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1976 280SE Fuel Leak or Evaporation - advice needed

Started by Cloughy89, 26 May 2020, 05:23 AM

Cloughy89

Wondering if someone might be able to help me with a problem. I took my 1976 w116 280SE out for a 20-25 minute drive yesterday, after 9 weeks of inactivity during lockdown. It was superb to be out again!

But ... for a while prior to lockdown, I had been noticing that I was getting very sub-optimal fuel consumption, sometimes less than 10 mpg. I had thought this must be incorrect running of the engine or D-Jetronic fuel injectors, but now it's clear that fuel is leaking or evaporating when the car is not being used. Before storing the car away 9 weeks ago it had a completely full tank, but yesterday the needle had dropped to about three quarters full, and it needed to some 25 litres to fill up after only 15 miles (!!). So it's been losing around 2.5 litres per week just at ambient conditions in its underground car park.

There are no obvious drips from the underside, and no obvious smell of petrol from the cap. Can anyone suggest a series of checks I could do to try and locate the leak? I'm obviously uneasy about driving the car until this issue is fixed, except to take it in for a service/repair. Thanks in advance.

Here's a picture of the car. 😊

Alec300SD

Occam's razor: opportunistic fuel thief, probably a young moped rider.
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Cloughy89

Thanks, this was suggested also on the Mercedes-Benz Club forum. I hadn't thought of this! I've invested in a lockable fuel cap, so I can test out this theory.

ptashek

Quote from: Cloughy89 on 26 May 2020, 09:55 AM
Thanks, this was suggested also on the Mercedes-Benz Club forum. I hadn't thought of this! I've invested in a lockable fuel cap, so I can test out this theory.

That was me. Don't look in the fridge, I'm everywhere ;)
1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE

mrkozzy

Try looking at your fuel line corrosion perhaps,
Changed mine in 2012. Had a similar issue. Look at the date stamp on the original lines I took off!

All looked good from the outside. The rot was from the inside.  Biggest rot was on the 50mm short bits on the injectors.

The last picture shows the new hoses on the injector supply line. I ended up changing EVERY bit of rubber fuel line starting from the fuel tank and working forward.
Got more pictures and info should you need it.
MrKozzy

Cloughy89

Very helpful indeed, thank you! I will take a look at the state of the fuel lines.

revilla

All good candidates for fuel leaking indeed. However, before getting into disassembling lines/components etc (unless you want to do it anyway for fun/renovation) I would focus on identifying the leak source(s).  If you start replacing things you might end up throwing darts in the dark which can be both frustrating and expensive.

Fuel leaks always leave a trace (smell, visible drops or puddles), especially in your particular case the leak seems to be significant!
I would run the engine, car stationary, and carefully look end-to-end for the leak(s). From hoses around the pump/filter/accumulator, 2 lines to engine bay, fuel distributor all the way up to metal lines to injectors.  Do this carefully of course with a fire extinguisher at hand all the time.

Good luck.