Does M30B32 Idle Speed Increase Near Overheat Condition?

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Joey28
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Does M30B32 Idle Speed Increase Near Overheat Condition?

Post by Joey28 »

My '83 533i currently has a bad fan clutch; I can stop the fan by hand when temp needle is at center. But as the car gets hotter the idle speed increases. Conversely when temp needle is more at approx normal 1/4, idle speed is slower. Condition has existed through two idle control valves (I replaced the first bc I thought it was bad, didn't realize clutch was slipping, and I'm still a little new to M30 idiosyncracies), but now I wonder...
I've got a Sachs fan clutch on the way and will report any differences after repair, but I wonder if anyone else has any experience like this?
There's many posts about idle control valve troubles, but none I can find relating idle to temp.
Thanks!
DFWTX85
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Re: Does M30B32 Idle Speed Increase Near Overheat Condition?

Post by DFWTX85 »

I'd assume the idle speed increase is due to the presumption that the fan clutch is working and would work to increase the cooling fan rotation speed. When you say that the motor gets warmer/hotter, is this during the warm-up phase or consistently as the motor is at NOT and cools and warms?
Shawn D.
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Re: Does M30B32 Idle Speed Increase Near Overheat Condition?

Post by Shawn D. »

Is the idle control ECU functional? Yes, it's separate from the main ECU on 533s.
ThirdPedalNirvana
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Re: Does M30B32 Idle Speed Increase Near Overheat Condition?

Post by ThirdPedalNirvana »

The 533i uses a different system for idle control than the later Motronic 1.1 ECU's. Unlike the 1.1, the idle control is not integrated into the ECU, so the temp signal the ECU reads can't be used to alter the idle speed.

When the engine is cold it needs a fast idle. In order to do this, there is an additional device screwed into the thermostat housing on 533i's that isn't there on 535i's. It's a switch that is open below 50C (or maybe 60C I don't recall specifically) and closed above that temperature. The idle control computer uses that signal to know whether or not it is in fast idle.

When your idle is high, its worthwhile to check the operation of this switch. If you have a multimeter you can check the resistance, if the engine is hot it shouldn't have much resistance at all. If you don't have a multimeter, try disconnecting the two wires attached to the switch. If the idle speed increases, then the switch is probably working properly. If the idle speed is the same, try putting a jumper wire across the two wires you disconnected. If this slows down the idle, you've got something going on with your idle speed control switch. Unfortunately this switch is very hard to find an ridiculously expensive if you do, but you can build your own by drilling out the housing of the existing switch, getting a generic temperature switch, and gluing it in place with thermally conductive epoxy (PM if you need more details on this).

When I got my car, someone had put in a fan control switch in it's place, so when my car was warmed up to operating temp it would alternate between fast and normal idle. If I was just climbing a hill and came to a stop, it would normal idle. If I had been coasting and came to a stop, fast idle.

Of course its very possible that your issue is unrelated to this switch. If you have no cold start issues and no fast idle issues when the engine is at a normal temp, its possible the switch is re-opening above a certain temp, but unlikely. But it's useful to know that the "fast idle" setting is controlled in such a simple way. If the two wires are connected, its standard idle, if the circuit is broken, its fast idle.
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