That isn't a vacuum line, but I don't know what it really is.

But a theory, since that line goes to the valve cover and it leaking causes low idle, maybe your oil filler cap is bad. I've had that problem on another car. I never would have guessed but the right crankcase pressure is important, I think it's a smog thing.

Thanks mate will check it again doesn't look like it's leaking though but it's trial and error currently! Idles nicely without the ovp plugged in around 900 in park and 600ish in drive. Cold starts suck though! My cis manual mentions a gear lever switch position switch but cant find any reference to it not sure if they mean the neutral safety switch?

And to theorize further
 

It might be the valve cover gasket. They do get old and hard and crack and need semi regular replacement.

Just got the car back from mechanic after installing new lifters mate. new Valve cover gasket was put in with the job.
I do have a tiny exhaust manifold leak could that be causing any problems?
have ordered a new gasket set just to be safe though.

Check the air meter plate for proper deflection and make sure it isn't binding in the bore.

Try inducing a vacuum leak by unplugging a line that isn't switched or valved. See if the idle picks up and smooths out, indicating that you are correcting a rich condition.

Verify your cold start injector isn't leaking. It's somewhat common for them to act up when the cold weather really hits. In warmer climates, they don't get used for most of the year. Then the first cold snap hits and they stick open or partially open after firing for the first time in a year.

Other possibilities:


1) Bad OVP relay. Though this usually shows up along with inst. cluster lights. However, rough, low idle symptom is identical as the IAC no longer functions.
2) Dribbly injector. Old injectors can start leaking when they warm up, or anytime for that matter. This would create a rich condition in one cylinder, the 02 sensor would tell the computer to compensate for this resulting in one rich and 5 lean cylinders.
3) Failing fuel distributor. The diaphragm inside the distro gets stretched to the point where it starts to block the outlets in the (upper I think) chamber.
4) A vacuum leak in some place you haven't yet looked. There is a hose that accepts the cold-start injector, and also connects to air intake ports underneath the exhaust manifold. It's hard to see and a total pain to replace. When it starts to leak, intermittently at first, rough low idle is common.

To follow up on what Dux said:

Put a small bit of vacuum line behind the idle microswitch contact arm (so the computer does not think the motor is at idle) and see if you still have the problem.

thanks guys will check the microswitch...
The hose from the ICV that excepts the cold start injector is new and i do agree total pain to replace, i ended up removing the air flow meter to get to it!
replaced the boot under there while i was at it.
unplugging a vacume line doesnt really do anything with the idle, maybe a little bit rougher but hardly any difference to idle.

unplugging the TPS increases the idle to around 900 and smooths it all out. not sure if thats relevent. checking with a multimeter seems to give me the correct results though.

Our M103 (300SEL) idles around 650 when warm (in drive).

The cold start injector gets used everytime the engine starts when at ambient temperatures, i.e. car not run for several hours.

mine idles around 550 in drive and pretty much the same in neutral (575-600 max) when shifting in to park it revs to around 800 and then slowly comes down to about 550.

Just did a cold start injector test and it works fine :( the injector stops spraying after 5-10 seconds.

just feels like a sensor thats sending the wrong signal to the ECU. unplugging the OVP gives me a nice high ~900-1000 idle in park and about 700 in drive, which is a bit high but good as a mechanical only idle i think? (i presume the ICV valve will be in open position with no electrical input from the ECU hence the higher idle?) cold start sucks with the OVP unplugged though!

Hmmm. That's odd. Mine (on my M103) doesn't fire unless its fairly cold at ambient. My temp sensors are tested and accurate. Strange. Perhaps this is why I often have to give it a little gas on startup.

Sorry if I mislead anyone.

thinking its the crank shaft position sensor now? probably the only sensor i havent replaced along with the EHA. might have to bite bullet and replace it? i'm completely lost for things to check now.

crap not the crank shaft sensor! :( i give up!!!
anyone know if theres a sensor for the CIS from auto box to indicate which gear its in? my problem is the idle speed in drive and park are exactly the same (~550rpm) when shifting from drive to park or neutral the idle doest change up liike it used to....

I would like to assure him that there is no need for him to worry about his idle. It is only natural that his idle stays around 500. In my case the idle normally is in the range of 500 to 700. When I showed it to a mechanic he told me that it is perfectly normal and that I need not worry about it.

vacuum leak
 

I would check for vaccum leaks especially at the breather hoses-I had one a bit loose and the car would not start at all. I read that one must always check for air leaks and in my case that was perhaps the best advice i have received. A vacuum leak does not have to have a raised idle if there is too much air. Consider what happens when the car sucks in too much air and not able to compensate by adding fuel...It stalls.