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transmission will only down shift when going very slow. like going around a corner at 20mph it will not go down a gear it stays in hi gear. and what kind of fule mpg should i get i get about 20mpg.
 

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The transmission, a 722.303, is acting normally.

You should get better mileage.

Install new filters, do a valve adjustment and an Italian tune up.
 

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+1 to what Rmac58 says (just look how nice his wheel is!)

I recently had air, oil, and fuel filters changed, and valves adjusted on my '83 300 SD. Car ran fine before, amazing now. Averaging 26 mpg city/highway/windy country roads.

Not sure what an "Italian tune-up" is, but I also ran some "Diesel Kleen" in the fuel.

If your air-conditioner's shot, you might as well remove the belt that powers (what I would call) the air-compressor.

Oh, and if you're getting the work done, be sure to ask for a "valve adjustment". If you casually ask for a "valve job", your heart will stop when you get the estimate.

Good luck!
 

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^ Yes, a valve job is quite involved, new seals, ported/polished etc.

An Italian tune up is a term used to describe driving at high rpm for an extended period of time. The phrase came about as Italian sports cars would carbon up at continued low rpm, or around town driving. So get on the interstate and go as fast as you can for a while. This will blow out some build up.
 
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