Idle fluctuates intermittently

[lbocock]

I have a 1991 190e, 2.6 Mecedes. Recently, I have experienced that the normal idle rate of 700rpm wil suddenly rise to 1200-2000 rpm and stay at that level. Sometime it stays at that rate unit I turn off the ignition. Sometimes it comes back to normal on its own after several miles of driving.

Sometimes when I start the car it starts idle at 2000rpm and then drops to normal on its own. After driving for a while it will suddenly increase again,

Any ideas what could be causing this? One suggestion was that the "MAS Control Unit" might be defective. Unfortunately, this is an expensive unit ~$400US and once the unit is purchased it's yours (no return).

 

[whitenemesis]

Malcolm! over to you...

MAF is not that expensive (in the UK at least)

 

[television]

You should have KE injection your fault should be the switching from the idle relay over to the main ECU, or the OVP relay could be playing up.

What are the first 6 VIN numbers WDB201---

 

[Ellsy Tanners]

Hello,
You may have a sticking adle speed control valve. Thats common on the 103 engine. Try a gently tap on the valve whilst engine running this can some times free it momentertly and adjust the idle speed, if this is ur fault!

 

[television]

Hello,
You may have a sticking adle speed control valve. Thats common on the 103 engine. Try a gently tap on the valve whilst engine running this can some times free it momentertly and adjust the idle speed, if this is ur fault!

This then will be the rotary Idle valve just to the left of the mixture control unit.
the idle switch is on the front of the air flow sensor, and should measure zero Ω in the idle position.

The over voltage relay should have 12 volt on pins 87 tapping this with the engine running could make the fault

 

[hmang]

This then will be the rotary Idle valve just to the left of the mixture control unit.
the idle switch is on the front of the air flow sensor, and should measure zero Ω in the idle position.

The over voltage relay should have 12 volt on pins 87 tapping this with the engine running could make the fault

could make the fault ...do what malcolm ?...happen ?or go away?

if this is a californian model car, will it be fitted with a load of ultra lean running emissions gubbins too ?

 

[Bob.R]

Had the same problem on my 92 300E-24 - Turned out to be the idle microswitch not closing properly, therfore not signalling idle to the ECU.
Took it off cleaned & reset it - problem solved - cost = £0 !