85 300TD weak acceleration below 2000RPM

[tomas_maly]

I've noticed on my wagon that the acceleration was always weak. It never got above 2000RPM. I replaced the throttle bushing and now once it hits 2000RPM it starts to have decent power. It gets zippy, but only above 2000RPM. Below that it's like a snail. Once it hits 2500RPM, it's like lightning. I've been managing by simply pushing hard on the pedal to get above 2000RPM more quickly.

I'll have to compare it to my sedan, but it just feels really different.

Is this perhaps a sign of weak compression? Or this maybe fairly normal for the age? I have a compression gauge to check it at some point, but am wondering if it's something to just live with.

I also notice the pedal seems stiff - I've already cleaned/lubed the throttle linkage above the valve cover but there still seems to be stiffness maybe coming from the IP throttle - it's really greasy and maybe worth cleaning. I think that'll make the pedal seem softer, but I'd imagine I still have the underlying RPM issue.

[Biodiesel300TD]

The first thing to check are all the throttle linkages, make sure they are all moving to their full extent when you press the pedal. Sometimes the pivot points and white bushings get wallowed out and worn, which causes a loss of motion at the injection pump. There are specs for the lengths, they are in the FSM.
Also check that the hard plastic line that goes from the intake to the IP is not broken or cracked, my 82 wagon was a slug until I discovered this broken line, repaired it and then she was back to her zippy self. Then check that the banjo bolts on either end of this line are clean. You'll have to remove them to do this.
Then I'd think about fuel and air filters, then a valve adjustment.

[tomas_maly]

Valve adjustment, fuel/air filters, etc already done. Fuel injectors replaced, too. New glow plugs. I cleaned the banjo bolts from the ALDA to the turbo. Oil got into the line. The line is hard and brittle at this point, so may be worth replacing anyway.

Is it possible adjusting the ALDA may improve things? Assuming I get the line replaced first.

[Biodiesel300TD]

It's not likely that the ALDA is the problem. Yes turning it up will give you more oomph but it's will be masking the issue. I'd still make sure you're getting full motion of the linkages at the IP. If you are, then you might have a clogged tank strainer, or your lift pump may need replacing. Try swapping the supply and return fuel hoses at the steel lines. Then run the car. If no change then your strainer is fine. If you get your power then it needs a cleaning. You could also have an air leak. Can you see any bubbles in the hard return line from the IP to the fuel filter stand?

Also make sure you're not starting from idle in 2nd gear. Put the gear selector into Low and shift manually through the gears. See if you get any power that way.

[tomas_maly]

I replaced the entire lift pump (with the new bosch leak-free primer) without realizing I could have just changed the primer. Silly me. And in the process I think I had to remove some of the hard fuel lines (the clear plastic ones), so I will check for bubbles.

[Zacharias]

Originally Posted by tomas_maly

The line is hard and brittle at this point, so may be worth replacing anyway.

Is it possible adjusting the ALDA may improve things? Assuming I get the line replaced first.

Replace the line for sure. Verify that you are starting from a dead stop in first. And make sure those linkages are moving.

Are you getting 'kick-down' at lower speeds? If you are putting down the pedal but the transmission isn't shifting down, that may be making the car feel like a slug.

[tomas_maly]

Does the little black electronic switch on top of the valve cover have something to do with downshifting/kickdown? (On the 81-84 models, it was a vacuum 3/2 valve) The connector was broken, so I just have it hang off. Could very well be a good acceleration on engine start and just bad acceleration after stopping, since it may be remaining in 2nd gear.

Next time I'm at a stoplight, I will put it into 'L' (or shift into '2' as it's moving off) to see if there's a difference. That should force 1st gear, right?

[Zacharias]

Originally Posted by tomas_maly

Does the little black electronic switch on top of the valve cover have something to do with downshifting/kickdown?

I'm not the local tranny expert but AFAIK kickdown is controlled by the Bowden cable.

Originally Posted by tomas_maly

Could very well be a good acceleration on engine start and just bad acceleration after stopping, since it may be remaining in 2nd gear.

Next time I'm at a stoplight, I will put it into 'L' to see if there's a difference. That should force 1st gear, right?

Remember that when you come to a stop first gear will only engage when the car is AT A FULL STOP (if you 'creep' at stops like I do, it will stay in second).

Yes, placing the lever in L should launch you off in first and hold you there until you upshift to S or D.

[Diesel911]

When my Car had the symptoms you described it turned out to be a lot of slop in the Throttle Linkages.
With the Engine off hold the Throttle lever down on the Fuel Injection Pump and have some one step slowly on the Pedal and watch the Linkages and see how much movement there is before it trys to move the Throttle Lever.

Remove your hand and have someone step all the way down on the pedal and see if the Throttle lever goes all the way to the Stop that limits it.

In the Factory Service Manual the list the lengths that the various linkage rods are supposed to be.

[tomas_maly]

So I decided to finally tinker with my throttle linkages. Often, a lack of full movement will give the illusion that the car is sluggish. Especially when you have to push hard on the pedal for the car to move at all. Didn't really pick up until at the half way point.

I used my zippy 300SD as a reference point; The IP linkage was off by 1/4 inch, so it required more push to get any acceleration.

What is the sideways (metal) box looking thing right next to the secondary filter housing? It has a little lever that moves counterclockwise as the linkage moves. On the front of the valve cover, there is a V-shaped joint that connects this mystery device to the main linkage. Seems that it should be at the 10-o'clock when full throttle - but turns out to have been at 1-o'clock.

I also felt really bad about my main pivoting throttle lever looking so crooked attached to the throttle bushing. It just seemed to fit so awkward at an angle, like it would create resistance to the natural pivoting of the bushing. So I decided to bend my pivoting lever to be more zig-zag shape so it naturally fit straight into the bushing and straight into the other end. Looks more like this now (the holes in the wheel rims become quite handy for bending pieces of metal, including my homemade valve adjustment wrenches):

-----\______

I can just imagine with that sideways tension that it either adds unnecessary resistance to the pedal or after a few years cause the bushing to wear out sooner than it ought to.

Anyway, as of right now, the pedal is so much more response, I'm happy.

I also finally fixed all the vacuum leaks, so the car shuts off immediately with no delay. Must be Christmas. Some previous moron / owner actually had the VCV leak line plugged into the vacuum system!