starting trouble 300DT - PeachParts Mercedes-Benz Forum

dies_Al · #1 11-16-2008, 03:03 AM

I recently bought a 1984 300DT grease-car with 240K. It had been starting consistently but recently I have had to hold the accelerator down to keep it from stalling. I read about adjusting the ALDA and began to do some minor work: checking the vac lines, changing the air filter, secondary fuel filter and glow plugs, to see if this would improve the cold start idle before getting further into it.

The secondary fuel filter replacement is where I believe I made a mistake. The first time I put it on, the filter was empty and the car wouldn't start. I then came to the forum and realized my mistake. I went back filled the filter, cracked the injector lines and had a friend of mine crank it over in order to purge the lines of air.

3 of the 5 lines purged and the other 2 looked dry. I tightened the lines back up and repeatedly tried to start it. I think my starter is dying now. The dash lights flicker on and off while starting.

Tried to jump start it - got nothing
Tested power to all 5 glow plugs - good

JimSmith · #2 11-16-2008, 03:17 AM

I believe you still have air in the fuel delivery system. I am not intimately familiar with the 300 D engine you have, but I do believe it has a hand priming pump on the front of the injection pump. The unit has a plastic "knob" that was knurled by some sadistic cretin in the engineering department at MB way back when. You unscrew the pump "knob" and I recommend a cloth to protect your fingers from the "knurling" and, once unscrewed you lift the pump by the knob and push it down to pump fuel through the system. You do this forever, which is why the "knurling" is so sadistic.

If you don't use a cloth to cover the sharp edges you will end up with open blisters bathed in engine grime and Diesel fuel. After many strokes you will hear a chattering sound of fuel activating a small pressure relief valve. Keep pumping for another thirty or so strokes. When you are done, push the handle back down and screw the knob tight, or snug. Too tight and you might have trouble unscrewing it the next time you need it, too loose and it will leak air, so definitely snug, but not redfaced tight or anything like that.

Your starter may be suffering but I would suspect it is your battery. Charge it good with a real battery charger, not a trickle charger. Jumping is not likely to be successful. These things draw a lot of current to heat the glow plugs and then turn the engine over against the compression. In the end the cables are usually too small in diameter to deliver the current and voltage to the starter, so you don't get the speed you need the engine to spin at to start.

I would also rig up a pure Diesel fuel source (temporary) to get the question of the viscosity of the fuel you have off the table if it continues not to start.

Good plugs, a healthy battery and starter, and fuel, and there is no obvious reason it shouldn't start. Good luck, keep us posted. Jim

tobybul · #3 11-16-2008, 11:10 AM

Nowhere in your write up did you mention pumping the manual fuel pump.

There is no need to fill the new fuel filter with fuel. After you screw it on, crack open one of the top or side bolts on top of the filter. Pump the the manual pump until the fuel seeps out of the loosened bolt (you will need to do this anyway even if you fill the filter with fuel). Tighten the bolt as soon as fuel seeps out. Now try starting.

With your situation now, air may have gotten into the IP. Per the FSM, you can get rid of the air in the IP by pumping the manual pump vigorously until you hear a "buzzing" sound off the IP. This means the air vented out of the IP. This make take several pumps, like around 30. If air got in to the IP, the car will not start.

vstech · #4 11-16-2008, 11:21 AM

I agree with the above, this is needed to get back to the poor starting you were at before you started servicing it.
now, as to why it was so difficult to keep it running when cold, there's quite a few reasons I'm sure.
first check with an ohmmeter the condition of the glow plugs themselves. http://www.dieselgiant.com/glowplugrepair.htm
if they all check out in the .6ohms range, you are down to compression as the reason for difficulty starting.
you could also have cold thick oil and a weak starter in combination with a weak battery... but the rough running after starting is a sure sign of compression or glow plug issues.

JimSmith · #5 11-16-2008, 11:46 AM

We ought to make a "W123 Diesel Cold Weather Starting" thread and keep it as a sticky since there are a lot of W123's out there and every Fall and Winter this topic becomes a "FAQ."

