low speed poor performance - PeachParts Mercedes-Benz Forum

rsmartin · #1 06-02-2007, 11:32 PM

85 300SD 231,000 - bought with 180,000 in Oct 2005 - used for communting from Tennessee to Maryland and back a few times each month. Just useless FYI!!

Problem is.. at low speeds taking off, it can hardly get out of it's own way especially with A/C - was going to give this to my son when he gets his license but i'm a little worried it is too slow and he'll have no power to get out of trouble if he pulls out in front of someone.

i hooked up a pressure gauge between the switchover valve and the line to the ALDA. when taking off i get 1-3 psi until about 2500 rpm when the pressure starts climbing. at about 3000 rpm i'm getting 7.5 psi. My 126 Service Manual CD tells me I should have more like 11-13 psi at 2000 rpm and around 6 psi at 1500 rpm. also hooked up pressure gauge so the switchover valve was bypassed (line from manifold was t'd with the line going to ALDA. same readings so it appears switchover valve is NOT at fault for limiting boost

i have 84 300SD that seems to have a lot more low end pickup AND you can hear the turbo whistle as you take off. i never hear the turbo on the 85

i get about 27 mpg on the highway going about 70 mph on my 10 hr drives

i've been working at replacing all the little rubber vacuum connectors, i've cleaned my line from the manifold to the switchover valve - it was never plugged up but cleaned it out anyway

both fuel filters were replaced in late December after performing diesel purge which resulted in NO perceptible performance enhancement. air filter replaced about 6 weeks ago.

i finally broke down and "molested" my ALDA. took the seal off and turned the screw counter clockwise 1/4 turn. made no discernable difference, turned it out 1/2 turn, made no difference other than started smoking on take off so i turned it back in to just a 1/4 turn

any comments? anything i should check to help figure this out?

R Martin
Chattanooga, TN
84 300SD
85 300SD

#2 06-02-2007, 11:37 PM

When was the last time the valves were adjusted?

rsmartin · #3 06-02-2007, 11:41 PM

just had the valve adjustment done maybe a two months ago?

#4 06-02-2007, 11:48 PM

Have you cleaned the tank strainer? You can see if it's the problem by swapping the fuel supply and return lines. If it's clogged, you will see a significant difference.

Is it starting in 1st? How many shifts do you feel?

rsmartin · #5 06-02-2007, 11:59 PM

All the shifts occur.. no transmission problems apparent.

How does one clean the tank strainer? although it doesn't seem to be lacking for fuel as it cruises non-stop at 70 mph for 10 hours at a time without missing a beat.

#6 06-03-2007, 12:05 AM

The tank strainer is above the right axle (At least on the W123, not sure on the W126). Remove the fuel line coming out of it and use a 46mm socket to unscrew it out of the tank. Just make sure it's nearly empty before you try to remove the strainer.

Cruising is no problem, it takes very little fuel to do that. You need the most fuel while you are accelerating.

rsmartin · #7 06-03-2007, 12:37 AM

Quote:

Originally Posted by ForcedInduction

Have you cleaned the tank strainer? You can see if it's the problem by swapping the fuel supply and return lines. If it's clogged, you will see a significant difference.

I don't understand what you mean by swapping the fuel lines.

#8 06-03-2007, 12:42 AM

Swap the supply and return line. Put the cigar hose and the hose before the fuel filter on the opposite metal lines.

vstech · #9 06-07-2007, 10:01 AM

since the cigar hose will not reach, go to a parts store and get 6' of 5/16" hose, cut it in half and replace the cigar hose with the 3' chunk of hose and reverse the lines. this will clear out the strainer, and give you decent performance until about 1/4 tank, where you will run out of fuel, so switch the lines back before this point. then once you have gotten the fuel deep into the reserve tank, (like 60 miles after you hit the R mark) take the other 3' piece of hose and stuff one end into the cap of a juice bottle filled with Marvel mystery oil. hook the other end up to the supply line, and take the hose off the tank and hook it up to the juice bottle too. you should now be able to do a "diesel purge" as in the DIY on dieselgiant's site.
enjoy the performance boost!
while you are at the parts house, get new fuel filters too.
John

winmutt · #10 06-07-2007, 10:11 AM

Does your IP linkage hit full stop?

rsmartin · #11 06-10-2007, 04:02 PM

swapped the fuel hoses and made way from Maryland to Tennessee. drove about 400 miles with them switched.

Can't say it made any difference. I think I could persuade myself there was a little difference in low end performance but I could just as easily be persuaded it is all in my head. Nothing dramatic.

maybe IP needs timed?

rsmartin · #12 06-13-2007, 10:30 PM

Finally got around to having mechanic clean the tank screen. It didn't appear to be very dirty but it's definitely not dirty now as we cleaned it up completely. Also change both fuel filters and replaced my leaking primer pump. Low end performance seems to have improved. We'll see.

Still would like to tweak my turbo boost up a notch. I think maxing out at 8psi is too low and would like to feel what the car would do with more like 10.

See my other thread on the fiasco we had with replacing the rear sway bar links.!!