Hey, All. Happy 2021! A question about acceleration (such as it is:) in my 1981 300D manual (580K km on the clock): When I get to about 20km/h in 2nd, 30km/h in 3rd, and 40km/h in 4th as I go through the gears, there's a jerkiness (reminiscent of a dud spark plug) for about 3-5 km/h in each gear. At about 90km/h the engine smooths out and runs well up to our 110km/h speed limit (and a bit beyond). A diesel purge and new filters don't seem to make much difference. I'm running biodiesel, but dino only seems to bump the jerkiness along by about 3-5 km/h. Is it perhaps an IP thing? Does anyone have ideas?
Cheers
Graham

No turbo and manual transmission?

This is only remotely possible if you have a manual transmission. I had a BMW Motorcycle that acted similar and it turned out the Clutch Friction Disk was saturated with Oil due to a leaking engine rear crank seal. It would slip at various points in acceleration.

There is no easy way to connect a Fuel Pressure Gauge and when you do the needle moves back and forth violently. Liquid filed gauges help a bit but some sort of restricted fitting works best.

The Banjo Bolt coming off of the Fuel Injection Pump return Fuel Line is a Valve called the Fuel Pressure Relief/Overflow Valve. That is a major part of the control of the fuel supply pressure inside of the Fuel Injection Pump housing. If you have one that can be taken apart you can take it apart and don't loose the ball bearing that is used as a valve and stretch the spring in it till it has a free length of 27 mm. That is a low cost thing the only expense being if you replace the crush washers when you assemble it.

The other things that effect the fuel supply pressure is the Fuel Supply/Lift Pump on the side of the Fuel Injection Pump. If you have the old style hand primer with the shiny Aluminum body and the hard plastic usually white knob they are notorious for being the source of an Air leak. An Air leak would certainly fit your symptoms.

Look at the plastic see through fuel filter. If you see air bubbles actually moving into the Fuel Supply/Lift Pump you need to find where the Air is leaking in.

Also if the valves inside of the Fuel Supply/Lift Pump need to be replaced that would also effect the Fuel Supply Pressure. In the US the kits for them don't cost much.

Those are the areas I would look into first before suspecting the Fuel Injection Pump.

I is not a good pic but the Fuel Pressure Relief/Overflow valve that can be taken apart looks like the one in that picture.

Try driving with your Fuel Fill Cap off. If that improves things you are having an issue with your Fuel Tank Vent; likely plugged or restricted.

When was the last time the engine valves were adjusted? Is there any fuel leakage on the Fuel Injection Pump?

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84 300D, 82 Volvo 244Gl Diesel

The problem is you're lugging the engine.


20km/h in 2nd= 1500rpm
30 3rd= 1400
40 4th= 1500
90 4th= 2750


At those low speeds you're fighting the injection pump governor for control of the fuel rack wanting to return the engine to idle. From 1500rpm to near maximum loaded speed, about 4500rpm, you are in direct control of the fuel rack which is why its smoother.

You need to revv the engine higher between shifts, at least 2500rpm, and downshift if speed goes under 1800rpm.


Attached is a ratio chart to help you figure out what RPM you're at a given speed since you lack a tachometer. Your car will have a 3.46 ratio. You will notice the "green" range is 3000-4000rpm. This is the most efficient speed of the OM61x diesels. RPM won't hurt the engines and in fact they need it to run well.

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Another way of putting it, replace Kph with mph and that’s where you might try shifting.

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1981 240D 4sp manual. Ivory White.

Thanks, Gents. I'll amend my driving style (Felching). Might be a good idea to get out of the idea of changing short. However, I'll also look at the lift pump and the Fuel pressure relief valve (Diesel911). There's no indication of clutch slippage, as there's no cvt effect as I accelerate.
Cheers

or shift right before the dots on your speedometer

No. The dots are not up shift indicators, the are the maximum speed at which its safe to downshift.

Just my .02 but when was the last time the tank strainer/filter was cleaned? If you are running biodiesel it will clean out a lot of crap in the fuel tank that will end up in your strainer and eventually the fuel filters.

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)

Thanks, psaboic. I thought that might be a problem, so checked it out. What a waste of time! Messy job, but clean as a whistle. I've been running B100 for most of the last 100K km (winters about 25%-33% dino), and really noticed the need for filter changes in the first 5000 km. Bio seems to be really clean, and diesel purges are (disappointingly) smoke free.
Delaying my changes and not changing short definitely seems to make driving smoother.