Hi, Have you discovered the source of the hissing?

I've experienced the same on my 1979 450SLC, in the past. Car would run OK, but would suddenly develop severe loss of power accompanied by loud hissing. Then the problem would go away.

Now, after sitting for a few months, the car will not start, and the mechanics found very low flow to fuel injectors (primary fuel pressure is 75 PSI, control pressure on cold motor is 18 PSI, both apparently in spec, ruling out the Fuel Pump and the Warm Up Regulator as culprits) along with polluted fuel coming from the one of the fuel distributor injector lines (the one they opened).

My guess is that the upper chambers in the FD contain polluted fuel with gunk and sediments. On a running motor, this might throw off the FD fuel metering and interfere with the control pressure, thus forcing the air flow meter plate into a different position (in relation to the air flow) – probably to be very close to where it sits at idle - by allowing the FD plunger to position itself too low within the FD in relation to the fuel demand of the running motor. The combination of fuel demand, at given RPM, with an almost closed off air-flow sensor plate is what I suspect is the cause of the hissing. This is consistent with a severely degraded power output at those times.

In my case, if the hypothesis of the problem is correct, I will need to bleed the FD in hopes of dislodging the polluted fuel. It is likely that the injectors have also plugged up and will need cleaning or replacement. In case of you motor, the cure is probably the same, assuming the diagnosis is correct, except that in your car the situation has not gotten to be as bad as in mine.

I’d appreciate any feedback.

Cheers

__________________
Michael Stoic

1979 450SLC
1989 300SE