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'83 300D
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638 Posts
Discussion Starter #1
My '83 300DT feels like the turbo is intermittently boosting when driving at highway speeds. For example, while driving at 65 mph and without adjusting the accelerator pedal, the engine feels like it's getting more boost causing the car to speed up 10 MPH. Sometimes after doing this it drops the speed back down to where it started, or it stays steady at the new speed until I ease off the pedal.

Any ideas what I should look for on this?
 

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1984 300D
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5,070 Posts
Try cleaning the Banjo Fitting and line that goes to the ALDA on the Fuel Injection Pump.

Not related to the Turbo but if there is something going on with your Fuel Supply system it can show up a higher speeds.

Filters getting plugged (change the Filters)

Fuel Tank Screen getting Plugged (you need a 1/2 tank of fuel; swap the under the hood Fuel Inlet Hose with the Fuel out let Hose; this bypasses the Fuel Tank Screen. If it runs OK like that you need to remove the Fuel Tank Screen and clean it.)

Fuel Tank Vent getting Plugged (do not do this on a full Tank or Fuel may leak out but, loosen or remove the Fuel Fill Cap. If it drives normally like that your tank vent is restricted.)

If the Primary/Plastic Filter has any slimey gray black substance in it you have something growing in your Fuel Tank and that is what is starting to plug up your Filters.

It is also possible for an Air Leak in the Fuel Inlet Hoses to cause your symptoms. Replace the Hoses with 5/16" ID Fuel Hose from a regular parts store. It is rated for Diesel Fuel.
 

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'83 300D
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638 Posts
Discussion Starter #3
I cleaned out the banjo fittings at the manifold and fuel pump, the tubes as well. Also swapped out the filters after a diesel purge. Both actions performed a week ago, primary filter still clean.

I'll try loosening the fuel cap and swapping the supply/return lines to see if that gives me an indication of the cause.

Thanks!
 

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'83 300D
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638 Posts
Discussion Starter #4 (Edited)
Update

I tried 300Dman's suggestions resulting in no change.

This morning, after swapping out a few other parts, related and otherwise, I gave the ALDA bypass a look.

With the power on I grounded the wire from the switchover solenoid to the pressure switch (simulating boost pressure), no sound from the solenoid. If the solenoid wasn't active, there "should" be boost pressure to the ALDA all the time, right?

I unplugged the solenoid to check for power, and heard a "click" from it. I disconnected the lead from the boost switch to see if it was shorting out and keeping the crossover open, plugged the leads into the solenoid and heard the same "click". Great, I have a short somewhere.

I checked out the wiring from the solenoid to the pressure switch, looks like the wiring between cylinders 3 and 5 got hot as the wire covering melted a bit at some time. When I unplugged the connector for the coolant and pressure switch (near the glow plug relay), the switchover solenoid disengaged. Cause found.

A trip out to the p-n-p in the afternoon and I now have the wire assembly for the glow plugs, temp sender and pressure switch. Cleaned off all the old gunk and will install it in the morning.

The original wiring harness:




The replacement installed:



I love how easy these cars are to work on!
 
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