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  #1  
Old 06-18-2015, 09:09 PM
t walgamuth's Avatar
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Location: Lafayette Indiana
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1985 300sd acting up

the car is my daughter's but I am taking care of it for her. It will run fine then sometimes have no power and be hard to start.

I changed the primary filter a month or so ago and the air filter and treated the fuel with a biocide.

I'm thinking to treat it again to start with.

What are the symptoms of a worn lift pump?
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  #2  
Old 06-18-2015, 09:20 PM
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I would imagine a worn lift pump wouldn't affect starting too much, but would be pretty predictable and replicable while driving. If the car sat for a while you might want to take a boroscope and look in the tank. Not only fungus but it's possible rust got in there I imagine. Honestly id just pull the tank screen and take a look, it's not too difficult.


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  #3  
Old 06-18-2015, 10:50 PM
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Intermittent pretty well lets out the mechanical side. Our MB's usually work or dont.

You might consider pulling the tank and having it flushed. The borescope is a good idea too.. get an idea whats happening in the tank
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  #4  
Old 06-18-2015, 11:11 PM
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When its not working properly try opening the fuel cap. Maybe the vent isn't working properly and is causing a vacuum in the tank.
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  #5  
Old 06-18-2015, 11:18 PM
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M first step is Startron and filters. My 85SD went from not starting to running fine with that process.
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  #6  
Old 06-19-2015, 02:04 AM
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Dude 99s suggestion is a cheap starting point. The consistency being when the car is turned off the existing vacuum in the tank may pull the fuel back a little creating harder starts. Or when the vacuum accumulates enough the engine is being partially starved.

Of course it can be other things but this is probably the easiest to check . Usually changing the fuel filters is always the first step with any suspected fuel supply issues though. Filters can fool a person in their behavior sometimes.

Much depends on if you are getting her to check things or doing it yourself. Leaving the fuel cap loose may be messy. As a test just drilling a 1/16 inch hole in a spare old fuel cap you have around might be used for a test.

Hard for this design of lift pump to become intermittent. Not impossible just unlikely in my opinion. Air leak into the system of a semi intermittent nature is a possibility as well I suppose.

You have been around these cars for a long time in comparison to most members. There is a fair list of possibilities. Some are just less probable than others. If she only experiences the loss of power after driving a fair total distance also increases the tank vacuum possibility quite a lot. On a totally obstructed tank vent the car will usually totally shut down in about 100 miles after fueling with a strong lift pump. With a less than totally obstructed tank vent running time until power decreases may not be that great a distance. Especially if the lift pump has lost some efficency with age. The primary problem still would be the tank vent.

The majority of these cars are still on their original lift pumps with original valves I suspect. They are not technically really bad. I suspect they are still quite serviceable but just do not pull as hard nor develop their original output pressure. They should be tested for their pressure output every few years. Few ever are.

Last edited by barry12345; 06-19-2015 at 02:29 AM.
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  #7  
Old 06-19-2015, 07:08 AM
t walgamuth's Avatar
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Thanks guys. I'll check the fuel cap, treat the tank and change both filters....in that order.
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  #8  
Old 06-19-2015, 01:53 PM
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Quote:
Originally Posted by barry12345 View Post
Air leak into the system of a semi intermittent nature is a possibility as well I suppose.
This is my bet. Source could be the primer pump or some other leak in the engine bay. Diesel fuel will fluoresce under black (ultraviolet) light, so you can start with that and maybe pressurize the system to make the air leak into a fuel leak.
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  #9  
Old 06-19-2015, 03:02 PM
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If the 300SD is acting up, you need to place her in the corner for a few weeks.
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