Fuel Distributor leak caused by clogged return line 1983 380sl
 

I got the fuel pump in, but car dies soon after running (briefly sounded good), so I wanted to check fuel pressure, don't have the 2 tube setup shown in the manual, but thought a plug on top of Fuel Distributor would be a port. As soon as I loosened it a tiny bit, fuel started leaking - when I tightened a bit, seemed to be leaking more. I couldn't find anything about this plug anywhere. There appears to be a thin black gasket underneath (0.010"?). Got a bad feeling it's a pressed in plug.... Does anyone know about this?

I think I found an answer. The hex cap is the closing plug for the 'pressure compensating valve'. It screws in, should have a gasket, and shouldn't leak. I need a very thin wall socket to remove this - it's tight between the FD and plug, and I may have moved it a bit sideways when loosening/tightening it. I'd like to put a copper gasket under it but not sure if that would change height/operation of the valve.

Edit: The gasket was copper. I pulled it out, looked at it (ok aside from getting crushed ~1/3 around it from mispositioning), and put it back - snug but not super tight (brass fitting in aluminum body). I turned on the FP and gas sprayed about 3 ft up (stopped by the hood) and out sideways. I don't think this is normal - will check pressure later today. At least the new fuel pump is working...

Return line to tank was plugged. Disconnected at firewall from cooler, put 4' fuel line on it into a gas can, put the copper washer back under the closing plug with scarred side facing plug - snugged it a little more than before (no strips or breaks), and it started/ran more than 3 seconds for the first time - then died, but no fuel leaks. Had to jumper the FPR. Seems to idle longer (maybe 45 secs) than run with throttle open (~20-30 sec) - will hook up some gauges and go from there.

you will have to now as you have altered the pressures values. also reguarding the leaks , you had no back pressure when you were shooting into the can, so be careful to not assume
you are leak free at this point

Will no back pressure affect system and control pressure readings? Will it eventually make the car die (after the WUR starts working)?

Is there a way I can simulate approximate back pressure? like clamping a piece of 1/4" ID tube inside the 5/16" line I'm using, then maybe pinching it a bit until I get the right flow/backpressure.

This is the way to check fuel delivery (~1 qt in 30 secs), but do you know what the flow rate is for the return line in the tank?

And I thought about the leak not showing now. I'm using tranmission cooler hose (no psi rating on it) that should be able to handle 100 psi (or more) from this fuel pump, but I don't trust the circle clamp connection enough to pinch it off and see what happens. I'll hook it back to the return line at some point to get max pressure (got to assume something could go wrong and get max pressure while driving).

I've never had anything like this happen before - I wondered if the FD was cracked by the closing plug, but it looks ok.

tough to say, I think the only sure way to test pressures is to close the system like designed. Do you think the return is clear now?

I agree about closing the system, but want to check pressure soon and not sure when I'll get the line cleared. I can run the pressure test and slowly kink the drain line while watching gauge.

The return is blocked solid, which surprised me because the strainer just had small "balls" of soft brown crud stuck to it, which would blow out with the pressure from the pump (100 psi+ ??). I read about guys blowing them clear with 10-15psi (after loosening gas cap). There were no major rust particles in drained gas, but maybe major rust stayed in there...

The manual (47.110) shows the 107.04 return line (70) going in from underneath - I see nothing like this on my car. I thought I read return line enters from top near sending unit (in that photo might be in the middle?) where it goes into the "swirl pot" - I assume this is accessed from the top well (I don't want to pull hardtop til I move car in garage).

http://www.justanswer.com/mercedes/2c1gz-11985-mercedes-380sl-spark-plugs-wires-cap-rotor.html
Saw this photo (about 1/2 down post), which I recognize, though it doesn't show where the return line enters the tank. I thought I read a guy cleared his by jamming a wire through disconnected line into tank.

I can look around car for the next connection past firewall. From diagrams, there may be a lot of rubber connecting hoses that could rot/gum/clog up before the line gets to the tank.

go from the swirl tank forward , if you have a way to create a vacuum you may get lucky , or try compressed air in the opposite direction. Did you remove swirll tank , sometimes they rust inside, it should have free airflow in and out when off

My 86 SL would start and run for about five seconds and then stop. I bypassed the fuel pump relay with an aftermarket one and it ran o.k. I later found out that the fuel pump relay gets a signal from the tach. which had a shorted wire caused by a rubbing hood spring which I later fixed, go figure! Good luck.

For vacuum, I just sucked on a rubber hose hooked to the line at firewall - no good; hooked up a fairly powerful vacuum cleaner (pulling through a small setup used to pull cleaner through fuel injectors) - no good. I just got a vacuum pump but will hold off on that for now.

