Intro and few issues on new to me MB 1984 300SD - PeachParts Mercedes-Benz Forum

jdkl81 · #1 02-20-2013, 12:32 AM

Hey everyone, I've enjoyed reading the good tech advice on these forums and have already learned a lot.

I just acquired a 1984 300SD from a friend who has had a lot of Mercedes diesels over the years.
Car is really neat to drive, but does have some minor issues which I will take on as they demand my attention.

The main gremlin that I notice with this car is an inconsistent idle and sometimes it is hard to start after sitting for a few hours.

I also noticed that the primer pump is leaking.
I did a search here on the forums, but didn't find too much about the primer pump, so I'm asking, for those who have done it, is replacing this a hard job or not too bad?

Is there a tech article here on it that I might be missing somehow?

I'm fairly handy and have done quite a bit of work on the Stanadyne fuel system on my 1986 F250, but I know these fuel systems are very different.

One more thing, should the low pressure lift pump be replaced at the same time as the primer pump?
I saw from another post that the lift pump at least on the OM603 is bolted on to the IP, I assume that's also the case with this engine...

Thanks in advance for any help I might get from you all...once again great forum!

kerry · #2 02-20-2013, 12:42 AM

Pretty simple replacement. No need to replace the lift pump. I think most people use a crow's foot wrench.

whunter · #3 02-20-2013, 03:26 AM

These links should have everything you need for the fuel system.

Diesel Injection

Fuel Delivery

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These links are for research, and later need.

Fast navigation Do It Yourself Links

The "Do It Yourself Articles - Mercedes Vehicles" are still here.
PeachPartsWiki: Do It Yourself Articles - Mercedes Vehicles

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jdkl81 · #4 02-21-2013, 12:38 AM

Thanks for the quick response guys. I guess this has been talked about somewhat in Whunter's extensive collection of tech articles. Now I'm a little worried with all this talk about low fuel pressure and the associated failures it can cause...I better get right on replacing this primer pump and see if it makes any difference and maybe check the fuel pressure as well. This car is intriguing to me because it seems like it can idle great, purr like a kitten at times and then idle rough enough to shake the car at other times. It gets acceptable fuel mileage starts fairly easily, and has good power. As I said, I have a lot to learn about these. Thanks for helping.

cooljjay · #5 02-21-2013, 01:24 AM

It takes a really dedicated person to own one of these beast and your going to be seeing that first hand. These cars are amazing strong machines and will last your life time with regular maintenance. People usually give up on them because after 30 years a lot wares out and seeing how most of the components were never replaced with then ware limited approached....time takes its toll....most people only ever did oil changes on these and if you were lucky valve adjustments. Things like injectors, starter motors, glow plugs, fuel lines etc were never replaced because they showed no signs of failing but they all add a strain to the motor and functionality of the car. If you have a combination of air in the lines, weak battery, worn injectors, clogged fuel filters, tight valves...you will then start to have hard starting problems....you then start to smack your head on the wall because you think the engine is bad. You just start replace worn component one after another and still can't get it to start.....then you finally get the last bad component out and it starts!! All because there was never any preventive maintenance done.

The best thing to do is get fimilar with your car, just look it over good....get an idea of what things are suppose to look like and once a week open the hood, check the fluids and do a visual inspection....you can then usually spot trouble before it happens....start to make a mental list of things that need doing asap and things that can be done on a nice warm day.

For instance since you have to replace the primer pump, do a whole fuel system over haul. Pull the sender out of the tank, clean it and be rewarded with a working fuel gauge....look inside the tank with a flash light and see if the tank screen needs to be replaced. If so, remove it and replace....then replace the rubber lines underneath the car....move onto the primer....rubber lines on the IP....replace the filters....then replace the injectors.....now you know your whole fuel system has received service for many more miles...

sleepstar · #6 02-21-2013, 01:48 AM

replacing the primer pump is about a 5 minute job with a crowsfoot wrench. i think its a 17mm if i remember right

jdkl81 · #7 02-21-2013, 01:49 AM

Yeah, this car has seen some neglect. Fortunately the guy that owned it before me knows Mercedes diesels and isn't a cheapskate, so a lot of PM has been done, however, it probably needs injectors, this car's odometer quite working at 184,000 and my friend said be sure to add at least 100,000 miles to that figure. Already did have the valves adjusted...4 out of 5 were tight. He thinks he cut out the fuel tank strainer but he can't remember for sure. The fuel filters have been changed but it's beginning to look like it needs the inline filter changed again. I'm happy with it so far. I know it will require some work to realize it's potential, for now it's getting me back and forth to work and drinking half the biodiesel that my F250 does. In other words, it's meeting my expectations, but I know some of these things really shouldn't be ignored. I'm trying to learn what those are...every engine has it's tricks.