PeachParts Mercedes-Benz Forum - View Single Post - "not enough (diesel) fuel injection pressure"

psfred · #33 04-27-2005, 02:05 AM

The overfilled crankcase was very likely a fuel leak in the IP flooding the oil with fuel. Pretty rough on the engine, that.

There are only two things that will prevent the IP from generating enough pressure to pop the injectors (assuming that the pump itself is OK) -- air in the pump or incorrectly installed pressure valves. If the pressure valves are in fact installed correcly, you will have very little fuel delivered for each pump stroke even with the control lever all the way over to full power -- total delivery is less than 100 uL per stroke at max delivery. This is a couple drops, so if you are expecting much more than that, you may think the IP isn't working. Fuel won't flow out of the IP with the pressure valves installed and the holders torqued down, it takes a couple hundred psi or so to lift them. This is so that the falling plunger will pull fuel into the sleeve while the line stays at pop pressure. Delivery will be restricted to the volume the governor is calling for.

If the pressure valve holder seals aren't sealing (this is a problem with this particluar pump not seen much in the ones on the 617 engines), you will get normal delivery with the injector cap nuts loose, but none to speak of actually injected, since the pressure in the line leaks off between IP plunger strokes. The line must stay at just below pop pressure so that the fuel is delivered as the plunger in the IP rises, otherwise injection will be very late.

I'm pretty sure you bleed the air out while testing the IP timing, but you need to be aware that this pump is known for holding air. It's possible that you have air in the lines as well, and this will prevent injection til you get it out.

Here's what I would do:

Install pressure valve holders by the book -- pressure valve goes in the holder with the spring on the stem, pointing up. New copper seals are a good idea, as they harden in use and can fail to seal if reused. If the pressure valve holders are factory installed, leave them be.

Loosen the injector lines at the injectors enough to let them leak (about 1/2 turn), and then crank the engine with the accelerator floored until no more air comes out. This may take a while, and at least one charge on the battery. When you have air free fuel, tighten the cap nuts (they don't need a huge amount of torque), recharge the battery, run the glow plugs, and attempt to start. Don't crank more than 30 seconds at a time, and allow the starter to cool off between cranking sessions, you don't want to have to buy a new starter, too!

It will eventually start.

I suspect your problem is air in the pump, lines, and injectors, and the only way to get rid of it on this engine is to crank it forever.

One other thing to check is the condition of the lines on the suction side of the lift pump -- they are old, hard, and likely full of cracks by now, and if so, you are likely sucking in as much air as fuel and pumping that directly to the IP. You won't get enough fuel delivery to purge the lines until you get the air out of the fuel filter, plungers, and IP body.

I wish Benz had left the lift pump on this model -- I'd rather hand pump for half an hour than crank the engine that long....!

Peter