1990 300se. Running nicely now with the new injectors and plugs but idle is REALLY low and has a hard time gaining stability on cold start.

Couldn't find any "idle adjustment" information in searches. Is there a simple adjustment on this car?

Thanks!

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'90 300SE --- 173k miles --- (Odometer just stopped working!)

'85 Cali 300D --- 193500 miles --- (Second one) Sold Nov '10

'82 300TDT --- 236,xxx miles --- (My first one) Sold July '09

There are a gazillion things on the CIS that can cause a rough idle, PLUS a vacuum leak of some sort.

I mention this one because I just went through it. If the airflow position sensor is not correctly adjusted, it will cause a rough or low idle.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

NOO! That's not what I wanted to hear! Just say, "Hey, no big deal! Just turn that screw next to the headlight switch 2 times and sprinkle some glitter on the radiator" or something!!

Now where the devil is that airflow position sensor? EDIT: my lizard-brain forgot that I knew what this was. Awesome. Thanks lizard-brain.

Anyway is adjusting it difficult? It does look a little cruddy... I wonder if a cleaning would help?

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'90 300SE --- 173k miles --- (Odometer just stopped working!)

'85 Cali 300D --- 193500 miles --- (Second one) Sold Nov '10

'82 300TDT --- 236,xxx miles --- (My first one) Sold July '09

There is NO IDLE ADJUSTMENT on the constant injection system (I'm pretty sure) you have. Idle speed is controlled by the system, as, indeed, are we all!!!

Others have made some good suggestions but you must make sure of some basics:

Good HT components: plugs, leads, rotor, cap. All MB original or Bosch/Beru. NO CHEAPO RUBBISH.

No vacuum leaks in the inlet - this is a 'must have' condition. If you have vacuum leaks, all other adjustments are a waste of effort.

Check the fuel mixture using a Duty Cycle meter ($40-ish? over there). Set it to 50%. Loads of advice on this on the boards.

Make sure your fuel filter and air filter are good.

Start with the cheap things before you start worrying about metering units, injectors and other costly stuff.

OVP relays can cause grief - another cheap and easy fix.

Bonne chance.

RayH

PS: Oh, I just noticed. You've replaced the plugs. I hope you got the cheap, copper ones from Bosch, Beru or NGK. Don't buy platinum ones and certainly not ones with resistors. Again, loads of advice on this here and elsewhere.

i had that problem on my 300se;low idle rpms and a cold start issue , turned out to be the OVP relay with a bad solder joint at the Zener diode , resolderd the joints and the rpm went up and one key turn cold start.
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something I just noticed: The "economy" gauge sits in the red until I hit about 1200 RPM at which point it falls to about halfway. I'm assuming this might happen since the pump is dumping a lot of fuel into the injectors to prevent the engine from dying? Any ideas? Should I grab that OVP and resolder it for good measure?

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'90 300SE --- 173k miles --- (Odometer just stopped working!)

'85 Cali 300D --- 193500 miles --- (Second one) Sold Nov '10

'82 300TDT --- 236,xxx miles --- (My first one) Sold July '09

The Economy gauge doesn't measure fuel useage, just vacuum depression in the intake manifold. You can be in Economy Gauge Green with some serious cruising velocity at the wheels
Your idle speed doesn't sound much different to the hot idle speed on my Euro 300TE (same engine), but cold idle should be about 1000rpm at first. The only time we didn't get this was when there was a difficult cold starting problem that turned out to be the OVP relay had failed and was not switching on the cold start fuel injector.
General rough idling is often worn engine mounts on those M103 engines.

i assume you have cleaned out the distributor cap and checked the rotor for tracking .The carbon contact can break .
Best to fit new original Bosch not Uro type brands . Fresh Non -resistor plugs and leads will help,the spark plug connectors can spark internally if dirty or ageing.
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I would check the distributor cap first. Cold Start valve is hardly EVER the issue on these and the OVP deal typically doesn't cure anything (I replaced mine and fzzzzt...nothing different). Take it out your OVP and try to run it. That is what it would run like if it was bad unless it does have a thermal issue or cold solder but just buy a new one vs. wasting your time with that. I wouldn't chase that gremlin yet. 1000 rpm when you first start cold. Idle should be around 600-700 RPM after closed loop cycle stops. It sounds like a vacuum issue. I had all of that idling problem, adjusted the air mixture, and threw lots of money at it. New vacuum lines and idle air valve hoses finally eliminated the problem. Hard starts are typically distributor cap (and mine looked fine when I first checked it). The idle would get rough when car got hot because the hose cracks would open up more as the engine got hot. To start check the neck of the hose that goes into the front of the idle air valve and see if it isn't hard and cracked where it fits on. Put some electrical tape around it and see if it helps.