These old engines will run in Winter provided they are getting decent compression, have a starting rpm that ensures the heat of compression is maximized, functioning glow plugs, and fuel delivery. Sounds simple but there are a number of systems and subsystems that have to work together to achieve all that, and poor starting can be any permutation of weaknesses in these systems that reaches a threshold on a certain day......

So, the issue of compression was brought up, and if your car has been treated reasonably well in terms of oil changes with proper grades of oil, the rings and cylinders will likely be ok. They have cast iron heads and are not as susceptible to overheating to cause head gasket problems, but, compared to newer Diesels, have a tedious requirement to adjust valves. Poor valve adjustment (they close with wear) and you won't be breathing right, and you will waste battery and starter oomph to turn the engine over.

Proper oil for winter is also critical. These older cars love Mobil Delvac 1, or mine did/do, but any decent Diesel rated oil for winter service will be helpful. Valve adjustments are critical. Good battery is critical. Healthy starter is critical. Glow plugs are critical. A "solid" fuel delivery system ("solid" meaning without any air from essentially the in-line filter to the injectors) and injector nozzles that produce a mist and not a spurt of globular, congealed Diesel fuel are critical.

I am not sure but I thought 300D's had the idle speed adjusting knob on the dash like a 240D. Maybe not on the turbos. I never owned one so I don't really know. If you have it, turn it "up" when the engine is cold and it does the same as you holding your foot on the pedal a bit. When the engine warms up and the idle speed is too high, you turn it down. The turbo may have something more sophisticated and I cannot offer any advice if it does.

Good luck. Jim

tobybul · #6 11-16-2008, 12:21 PM

I don't advise messing with the ALDA. If you meant cleaning the ALDA boost plastic line between the manifold, switchover and ALDA, by all means. The ALDA was factory set and better to not mess with it.

However, it should not affect the starting of the engine but it affects the performance of the engine.

Its not unusual to have difficult starts in the winter especially with very high mileage D's. You can help it along by following the suggestions mentioned above by the other members, such as putting in lighter weight oil. I'd start with that b4 messing with compression testing. Simple stuff first

Oh, BTW, does "grease-car" mean a WVO conversion?

dies_Al · #7 11-16-2008, 11:32 PM

tobybul,
It has two tanks one for diesel and one for WVO.

Jim,
I have since filled the secondary filter, loosened the top bolt, purged the filter housing, pumping until I had diesel coming out of the top and tightened the top bolt. The pump now makes a "pfft" high-pitched fart type noise when fully compressed. Is this the "chattering" sound you described?

I will attempt to purge the rest of the injector lines and start her up tomorrow afternoon.

Thanks guys!

JimSmith · #8 11-17-2008, 04:45 PM

Yes, the noise you describe is the little relief valve chattering to let fuel through, once that part of the circuit is free of air. I usually keep pumping for thirty or so strokes because to get that much pumping of fuel through the system by turning the engine over with the battery and starter is about the most inefficient possible way to move a thimble full of Diesel fuel. So, I try to make sure the system is really free of air before trying to start the engine. Good luck,

Jim

curtludwig · #9 11-18-2008, 05:14 PM

You don't say where you are or how warm it is outside. Could you have some gelled fuel?
I'm thinking where its a WVO car maybe you've gone several months on one tank of diesel so you've got summer fuel and its cold out and its gelled. That might explain why it'd run but not idle well. If that were true you'd probably be down on power as well.

Its also quite likely you've got an glowplug or two not up to scratch, if the car idles well once its warmed up thats what I'd expect.

Finally, and I almost hate to mention this, you're absolutely sure your WVO system is warming the oil enough? Try running on diesel for a week or two and absolutely running the stink out of the car. Fill the trunk with junk to put some weight on it and find some big hills to climb, make the car really work. Might burn some carbon out of the rings, raise the compression and fix some issues.

A friend has an '80 300SD his wife drives around town mostly. This time of year he usually gives it to me for a weekend, I drive the snot out of it on the highway for a weekend (no idea why he can't do this himself) and when I give it back he's always amazed how much more power it has. The first time I did it I also replaced the thermostat which had been sticking a little open. That made a HUGE difference on his car...