W124 300TDT Diesel wont start 1991

Check if the glow plugs are working. There's a fuse / fusable link on the glow plug relay that must be intact for the glow plugs to work. Replacement links can be bought from dealer and on line.

If the motor runs well otherwise the motor and its fuel system are fine, starting is dependent only on the glow plugs and the engine's compression.

-->> T.G. Lambach

Black box... with a cover on all the wires... located at the front of the wheel well.

I think it looks like this one:

Ximinez

Yep. that's it.

Thanks everyone, the relay seems to be working fine, It seems the fuel system somehow keeps developing vapor lock (air bubbles).

Air bubbles.... That's the problem.

Your fuel system has an air leak - at a filter, hose clamp or just a deteriorated fuel hose. Once air creeps in, usually overnight, starting is difficult. That's because air can be compressed (fluid cannot be compressed) so when the injection pump pushes 2,000 psi pressure to the injectors the air absorbs the pressure and the injectors don't open -- until the air has been expelled from the high pressure lines by your expensive cranking with the starter.

If you changed a filter check that it's really tight, then check if all the fuel hose clamps are tight and wipe the fuel hoses with a medium dark, not black, rag so any leaks or weeping really show up on the dark rag. You should know the the new ultra low sulfur diesel #2 affects some old, dried out hoses and sealing "O" rings and so, causes leaks.

-->> T.G. Lambach compressed) so when the injection pump pushes 2,000 psi pressure to the

Yes, this will be a bit of fun tracking down the source... Also I noted that one of the fuel filters has an air bubble in it, I suppose that this indicates the air ingress is somewhere behind the filter then. I have not tried to start it today (the weather totally sucks here).

Prior to starting you can push the air past the injection pump with the hand primer pump. Pump, pump until all the bubbles pass into the fuel return line. Incidently, the hand primer pump is also a possible culprit.

Otherwise, flooring the accelerator will shorten the cranking time as the full throttle fuel flow will expel the bubbles sooner.

On Mar 23, 11:44 am, "-->> T.G. Lambach

To both of you, I suggest you first check the highest fuel line / filters because the air enters the system when the fuel flow stops after shut down. That's because the fuel wants to be at the lowest level so a slight vacuum is created at the high points of the fuel lines like the spin on filter for instance. Think of a siphon - it works so long as it's intact. A bubble inside the small clear plastic filter is OK for it stays there. I'd start at the fuel input to the injection pump and work backwards because the air is probably getting into the IP while the engine is shut down - that's why starting is the problem.

On Mar 27, 9:19 am, "-->> T.G. Lambach

Possibly, but don't forget that air expands with temperature so a small cold bubble will be larger when it's warm, but I'd check the hose clamps near that filter. If the ONLY problem is during starting the problem is very close to (or at) the injection pump but if there's sputtering DURING driving the air leak could be back at the tank's hoses.

-->> T.G. Lambach filters because the air enters the system when the fuel flow stops after

Sounds logical, thanks!

On Mar 27, 3:05 pm, "-->> T.G. Lambach

No bubbles? If that's so then two new suspects come to mind - - glow plug system and engine shut down device. The shut down pulls the fuel rack inside the IP to OFF. These devices in the old 5 cylinder motors are vacuum powered (a bellows) that eventually wears out and gets lube oil logged and sluggish. It could be the cause. The glow plugs depend on a relay and a fuse. The IP is the very last place to look, IMHO. That's Bosch diesel specialist work so be very sure EVERYTHING else is OK before taking out the IP for service.