What is it about a Diesel IP that it can't self prime by cranking? Do later IDI Diesels (OM60x and beyond) still require hand priming?

Thanks,
Sixto
91 300SE
81 300SD

The diesels do not have tank fuel pumps, so air in the fuel line is not pushed out at any great speed. The primer pump moves more fuel than the mechanical fuel pump (works at engine speed) at the injection pump and saves a lot of time; if you cranked the engine to get fuel up you'd be through ten batteries and a few starters . Because of the diesel injection system (not rail) a metered amount of fuel is taken from the fuel pump and pressurized to incredible pressures and then pumped through those metal lines into the injectors of your engine. If you cranked your car, air would get into those small lines, and since air compresses much easier than oil, the air would not be easily removed, as you would have to crank you car even longer just to evacuate the air even if you had fuel in your lines. I imagine that this problem could have been solved by using an in-tank pump, but MB probally didn't because of the nature of diesel fuel and it's gelling etc. (also that cars seem to have older technology engine components than MB gas cars).

Dr. Adam T. L. Delecki

I know that my 603 engine in my 300SDL does not need priming.. just crank it until it turns over.

John

The older MBs use a separate piston type pump driven off the IP. These aren't too good at suction, and will take forever to get enough fuel through the filter and into the IP to purge all the air out. The IP is also somewhat difficult to purge of air, too.

The newer (60x and up) IP has an internal gear pump that produces somewhat better suction and better pressure flow, and the IP purges better, so no priming pump.

My Volvo has a VE type pump (single piston, rotating delivery sleeve, and also has and interal gear pump and no priming pump.

Always helps to fill the fuel filter before attempting to start dry, it still can take a long time to get a 603 pumped back up if it runs out of fuel or has a leak -- I've heard of the 0-ring at the pump intake leaking all the fuel back into the tank overnight and taking a long time to crank up next start!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I think the elimination of the hand pump in the later 60x diesels had more to do with cost containment than any change in technology.

I rather liked the way it worked in my old 123. Change the filter. Pump the primer pump until you hear the popoff valve in the IP. Go turn the key and the car starts instantly - no more cranking than normal. There's something elegant about doing it this way.

Note that the hand pumps can fail and feed air bubles into the system. Depending on how much air, the engine may fail to start or start and run poorly.

In the later cars some cranking is required even if you fill the filter with diesel fuel, and there's no way around doing so.