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Car Forum / Mercedes-Benz Cars / March 2007W124 300TDT Diesel wont start 1991
Hi all, my 300TDT wont start up. It turns over and the engine wont catch. I got it going once yesterday and it ran while warm. It was a pig to start though. What are the likely causes? 1.) switch to ULSD? -if so would an additive help? 2.) fuel system? 3.) other What are the appropriate diagnostic steps to try here? Any pointers to get it going? 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.  Signature© 2007 T.G.Lambach. Publication in any form requires prior written permission. > 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. Thanks! I can't seem to locate the glow-plug relay (I thought it was near the drivers side headlight) any pointers? Cheers Black box... with a cover on all the wires... located at the front of the wheel well. > Black box... with a cover on all the wires... located at the front of the > wheel well. I think it looks like this one: http://market.autopartsfair.com/mercedes-engine_parts/item-u8898_39579ylh.html (at least there's a box on my wheel well that looks like it) Ximinez SignatureOur three weapons are fear, surprise, and ruthless efficiency... and an almost fanatical devotion to the Pope.... http://www.ai.mit.edu/people/paulfitz/spanish/t1.html http://www.youtube.com/watch?v=gldlyTjXk9A Yep. that's it. > 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. Signature© 2007 T.G.Lambach. Publication in any form requires prior written permission. > Air bubbles.... That's the problem. > [quoted text clipped - 11 lines] > 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. 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. Signature© 2007 T.G.Lambach. Publication in any form requires prior written permission. On Mar 23, 11:44 am, "-->> T.G. Lambach <<--" <"T.G. Lambach at NoHamorSpamcomcast.net"> wrote: > Air bubbles.... That's the problem. > [quoted text clipped - 14 lines] > © 2007 T.G.Lambach. Publication in any form requires prior written > permission. I have been having the same problem with my 1982 300 TDT. Air appears in the in-line filter that is before the priming pump. The air appears to be getting in somewhere between the tank and the engine compartment. I have replaced all the hoses under the hood and checked the hoses between the tank and the metal lines from the tank to the engine compartment, but the air continues to get into the system. It appears that I am going to have to remove the tank from the vehicle and replace all the hoses between the tank and the expansion tank. Does anyone know if bad hoses between the big tank and the expansion tank would put air into the fuel lines that run between the tank and the priming pump? > On Mar 23, 11:44 am, "-->> T.G. Lambach <<--" <"T.G. Lambach at > NoHamorSpamcomcast.net"> wrote: [quoted text clipped - 29 lines] > tank would put air into the fuel lines that run between the tank and > the priming pump? Sounds like we are similarly afflicted. I am going to replace the fuel lines, and the primer pump on mine and see how that goes. 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. Signature© 2007 T.G.Lambach. Publication in any form requires prior written permission. On Mar 27, 9:19 am, "-->> T.G. Lambach <<--" <"T.G. Lambach at NoHamorSpamcomcast.net"> wrote: > 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 [quoted text clipped - 9 lines] > © 2007 T.G.Lambach. Publication in any form requires prior written > permission. I seem to have eliminated the problems with leaks on the return lines and the filters. If I park the car facing downhill it seems to help with the starting problem. The other day I checked the in-line transparent filter when I started the car and there were a couple of tiny bubbles caught in the two high spots. Then I drove 40 miles and opened the hood and looked at the filter and it had large air bubbles in it. Does that seem to be saying the leak is back by the tank someplace? Paul 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. Signature© 2007 T.G.Lambach. Publication in any form requires prior written permission. On Mar 27, 3:05 pm, "-->> T.G. Lambach <<--" <"T.G. Lambach at NoHamorSpamcomcast.net"> wrote: > 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 [quoted text clipped - 5 lines] > © 2007 T.G.Lambach. Publication in any form requires prior written > permission. Once the car is running the engine runs perfectly with good power and no misfiring. I have replaced all the hoses except the semi opaque plastic ones and have replaced and tightened all the clamps and installed a new priming pump. Those things were all done some time ago, and I have stopped getting air bubbles in the semi opaque lines that run between the big spin on filter housing and the injection pump. Are there seals in the pump itself that may have begun to fail? Paul 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. Signature© 2007 T.G.Lambach. Publication in any form requires prior written permission. > 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 [quoted text clipped - 6 lines] > because the air is probably getting into the IP while the engine is shut > down - that's why starting is the problem. Sounds logical, thanks!
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