124 300e idle problem

[Flat6]

Hey folks, I am having a problem with a 1989 300e. It won;t idle when cold, and idles at around 1200-1400 rpm when warmed up. I cleaned the idle control valve, tried to make adjustments to the cis, but no luck. The Idle valve is recieving a signal, but will not make a difference to the engines condition if plugged or unplugged. I tried actuating the valve by using small jumper wires, and it clicks.
Now, I am wondering if it is possible the idle valve is not working, even though it clicks when powered, OR, if the signal to the idle valve is inapropriate. Any way to check the signal? What else can be causing this?
Thank in advance for helping a newbie!
Mike

 

[kid-jensen]

flat6,

Sounds like something is stopping the throttle settling back to it's proper position that would give you something below 500 RPM.

If there is something mechanical that causes a 1200 to 1400 tickover, the Idle control Valve won't be able to do anything about it.

It's worth checking that the throttle actually returns properly to it's stop. A tight or incorrectly adjusted cable (throttle or autotrans) could cause this.

 

[Arnie]

1200 - 1400 rpm idle is about 2x what it should be. Check the throttle and the cold-start valve.

 

[Flat6]

Thanks for the rplies guys. I checked the throttle linkage, and it is perfect. The idle switch clicks when the throttle is released and at it's proper stop. When the engine is cold, it won't idle, I have to hold the accelerator pedal for about 30 seconds and then only idles at around 500rpm. It feels as if the idle valve isn't letting enough air in the engine when cold, and too much air when warm.
Mike

 

[GaryC]

Hello,

How about when the engine warm? is the idling speed be normal???

I also have a 300E, but is a 1992, slightly different, but the idling prinicple was simular, not totally the same.

The idelling of the engine when cold, should be around 1000rpm, as the thermo time switich allow more air to flow into the engine, in order to rise up the idling speed, to let the cat warm up, after about 1 to 2 min, it will back to 500 rpm. Cold idling and hot/warm idling control mechanism was different.

I think your model 89' was using a mechanical thermo time switch, I not sure, as mind using electronic control. The thermo time switch have a bimetelic strip inside, as it get hot, it will block the passage, inorder to lower the idling speed. If not the case, it will be using thermo sensor and control the idling control valve, like mind.

For the idling control valve, if it fail, it won't even control the engine speed when it's hot as well, so I think the problem is not the idling control valve; normally if it fail, it can't hold the idling speed, the idling speed will be like 1000 to 2000 rpm even when hot. It principle was like, when the ECU give it voltage signal, it resists the internal spring, to close the valve.... if u remove the power or take it out, you found it will be totally open.

So, please check whether is the Thermo time switch cause you problem, or if is electronic control, check the temperature sensor...

Hope this could help!

Gary!

 

[Erop]

Vacuum leak?

If I were you, I'd check for a vacuum leak - tru to check all the tubes for integrity. Look carefully inside the air filter box. One way is to spray carb cleaner in and see whether the revs go up/down.

 

[Flat6]

Gary, thanks for the reply. if you check my first post, you will see that the car DOES idle very high when hot, and the car runs exactly the same if the idle air control valve is plugged or unplugged. (it does have a electronic valve, like yours) So maybe it is the valve .

Erop, thanks for the reply. I checked for vacuum leaks, but I found none.
Thanks! You guys are great!
Mike

 

[kiwi_bloke]

advice for free

i had the exact prob in a 124....cleaning the idle control did nothing and touching any other setting imaginable didnt do anything...the car had 144 km on the clock...replaced the idle control valve goes like a dream

josh

 

[BadTasteUK]

Would something like Redex or 10K boost resolve such a problem, or does it do more damage than good?

I'm also having idle problems; it sits at around 600rpm but every now and then it dips slightly, just for half a second.

Someone suggested this 10K boost or Redex to sort it.

 

[dieselman]

I'm not sure of the exact layout of the induction piping on your car, but why not try pluging or crimping the pipe from the idle control valve. This will give you the base idle speed with no idle valve in the system.
If the idle is still too high either the throttle is too wide or there is an air leak.

 

[Neilc]

First check for vacumn leaks. I would then check the injector duty cycle. It is possible that your mixture is out. You'll need an automotive multimeter with a dwell % setting to do this. You read pin 3 of the round X11 connector in the engine bay. You should see a fluctuating reading of around 45-55%. Higher and the mixture needs to be leaned out, lower and it is too rich. You adjust by using a long allen key down the lamda tower pushing against the spring action turning clockwise to richen and anti-clockwise to lean out. If you only have a voltmeter you can check duty cycle that way as well - just ask and I'll post.

Could also be a failed coolant temperature sensor. You will have hard starting problems and a cold rough idle and the idle will be kept high, ie the max. opening of the idle regulator. You can check this with a multimeter and see the resistance alter as the engine temp increases. You may have a 4 way temp sensor, the two pins opposite each other you need to measure. One set is for ignition other for CFI. If you see infinity across these pins then it's time to replace the sensor. You will also need to know the resistance values wrt temp to see if the sensor is still in range. I'd suggest searching the archives at www.mercedesshop.com where you'll prob. find the information - or buying a MB CD for your car.

HTH
Neil

 

[Neilc]

Oh and I gather you don't have the ABS light illuminated when the car is running? If it is, then the OVP relay is toast. With no OVP there is no mixture enrichment at cold idle or idle speed regulation.