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  #1  
Old 07-26-2020, 12:11 PM
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What happens when running out of fuel?

Seeking help trying to diagnose an issue with my 300CD.

I've had no mechanical issues, until the other day I had some increasingly violent shaking every time I idled. 2 red lights later the engine shut off. Was able to turn it on for just long enough to get to the gas station across the street.

I was low on fuel but not at minimum according to the gauge, maybe 10% above. That said, I replaced the cluster a few months ago and it might not be entirely accurate.

I filled up the tank but still could not get the car to start. "Choking" sound and shaking when firing the ignition.

Towed it to the mechanic where it sat for 3 days, lo and behold it starts perfectly, runs perfectly, drives perfectly.

I'm nervous to drive it without knowing what the issue was. Mechanic can't find anything wrong.

Is it possible that the tank was so empty that it needed time for the fuel to re-enter the system? I'm not sure if that makes any sense... but it's the only clue I have at this point. Any ideas are welcome and appreciated -- thanks.

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  #2  
Old 07-26-2020, 12:53 PM
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May need tank and tank screen cleaned.
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  #3  
Old 07-26-2020, 01:05 PM
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X2 on post #2 Did you use the primer pump to rebuild pressure? Then take it out and see if it will run at say 65MPH easily.
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  #4  
Old 07-26-2020, 01:07 PM
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Did the mechanic possibly use the primer pump to pump up fuel? Where you there when he initially started it? If he did pump up fuel with the primer pump you know what the fault was probably.

If it is running well right now I cannot think of anything to check particularily. It may or may not happen again.

Basically it does sound like you might have been out of fuel. A very tired lift pump may have had an issue with drawing fuel from a low level in the fuel tank or an issue with the screen in the tank.

It sounds like you do not do your own work. So I resist giving a course of action. That you have to pay to have done.

When I purchase any old car I go over a lot of things myself. Usually doing everything I feel is needed. Costs far less than one road breakdown. As so many old cars have not had reasonable care in respect to their age. By doing this reliability is vastly increased.

At one time the cost of paying for this service was reasonable. Almost forget it today unless you do it yourself.
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Old 07-26-2020, 02:09 PM
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Was it towed with the rear in the air?
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  #6  
Old 07-26-2020, 02:21 PM
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Thanks all. I wasn't there when the car started running, nor do I know if they used a primer pump. I had never even heard of a primer pump before - new to all of this and learning/researching as I go, hence why I'm leaning on a mechanic for the foreseeable future, and asking you experienced people to learn what I can.

Helpful to know that the sequence of events is potentially consistent with an empty fuel tank - now I have a working theory.

Quote:
Was it towed with the rear in the air?
Yes it was. Could that have solved the issue in the fuel tank? Though right when it got to the shop we tried turning it on and it had the same issue. Only started working after 3 days of rest.
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  #7  
Old 07-26-2020, 02:29 PM
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Sure sounds like you had air in the fuel system. Diesels REALLY don't like that. Whenever I break into the fuel system on Mutt the Race Truck I know I'm in for a serious project of priming and working fuel back up to the injectors - you need a solid column of fuel from the tank to the injector nozzle.


These long distance analyses are always to be taken with a grain of salt, however.



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  #8  
Old 07-26-2020, 02:57 PM
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Separate issue since the car is new to you, buy and store in the trunk: both fuel filters (2 sets), 3' of return fuel line, wrenches, cutter, pliers and screw drivers to install the filters and return line. Also, you may need to be able to crack the injector hard lines at the injector. Add some shop rags. Buy a small inexpensive storage box (small fishing tackle boxes work) in which to store everything.

NAPA has a 1/8" fuel line that is locally available and seems to hold up better than the covered line available from MBZ parts suppliers.

Be sure that the filters are changed before taking a long trip.
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  #9  
Old 07-26-2020, 03:32 PM
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Old cars supported by sites like this one are a very good learning experiences as well.


I support the majority of maintenance requirement on all our cars. As cars got newer they also became much more complex. In my opinion they are far less desirable to learn on.


A lot of my older generation started to repair our cars as was quite common all those years ago. In my case it was over sixty years ago. I still do most of it.


The manual primer pump is located at the base of the injection pump. The original pumps had a white knob you unscrewed and pumped until you you heard a noise. That is the relief valve opening. The push on the pump with your fingers becomes more resistant as well as you pump. A lot of these primer pumps where replaced with a unit with a black knob. These you do not unscrew to use and screw down once used. You just finger pump them.


