High-Low-High Idle on Startup-350SLC - PeachParts Mercedes-Benz Forum

SatRoc · #1 08-29-2010, 07:58 PM

When starting my '74 350SLC, the idle will fluctuate between 750 and 2000 RPM and then back to 750. In other words, it's running up and down until the engine warms up. Removing the air cleaner and pinching the hose running from the throttle body and the supplimentary air valve will give me a steady idle. Pinching the hose between the supplimentary air valve and the idle speed air distributor gives the same result. Replaced both parts with the same from a 450SL motor that was running normally when removed from a salvaged vehicle. It had no effect on the problem. Any suggestions would be deeply appreciated!

mbdoc · #2 08-30-2010, 09:23 AM

Most likely the idle speed warm-up thermostat is bad. (air by-pass)

alabbasi · #3 08-30-2010, 10:30 AM

Yep, what he said. Try squeezing on the hose that goes to the idle air valve and see if it makes a difference.

SatRoc · #4 08-30-2010, 05:32 PM

Thank you both very much! Am I to assume that this is the "cold start valve", located on the idle speed air distributor, p/n 0 280 170 022?

Tomguy · #5 08-30-2010, 07:29 PM

Not the cold start valve itself, that's an injector. The air valve itself.

This could be bad, but it could also be a maladjusted throttle plate (not closing too much, letting too much air in at idle), idle air speed screw (again, too far open), or bad idle mixture. My problem on my 4.5 was getting the mixture "Just right" without a CO meter so that cold idle didn't hunt, warm idle wasn't too high or low, and hot starting wasn't a prolonged affair. It took about 3 months of fiddling to get it just right (although, to be fair, I was adjusting the MAP too which does throw off the idle mix without even touching the ECU screw). I also needed my idle air mix valve open a little further so transmission fluid didn't get sucked into the engine in massive quantity once the diaphragm on the modulator blew.

SatRoc · #6 08-30-2010, 08:25 PM

Thank you Tomguy. Unfortuately, I'm really confused now. Are you referring to the supplimentary air valve between the throttle body and the idle air mixture control unit? By the way, the idle adjustment screw has no effect on the idle speed.

Gurunutkins · #7 08-30-2010, 10:30 PM

yes he is the supplemental or auxiliary air valve has a hose from the throttle body to it and from it to the idle screw block. when cold its open and allows extra air in to keep the engine running with the cold start valve open, once the engine starts to warm up the aav closes and reduces the air and slows the engine. when its stuck open it lets too much air in and the engine starts to race at 1800 rpm with no throttle showing on the tps the relay will cut the fuel pump, fuel pressure drops the engine starts to die the sensor says revs are below 1800 so starts the fuel pump again etc etc and you get the rev to 1800 and then try to stall effect.

as Al says squeeze the pipe from the throttle body to the aav and the engine should slow. if thats not it try the throttle plate as Tom says and also check the tps is set correctly

it looks like this on a 74 d jet car
cheers
barri

SatRoc · #8 08-31-2010, 12:37 AM

Thanks Gurunutkins! That's exactly where the problem is. I'm going to try the "boil/freeze" trick and see if that cleans her up. I still think that there might be problems in the idle block which I'll deal with next. This car has the electronic injection system, but will be converted to Mega-Squirt when I have the time to do it. She still has the 3.5L Euro engine which should handle the upgrade well. Will be using your conversion article as a reference guide. Thanks again!

Gurunutkins · #9 08-31-2010, 02:10 AM

just saw in your first post that you have replaced the aav with a probable good one so try the freeze / boil and if that doesnt work use a hot plate to heat some kerosene or diesel (not above 60 deg C - I prefer diesel and its safer) and see if it will move with those.

if the little slider in the aav is moving but the searching remains and everything else is OK at warm idle - test your vacuum at idle once warmed up and let us know also check your tps is set for idle as per the 450 manual instructions - after those simple things you are into the work Tom had to do.

I also converted my euro 350sl - 116.982 - and she runs like a rocket so if you need the msq file just shout. I will eventually get to EDIS it one of these days - have the MS2 kit - the edis module - the 36-1 wheel etc etc just need time.
cheers
Barri