IAC stepper motors question

[Joe Frickin' Friday]

Oilhead bikes did not have any electronic control over engine air flow at all. Working in your garage, you set the idle speed to ~1050 RPM by adjusting a big brass air bypass screw on each throttle body, and that was that. The amusing thing was when you left your low-altitude home (for example, here in Michigan) and went cruising at high altitude (for example, summiting Pikes Peak), and the reduced air density left the engine gasping: idle speed fell as low as 900 RPM.

 

I have not yet had the pleasure of high-altitude riding on my R1200RT, so I'm curious to hear from others. Are the hexheads designed/able to keep their idle speed despite wide variations in altitude, or does their idle speed fall off at high altitudes like the oilheads? Or are the IAC motors strictly intended to balance air flow to both cylinders, and aren't intended to also control idle speed?

[dirtrider]

Morning Mitch

 

The IAC’s or stepper motors have the ability to control and hold idle RPM at varying altitudes as long as the amount of air needed to do so doesn’t exceed the air passage size and/or total movement of the stepper pintle. This is built into the basic design of computer controlled active idle system. In closed loop hot idle the system will just command the proper fuel to be added to the incoming air.

 

What I can’t tell you is how active the cold start IAC control is. In open loop cold start at altitude the steppers will go to a pre set count open from seated for cold start enable. This is usually based on engine temp/AIT temp and battery voltage. What I don’t know is if the BMW BMS-K has the ability or programming to also use the BARRO or atmospheric sensor to attenuate the pre crank stepper position to facilitate high altitude initial fast idle. Once started and running it should be able to keep the engine running at high altitude but it’ possible a rider might have to manually hold the throttle slightly open at high altitude to get a stall free cold start.

 

Most computer controlled fueling systems have the ability to lean out pretty far at altitude for fueling compensation, same with spark control. Where the problems are usually encountered are at very low altitudes as that takes a richer mixture and the rich end is usually capped at somewhere around 2-4% fuel trim to prevent cat. damage if the system defaults on a defective sensor. This isn’t usually a problem for most land based motorists as there just aren’t that many below sea level extreme cold start opportunities.

 

BTW, did you get your FPC replaced without any problems?

 

[Joe Frickin' Friday]

BTW, did you get your FPC replaced without any problems?

 

It's at the dealer right now; I'm expecting a call from them today or tomorrow.

[Twisties]

We live at 4400' and typically ride at 4000' to 11000'. When in CO we can exceed 11000'. We've never noticed any change in operation with altitude, other than the expected power loss. I'm sure we have experienced cold starts at 7000' to 8000'. No issues. The only times I have ever felt the need to apply gas after a cold start was below 40 F with 20W50 oil. I now run 15W50 or, when I can find it, 10W50 and avoid that problem and improve cool weather cold start lubrication.

[Twisties]

P.S. Get that bike out here already!

[KDeline]

One of the few things that we did not have problems with on our trip this summer was running and idle. We were in temperatures from 103 to 45, and sea level to 13000 feet and the engine never hiccuped once, all very smooth and in normal range.

[lawnchairboy]

Haven't ever noticed much difference riding where I live at sea level or on CO-550 or over the mountain in Torrey. Temps from !rackin' hot in Gila bend to !rackin cold at 10000+ getting snowed on. Computer must be smart.

[Grumpy-ol-Fart]

I've ridden from here in Oklahoma at 900' to Colorado-Wyoming-Idaho-Montana, traversing passes as high as 12000'...I haven't had any issues or noticed any wierd behavior at all...sure do get great meileag at those altitudes...