They refill the fuel filter and base of the injection pump with fuel. It is better to not run out of fuel though. As some of these cars present more of a problem getting them running again. If you run them out of fuel.
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  #10  
Old 07-26-2020, 03:48 PM
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What does everyone make of the fact that the car seems to now be running fine? Presumably the primer pump hadn't been tried yet, but I'll confirm tomorrow.

Should the pump still be used? Could the issue have resolved on it's own (during tow or otherwise?) At what point should I feel sure that the car is back in working order?


Quote:
Originally Posted by barry12345 View Post
The manual primer pump is located at the base of the injection pump. The original pumps had a white knob you unscrewed and pumped until you you heard a noise. That is the relief valve opening. The push on the pump with your fingers becomes more resistant as well as you pump. A lot of these primer pumps where replaced with a unit with a black knob. These you do not unscrew to use and screw down once used. You just finger pump them.
Very helpful. That gets me on the right track and I'll be watching videos and reading more so I have more of a grasp on this mechanism. But it sounds like something I can manage on my own.

Quote:
Originally Posted by Junkman View Post
Separate issue since the car is new to you, buy and store in the trunk: both fuel filters (2 sets), 3' of return fuel line, wrenches, cutter, pliers and screw drivers to install the filters and return line. Also, you may need to be able to crack the injector hard lines at the injector. Add some shop rags. Buy a small inexpensive storage box (small fishing tackle boxes work) in which to store everything.

NAPA has a 1/8" fuel line that is locally available and seems to hold up better than the covered line available from MBZ parts suppliers.

Be sure that the filters are changed before taking a long trip.
Thank you for this. I have most of these already, except the fuel filters. Great to know that it's recommended to replace before a long trip. For all the research I've done, this sort of tip never seemed to come up. Much to learn.
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  #11  
Old 07-26-2020, 04:21 PM
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All you need to do is watch the mileage and the fuel gauge. When you get about 250 miles on the tank or the gauge drops to maybe 3/4 empty, fill it up. Do not depend on the low fuel light and you will be OK.

Symptoms of running out of fuel - it will lose power, then pick up again; will do this several times. It may cough. Then it will just stall and not restart no matter how strong your battery is. Just fill it up, pump the pump, and grind the starter until it starts.
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  #12  
Old 07-26-2020, 06:39 PM
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Quote:
Originally Posted by tyl604 View Post
All you need to do is watch the mileage and the fuel gauge. When you get about 250 miles on the tank or the gauge drops to maybe 3/4 empty, fill it up. Do not depend on the low fuel light and you will be OK.

Symptoms of running out of fuel - it will lose power, then pick up again; will do this several times. It may cough. Then it will just stall and not restart no matter how strong your battery is. Just fill it up, pump the pump, and grind the starter until it starts.
Provided the odometer still works. My last W123 had odometer stop at 268k and fuel gauge sender after cleaning was good until around 1/2 tank.
Three years or so, as stated 250 miles top it off.

Last edited by sloride; 07-26-2020 at 07:10 PM.
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  #13  
Old 07-27-2020, 08:47 AM
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Ok, time to discuss one of the oddities of the Mercedes fuel system. If your tank was "simple", then fuel would slosh around as you turn right and left. As you reach the bottom of the tank, you would alternate between running and stalling. To prevent that, there's a snail shaped swirl pot which surrounds the pickup screen. Excess fuel returning from the engine squirts into the opening of the snail, and creates a sort of pumping action that continuously pushes fuel into the cup, so that at low fuel levels, you have about a quart of fuel surrounding the pickup regardless of the attitude of the tank.

What happen over time is that debris accumulates in the cup, allowing the car to seemingly run out of fuel, even though the gauge tells you you have a few miles to go. So yes, cleaning out the screen helps. And keeping the tank above 1/4 full will avoid the problem. But the real deal is to remove the tank and have it boiled out.

As for the rest, once you ran low, air accumulated in the system and kept you from starting. With a full tank and repeated attempts, than can clear up without working the hand pump.

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  #14  
Old 07-27-2020, 08:52 AM
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U just ran out of gas. That's it. Don't over think it.
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  #15  
Old 07-27-2020, 09:50 AM
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Hopefully he didn't run out of GAS! We all know you meant diesel, right??

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