Can anyone suggest what I might do to diagnose and eventually solve the following problem:

My 1981 300TD stops running when the tank gets to a little under 1/4 full according to the gauge. If the problem were just the gauge I wouldn't be concerned, but removing the drain plug at this point and a good 10 or 15 litres of fuel came out, indicating that the gauge is correct. This means I'm losing about 100km range on my tank which is not insignificant doing country driving here in Australia.

I suspected/hoped that the fuel delivery and return lines had been swapped (the person I bought it from had it serviced pretty regularly but not by a Merc specialist) but have confirmed that this is not the case. I'm assuming the problem is with the tank, but maybe it could be further up the fuel line? Though that seems pretty unlikely as the fuel gauge reading is very consistent when it happens. Other information: I'm running it on veg oil, but the behaviour began when it had only ever been driven on Diesel, and still happens exactly the same when it is running on Diesel.

I'm hesitant to drop the tank out of the car, given the reports of how difficult a job this is, and having no guarantee that it will accomplish anything.

Any suggestions would be greatly appreciated!

Had the EXACT same problem with my 240d. However, keep in mind the sedan has a different tank than the wagon. I think. Anyway, only thing that finally stopped it, unfortunately, was taking the tank out of the car and having it cleaned out as well as replacing the strainer screen and the hoses. I also cleaned out the steel lines with some mineral spirits.

I had also used bio fuel, which to my understanding has properties of a detergent. What i'm guessing happened was that after 30 years of being driven with petrol diesel, as well as periods of sitting, a fair amount of residue accumulated on the inside of the tank. That in itself can cause driveability issues. When that biodiesel is added to the mix, this substance in your tank is loosened off the walls and sent into the strainer. What I think happened with our cars is that the screen was partially blocked, so that the fuel could get through the top but with lesser amounts of fuel was restricted through the dirtier parts on the bottom, which would then send particles through the fuel filter.

It wouldn't hurt to do, even if it's not the cause of this problem. I would check the fuel filters first if you haven't already, a clogged one can cause mysteriously intermittent symptoms in my experience.

oh yeah.

even though it probably doesn't matter, you could also put this in the diesel forum. more people might check it out.

OK thanks again, that sounds promising.

I should have mentioned that when I drained the tank, the strainer was very clogged. After cleaning it, I was pretty sure that I'd solved the problem, and was surprised when it made no difference whatsoever. I guess I should take it out again just to see that it hasn't immediately reclogged.

Cleaning the tank is sounding like the best option, and probably best done with the right equipment. I'll see if I can find a friendly mechanic who'll do it with the tank still in the car - I'm not too concerned with the state of the carpet in the back.

I'm a complete newbie here at PeachParts - not sure whether reposting on another forum might be considered bad form. Maybe I'll give it a few days here first.

Well in that case welcome to the forum!

If you have the advantage of owning a diesel I'd post in the diesel forum as it is a vibrant community over there - on the whole I think you'll get a quicker response.

X2 on the tank screen - that's a good place to consider first.

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I wonder if you could have an unusually large vacuum as a result of a restriction in the tank breather system?

I recall an old caveat that it's easy to reverse the fuel hoses on a W123, and the return line doesn't reach as deeply into the fuel level as the supply line. The car runs the same until the fuel gets low in the tank. Just something else to check.

clogged vent pipe: i resoved it by spraying in the vent pipe ,no more vacuum whoosh on opening the cap.
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I'll assume you've got correct usage of the model designation and by TD you mean "wagon" and not turbo diesel. The wagons have different tank layouts and, if I'm not mistaken there's some sort of "tank within a tank" for them. I think chad300td had some posts/threads that described this.

And, when dealing with these cars, age and condition, are you sure about the accuracy of the gauge? On my coupe I drained the tank with the low light on and there was still 5 gallons in the tank. 10-15 liters sounds like less than 1/4 tank. The service manual says 70 liters full and 11 at reserve for the T-sedan (wagon).

If you've got a 300TD wagon, I'd recommend doing a search for "tank" in posts by chad300tdt and review a few of those threads. He's sketched the wagon's tank and strainer. It's significantly different from the sedan/coupe.

How does the tank screen work on a 123 wagon

Also, how much fuel do you add to full when at 1/4 and having fuel starvation? If it's close to the 3/4 quantity then your gauge should be okay, but if it's more than 3/4 of the tank capacity then your tank is probably okay and your gauge is off and you're running out of fuel. 10-15 liters is close to the reserve limit. The fuel senders can really gunk up, so cleaning one may be a reasonable step in any case.

W123 300D Fuel tank sender question

Oh - and welcome to the forum.

Should the cap not "whoosh" when you take the gas cap off? My 300E gasser is under pressure and makes that sound when I remove the gas cap. I have read threads that it should...?

Thanks for all the feedback. I haven't reposted on the Diesel forum as I seem to have got a good response on this one.

In reply to the various questions, the car is a real 300TD, ie a non-turbo Diesel station wagon.

I've never noticed any sign of pressure when removing the fuel cap, so have discounted the blocked breather hypothesis.

I've never made an accurate measurement of how much fuel the tank takes once the car stops running, but have confirmed 10-15 litres remain in the tank at that point. So the fuel sender seems to me to be pretty accurate.

Those diagrams of the inside of the tank certainly make things clearer. I think the next step is 1. confirm (again) that the fuel delivery and return lines haven't been swapped and 2. clean the strainer and possibly the tank. But I'm unlikely to get around to that before next weekend.

I know this is off topic but I'm trying everywhere to find an answer to this sending unit question...Does anyone have a diagram or know what wires get connected to which terminal on a 1980 300TD sending unit? I took mine out to clean and the connector fell apart. I tried to put them on where I thought they were, but now the gauge doesn't work at all. If I touch the unit to the side of the hole it will spike the gauge up to full. I'm thinking I have the ground wire connected to the wrong terminal. The reserve light now works though. Any help would be appreciated.

I still haven't looked any further at mine, but if and when I do, I'll try to remember to take a photo of it for you. But you'd probably do better to keep looking for answers as I have many more urgent tasks.

I took the sending unit out again and I noticed that the 2 thin wires that worked to measure the height of the float were not centered between the 2 metal tabs on the either side of the float. I reguided the wire between the tabs and put it back together carefully without jarring it around like last time and all works perfectly. Thanks for everyone's input!