We have a 1982 300D with about 220,000 miles, we've been driving it mostly on biodiesel for about 5 years. About 10 weeks ago it started running badly, intermittent rough idle, sometimes no power. Coughing and stuttering. I thought we got a bad batch of fuel. Our mechanic tested our injectors and replaced a couple and it ran better, but only on diesel

Since I have an 84 I ran on SVO, I "know" what fuel supply issues feel like, so after checking and replacing injectors, I decided to drain the tank & change the filters.

I couldn't get the tank strainer off - my 46mm socket is too tall, but I drained the tank - with 5 gallons in the tank it was a fast hard stream of fuel, filtered the fuel -nothing to speak of in the sock filter, did a diesel purge, changed fuel filter & pre-filter. Engine ran noticeably better, smoother. New air filter - small improvement.

After all this it runs OK on diesel, its a bit rough idle cold, once warm there not much power at highway speeds, and the car labors going up hills, very smooth going downhill . When the car is warm the exhaust isn't smokey, but smells bad. When we put biodiesel in the tank it idles rough and runs rough and we get smoking at idle even when the engine is hot.

I read all I could find on the forum going back 6 months, my apologies if this was asked and answered before that.

I'm wondering if I have a 'tired lift pump'? My mechanic suspects the injector pump. One website suggests a booster pump for biodiesel and SVO for these older Mercedes.

Any suggestions for diagnosing and repair? I couldn't find the $10 Harbor Freight fuel gauge ...

Thanks

It's very possible. These cars are getting quite old, and the lift pumps have completed millions of oscillations.

I doubt the injection pump is bad if all you ran it on is biodiesel.

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'84 190D 2.2 5MT (Red/Palomino) Current car. Love it!
'85 190D 2.2 Auto *Cali* (Blue/Blue) *sold*
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Long Fuel Pressure Relief Valve/Overflow Valve Thread Cervans long thread
Fuel pressure relief valve adjustment - PeachParts Mercedes ShopForum

There is also 2 other significent threads on the Long Fuel Pressure Relief Valve/Overflow Valve.

There is both generic and real Bosch rebuild kits (were about $17 and that price included shipping) for the Fuel Supply/Lift Pump (where about $30 shipping may or may not be free).

It is extremely rare but I have seen Fuel Supply/Lift Pumps with a broken Piston Return Spring. The little springs on the set of valves can also break or rust through (you get both new in the rebuild kit).

People have also said that a restricted Fuel Tank Vent can cause fuel supply issues.

When it is running good does the idle speed fall within what is on the Emission Sticker.

If you Throttle Linkiages get sloppy due to wear (especially the part on the Fire Wall) it can be extrmely slow during acceleration but once up a good speed will drive nromally.

With the Engine cold grab the Throttle Arm on the Fuel Injection Pump and have someone slowly step on the accelerator pedal and see just how much play there is in the Linkages and see if you can see what is causing the play.

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84 300D, 82 Volvo 244Gl Diesel

Fuel Supply/Lift Pump rebuild kits. For the Mw Fuel Injection Pump for my 84 300D the real bosch rebuild kit is DGK-301. The real bosch kit comes with some stuff you won't need in it as you have no glass bowl filter on it.

Generic Lift Pump Kit as on eBay. Complete kit. Gasket kit to fit Bosch* Supply Pumps ADS-DGK-301

There is no easy way to hook up a pressure gauge and even with a liquid filled gauge the needle puslates a lot making it difficult to read. Member funola had a setup up that used I think it was Oil Pressure Gauge Nylon tube if a small enough ID that it apparnly damened the pulsations.

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84 300D, 82 Volvo 244Gl Diesel

The lift pump was the source of the problem.

I was in a hurry so I bought a new lift pump and installed it, then adjusted the valves and the engine runs smoothly.

Thanks to everyone who posted information and advice.