkbox ALDA with an unbroken seal that I could put on in place of the one on my car. If I could do that, I'd have the correct setting without having to do any more experimenting.

Jeremy

I suppose that would work, adjusting it clockwise, the car then should do what you won't.

Turning the ALDA clockwise will simply eliminate more fuel at part power levels before the boost kicks in. The vehicle will accelerate slower and the pedal will feel less responsive.

This "might" be a benefit to fuel economy if you typically drive it below 2000 rpm. It effectively forces you to drive it easier.

Of course, if you simply don't press the pedal as far............you accomplish exactly the same thing. Less fuel is delivered to the engine and the vehicle accelerates slower.

My personal opinion is that it will have no benefit on fuel economy. You'll simply mash the pedal to bring the boost up faster and the ALDA adjustment is meaningless.

I'm getting a bit less mileage with the SD now that I've adjusted the ALDA, but at least it starts in first now and can get out of it's own way. If I alter my driving style, I'm sure I would better mpg.

I had the same experience.

I finally found a setting where a medium acceleration won't cause smoking. My mileage went up also.

I have to admit, I never really expected the catalyst to hurt performance according to the numbers relevant to HP output. But putting a test pipe on added at least 2 mpg. (there may have been a small exhaust leak)

There should be some discernable difference. Any chance the ALDA is leaking?

I haven't tested it, will pull out the Mityvac and see what happens.

I'm also tempted to try the spare ALDA from my parts box. It's top seal is intact so it might be set correctly, would eliminate a lot of tweaking.