Traced it back, disconnected rubber hose at tank line (goes up and over - can't see where it goes in) and was able to blow line clear from firewall to tank connection.

I thought the swirl tank was inside the gas tank and can't be removed unless tank is cut open. This has good photos.
Fuel Tank Strainer, Fuel Tank Insides, Swirl Pot, Return Line Venturi - Mercedes-Benz Forum

Ah... I jammed a wire (with a crimped end so it'd slide) about 16" into the line (had to pull-push it some) - about 10" was covered with gum. Connected the vacuum setup and started pulling fuel and what looked like chunks of escargo through the return line... bingo! The fuel was dark brown, but eventually cleared (mostly - I only drained, never flushed, tank).

I can connect lines and run it, but would rather not blow this gunk back into the tank (and into my new fuel pump). Or maybe it'll be okay as long as I don't let it set for a long period - and pull pump/drain tank if I do.

I sprayed some carb cleaner into the return line at tank (using paper towel to seal spray tube), could hear it bubbling in tank, and am letting it set. I can also pull carb cleaner from back to firewall to clear gunk (should have vacuumed at tank fitting so as not to contaminate the long steel line - maybe this is what you meant by "start at the swirl tank"...).

Vacuumed after carb cleaner sat and got some "balls" of gunk - there must be a lot in there. If I put some Lucas in the tank, will it eventually/slowly dissolve this stuff (though maybe it would gum up fuel distributor, which is working at this point)

Note: I cut open the old fuel pump and it was full of gelled gunk - motor was stuck but broke free. If this stuff was in the bottom of the tank, it wouldn't have come out when I drained it - wish I'd have stuck finger in the strainer hole and felt around, but this still wouldn't have cleared the swirl tank/return line.

Done for now - will maybe check fuel pressure another day.

Pulled carb cleaner from back to firewall to clean out return line (cleaner was black) - did this until clear vacuum hose was clean. Hooked up return line at firewall, ran fuel pump, and blew gas into a can at other end until it only had a few black flecks in it (varnish loosened from return line?). Replaced rubber hose from tank fitting to main return line. Jumpered fuel pump relay - no leaks (but still need to check with blocked return to ensure closing plug holds pressure). Car started/ran ~10 minutes, died, jumpered FP relay (that relay has intermittent connection), popped right off (need that initial 1.5 sec prime or is harder to start), ran about 20 min (hit about 100C), revved to 2000, temp climbed (think thermostat is stuck), abruptly let off throttle, went back to idle (was surprised it didn't die), shut it down.

Idles/runs fairly smooth (need to check plugs/injectors), light lifter noise comes and goes, intermittent alternator bearing noise, and the power steering pump was buzzing (foamy fluid/pulling air, found loose clamp). Goes into fwd and reverse normally, hunts for idle occasionally, brakes feel good - WUR must work (was concerned about that). I am amazed the FD wasn't gummed up.

Ah - when engine's hot/off, there's a hissing noise up by the thermostat - couldn't pin it down (using gas line "stethoscope") - did not sound like it was way down/under the intake.

Thanks Panda, for directing me to fix that return line. I can spend half as much time rigging something as it takes to fix it (thought I was going to have to pull the tank...).

Good point - will monitor voltages/signals at relay connections while the car's running, then dies...

Edit: Found +12v at socket pin 8 (battery+) drops when relay wiring is flexed. I posted a question elsewhere of how to get the relay socket apart to check connections (don't want an "iffy" connection pulling 6-7 unfused amps).

Just found out this pump pegs my gauge/150 psi max when hooked to (blocking) return line - maybe 200 psi. Surprised it didn't blow the rubber hose from main line to tank return line when it was clogged. There is no leak at the closing plug.

wow , you are lucky, is it an aftermarket pump?

Are you serious? I thought I read a pump should be within MB spec, but can't find a max limit - saw 2 (of 10) that listed specs - 100 and 135 psi max. The 380sl manual only has a min flow test (1 L in 40 sec); most say 1 L in 30 sec - mine was 1.3 L in 30 sec.

The high pressure is why the closing plug leaked when the return line was clogged. When gauge blocked fuel return, I couldn't push the air flow plate down (didn't force it), and it looked like it was higher than normal. Hopefully didn't bend something in there - runs about same, but who knows...

Maybe not an issue as long as the return line is open, and if it ever got clogged, the engine would die/fuel pump would shut off. I wouldn't want such high pressure in fuel system for very long. If I test again (with a gauge that won't max out), I'd hook it to the FD's supply line.

Below is my gauge - past 100 psi, the next 50 psi is equal to about 15 psi on the 0-100 scale - the needle was pointing straight down.

The pump is a Hayg (made in USA). You can find it with a google search.