All of a sudden reving up and down when cold
 

On my 72 450 SL - Djet, when cold ,and cold only ,it revs up and down like someone is pumping the gas pedal. When is warms, - no problem - Strange . Any ideas

Give the idle screw a tweak clockwise, the idle is too high and the ECU thinks you must be engine braking and is cutting the fuel.

ahh , engine idle in park should be what cold?

Should be same idle speed cold or hot,
that is the purpose of the air valve (the can type thingy at the front of the intake manifold) which gets its input from the coolant passage, The thermostat can affect this.
This system relys on everything being correct.

I humbly disagree. My '73 D-Jet revs around 1200 cold then gradually decreases to 750 or so. That is the function of the Auxiliary Air Valve that you described. It allows more air into the throttle body when cold then closes as the engine warms.

Sorry I don't have an answer to the unusual idle you are experiencing. I will do some research.

That is also an option. Screw it in all the way counting the turns so you can return it to the original position as a starting point. Then unscrew it and clean it. I have heard of contamination on the adjuster causing problems. Idle warm should be around 700 to 800. I like to split the dif at 750. Hopefully when clean the screw will need adjustment engine warm.

This isnt a modern idle control valve managed by the ECU its a wax filled piston that expands as the water temperature increase and slowly reduces the air bypassing the main throttle. The idle screw sits alongside and allows additional air to pass. It's basic and doesnt provide a perfect idle under all conditions.
From memory I think the overrun circuit kicks in at 1,500 rpm but as already said 800 should be about right when cold and the outside temperature is low, any higher and your giving the trans a big thump when you engage drive.
I'm guessing your local weather has improved recently and this has caused the cold idle to be higher than during the winter.

yes it has warmed up here, I will give it a cleaning and adjust it down, just gotta find the time (l0l)

Ron,

Did you figure it out?

yup, a slight turn of the screw did the job. I actually turned it too much at first, stalled at the first light?
Jumped out with my trusty screwdriver , backed it off, all good
I love these cars.....

The surging is a common problem on the D-jets. Mine used to do that. You do have to reduce the revs to below 1500rpm when cold, but it would be better to get them down to about 1100rpm or less. Then, sometimes the rpm with the engine hot is too low.

The problem is usually a sticking AAV. I had mine out a few times, cleaned it, lubed it, squeezed it etc. I now squirt some WD-40 or similar in before I put it to bed for the winter. I also drive teh car more. All of this helps. If the AAV is free to move, it soon adjusts the idle as it should.

My car is now at ~1100 rpm cold and 700 warm. In drive it is lower - maybe 550rpm. BUT, if I stop for a short while, say for a coffee, the idle sometimes goes way too low - less than 500rpm. I think what happens, is that without any coolant flow through the block, the bulb on the AAV gets hotter and cuts off more air than when running. Have to pump pedal to keep her running sometimes, but only for a few minutes.

Gurunutkins suggested putting in a manual air control valve in series with the AAV - I still need to find one - they have them on mid 90s Volvos and Saabs that are motor driven and can be adjusted using a dash mounted switch.

AAV
 

Sir, please tell me what part is the AAV ? thanks Richard

See below - It is an air slide valve that is actuated by a bulb that penetrates into the coolant space at the top of the block. As the bulb heats up, it pushes the slide upwards so as to restrict the air flow.

when the screw is removed to clean , will coolant leak? or is it sealed in some fashion

The idle screw restricts an air passage only, no water involved. The AAV is the bit sitting in the water.