Intermittent idle problem

[Chrisheg]

'69 280SL, mechanical fuel injection, Version 1 electric fuel pump

I have a problem that only occurs after the car has been driven for some time, say 1/2 hour or more. If I come to a stop with the car in gear it starts to drop RPMs in a cyclic way. Every 10 seconds or so it lugs down to about 3000 RPMs for a couple of seconds. Eventually it stall. It idles OK if I take it out of gear. If I shut down for 10 minutes or so is runs fine again.

This car was driven very little for about 10 years. I have done the following things to try to correct it he idle problem:

Drained the fuel tank and lines and replaced the fuel filter
Replaced the fuel tank strainer
Added injector cleaner to the fuel
Adjusted the linkages per the manual
Adjusted the idle air screw and injection pump idle screw with a tach and a vacuum gauge.

I have the original tall fuel pump and it is making a sort of high pitched cyclic whine after running for a while.

Any advice would be appreciated.

[jameshoward]

I had a similar problem with my car. It occurred after the FI pump was rebuilt/altered to solve a separate problem.

I did the linkage tour (JA17's tour on this site) and it wasn't much better. So I did it again, enriching the mixture each time, as my pump was adjusted to lean it out by the shop. By the third linkage tour attempt, and having made (I think) 4 turns of the idle mix screw at the back of the FI pump (WITH THE ENGINE TURNED OFF) I could set the idle very accurately indeed using the idle air screw by the butterfly.

I would recommend having another go at the linkage tour. Read the plugs first and let us know if the car is running rich or lean. I found I was running very lean after I reinstalled the pump. That could be your problem.

I also have the old fuel pump; it has a very discernible whine when running.

J

[Chrisheg]

I would recommend having another go at the linkage tour.

Quite right, I had not set it up right on the first pass. I readjusted everything including the constant speed solenoid. It's idling better now. If I stop in gear after a fast run it does not stall but if I shift to neutral it stalls in about 1 second. Does not stall if I shift to park or any other gear except neutral. Does that mean something in particular?

BTW, is the venturi linkage arm supposed to be perfectly straight? Mine is not straight.

Forgot to look at the plugs, I'll do that next time I work on it.

[Cees Klumper]

This may sound too obvious, but if yuor engine stalls if you put it in neutral, and not if you keep it in whatever gear, then the solenoid is doing its job, but the idle setting without the solenoid (i.e. neutral) is too 'low' i.e. you need to increase the idle speed in neutral, and decrease then the amount of action you get from the solenoid when you put the car in gear (or else the idle speed will be too high when you put it in gear).

[ja17]

Hello, 
Don't over look tuning issues like ignition timing. Also the engine should idle nice and smooth. Check the engine compression if you are not sure about the engine's health. I always do the final idle check , with the car in drive (someone holding break).