Idle air m116 3.5, how's it work?? - Mercedes-Benz Forum

 
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post #1 of 9 (permalink) Old 01-25-2014, 04:39 PM Thread Starter
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Idle air m116 3.5, how's it work??

Hi fellow enthusiasts, I'm wanting to know how an m116 controls idle speed.
The throttle body on my car has I port above the butterfly and 2 small drillings below. The drilling at the rear is being used for vac advance for the ignition system and the one in front is blocked off with a plug.
The main port runs to a valve which looks thermostatically controlled. The valve is always open and does not close, possibly u/s.
There is another valve with a mechanical screw. I think like a power valve. Fully closed this valve allows air to pass also. I would think you could screw this closed and stall the engine but it's not the case, anyone know about these components??


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post #2 of 9 (permalink) Old 01-26-2014, 11:43 AM
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The big one with the screw is your idle adjust. The other one acts as an air bypass valve, though the nomenclature escapes me. If you go to the online resources portion of the forum, there are articles uploaded that will give you an in depth overview of the engine.

Aside from obvious displacement differences between the 4.5 and 3.5, both are nearly the same, and the fuel, gas and air management systems identical.
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post #3 of 9 (permalink) Old 01-26-2014, 07:05 PM
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cold start air is thermostatically adjusted as the engine warms up the thermostat closes off the extra air and idle speed returns to normal.The idle screw is used to fine adjust the correct idle rpms ,likely 750.
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post #4 of 9 (permalink) Old 01-26-2014, 07:09 PM Thread Starter
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So if the thermostatically controlled valve is not working, I will likely have a fast idle hard to slow down? The idle races a bit but it does slow down as the car warms up


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post #5 of 9 (permalink) Old 01-26-2014, 09:58 PM
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[QUOTE=Pizing82;7546305]So if the thermostatically controlled valve is not working, I will likely have a fast idle hard to slow down? The idle races a bit but it does slow down as the car warms up

I assume you have checked for leaky air pipes, and quite a few on the top , likely the thermostat is faulty or binding due to scaling .
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post #6 of 9 (permalink) Old 01-27-2014, 09:34 AM
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the air slide valve goes bad with time , it does no longer shut off on a hot engine , so you end up with a high idle .
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post #7 of 9 (permalink) Old 01-27-2014, 06:15 PM
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The engine rpm may hunt up and down as well, due to a bad valve.

Before replacing that thing, check/replace vacuum lines and their connections. This fuel delivery system relied on vacuum to feed input to the engine control unit. When I bought my 108, it ran like crap. Replacing the vacuum lines made a big difference (replacing the condenser with a pertronix unit was another big factor).
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post #8 of 9 (permalink) Old 02-08-2014, 06:29 AM Thread Starter
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I boiled some water and popped the valve in. I saw the plunger move up but when I blew through the valve it was still flowing quite easy. Any ideas who sells a replacement valve at a reasonable price??


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post #9 of 9 (permalink) Old 02-08-2014, 09:29 AM
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I bought mine at Napa Autoparts. I'd suggest calling around to see who might have them stocked.
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