I have a 74 450 SEL. Finally have gotten it to run really good- replaced 2 injectors, advanced the timing (doesn't ping).
But there's one problem ( I can live with , but it isn't right)
When it idles it surges dramatically. Once before this was "cured"by retarding the timing alot, but then the car was almost undrivable(missing, sluggish etc.) I'm happy with it now except for the surging (does it cold and hot if it's in neutral)
Also turning the idle screw has virtually no effect.
Any thoughts? Like I say the car has plenty of power now
(don't think it's starving for fuel).Haven't played with fuel injection box on the right passenger compartment. Should I? (The car no longer has to pass emissions)

You are probably experiencing a proper condition called decel cut-off. The conditiona are something like this: the idle switch is closed and the idle is above some amount (the amount depends upon the temp).

It is probably about 1500rpms on a hot 4.5 (was less on 3.5's and they always experience this phenomemon. When these conditions exist the fuel is shut off because the car should be driving down the road with a decel in mind to have those conditions. After the fuel is cut it is restored when the idle drops below say 1200 thus the up and down in an exact manner.

You will need to get the rpms down. I like timing and since timing will make sure a 3.5 decel cuts off at idle when cold, I have done a run around on a couple of them. One can disassemble the throttle switch and disconnect (break) the idle switch contact. After this the throttle will never be seen as closed and never will decel happen.

Or..... you find out why your idle is too high at that timing. 4.5s don't have this problem unless way out of spec. I would look at the auxliary air valve if you can't turn the idle down. Either that or an air leak somewhere.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanks Steve. Would the auxiliary air valve be the same as the supplementary air valve- the "column" thing next to the idle adjust. screw?
Where is the throttle switch (with the idle switch contact)?

Yes, thats the one.

The throttle switch is on the throttle shaft straight below the aircleaner. To prove the concept just unplug the switch. To separate out just the idle contact will require disassembly or removal of that pin from the connector.

The car will work fine for testing with the switch unplugged.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

If you can't get the idle down with the idle air screw adjustment, I'd say you have an air leak somewhere, perhaps the AAV itself, or the hoses between the idle control system (AAV and idle air screw) and the intake manifold.

might look here:

http://www.peachparts.com/shopforum/showthread.php?threadid=74837

Can you buy these hoses somewhere ? I would like to replace the Y hose as well as the other emision hoses. Mine are getting kind of old and maybe causing my idel problem ?

I'd pull the manifold. Replace injector seals on all the injectors, check out the AAV (probably stuck), replace the idle air hoses and the seals between the manifold halves. Replace the rubber parts of all the vac lines, and replace the hose to the MAP -- these are all probably leaking to some extent.

Clean the idle valve, and when you put it all back together, set the idle to keep the fast idle with the AAV all the way open less than 1500 rpm. With no vac leaks, this should be possible. May lower the idle hot in gear to about 500, but if you don't have stalling problems, this will work fine.

That surge is a real PITA, eh?

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!