idle control unit — a 2-minute freakout
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Joined: Jan 2007
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From: Mercer Island, WA USA
idle control unit — a 2-minute freakout
I was holding a steady ~80mph on the highway for 45 minutes yesterday and then pulled off to fuel up and get some lunch. When I finally got out of gear, idle shot up to ~3100rpm and pegged there. I'm just glad it didn't head for the redline. I flicked the throttle, futzed with the cruise stalk, and put her in/out of gear several times but nothing brought the revs down. It was like doing a run-up on the taxiway in a Cessna!
Finally, after killing the ignition and letting the car sit for a couple of minutes, idle settled out nicely back at the traditional ~850 level. So, granted, it was COLD here yesterday — right around 0F — and this was the first long trip I've taken in such bitter conditions. The idle control valve is supposedly two years new, so I think the part is probably decent, but what about its mechanical action would be prone to freezing? I'm grabbing at straws here.
In other, contrary experience, I had no problems at all with my trip home in virtually identical (but, uh, dark) conditions. Love to know what y'all think happened.
Finally, after killing the ignition and letting the car sit for a couple of minutes, idle settled out nicely back at the traditional ~850 level. So, granted, it was COLD here yesterday — right around 0F — and this was the first long trip I've taken in such bitter conditions. The idle control valve is supposedly two years new, so I think the part is probably decent, but what about its mechanical action would be prone to freezing? I'm grabbing at straws here.
In other, contrary experience, I had no problems at all with my trip home in virtually identical (but, uh, dark) conditions. Love to know what y'all think happened.
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Joined: Jan 2007
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From: Mercer Island, WA USA
Good theory. Humidity was very very low yesterday and no precip was falling while i was driving, so unfortunately, it's unclear to me how moisture would've found its way into the unit. (Car is garaged when parked, too; I'm no cruel master.)
It's the cylindrical unit atop the intake maniifold, mostly sheltered beneath the rear decklid opening. (I think.) — Just making sure I'm referring to the right part, exposure-wise.
It's the cylindrical unit atop the intake maniifold, mostly sheltered beneath the rear decklid opening. (I think.) — Just making sure I'm referring to the right part, exposure-wise.
Nordschleife Master
Joined: Sep 2004
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From: not in HRM
Could be just a little ice on the butterfly / bore .
General Aviation
Accident Prevention Program
U.S. Department of Transportation
Federal Aviation Administration
Tips On Winter Flying
"Throttle ice is formed at or near a partly closed throttle valve. The water vapor in the induction air condenses and freezes due to the venturi effect cooling as the air passes the throttle valve. Since the temperature drop is usually around 5°F, the best temperatures for forming throttle ice would be 32-37°F although a combination of fuel and throttle ice could occur at higher ambient temperatures. "
General Aviation
Accident Prevention Program
U.S. Department of Transportation
Federal Aviation Administration
Tips On Winter Flying
"Throttle ice is formed at or near a partly closed throttle valve. The water vapor in the induction air condenses and freezes due to the venturi effect cooling as the air passes the throttle valve. Since the temperature drop is usually around 5°F, the best temperatures for forming throttle ice would be 32-37°F although a combination of fuel and throttle ice could occur at higher ambient temperatures. "
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Joined: Oct 2003
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From: PA
I had a very similar experience with my 96 993 when I first purchased it. I was driving through northern IN for a couple of hours with the cruse on. It was February and probably less than 20 degrees. Approaching a toll both, I pushed the clutch in and the revs shot up to over 5K. It freaked me out and I shut the car off and sat there a bit while my heart rate went down. I started the car back up and all was fine. I never had it happen again for the three years I owned it.
Racer
Joined: May 2007
Posts: 274
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From: Malaysia
Carburetor ice on aircraft is an old problem and has killed quite a few people over the years. 
The ice forms downstream from the butterfly and basically constricts the intake passage. The cure is carburetor heat to melt the ice. The heat comes from a heat exchanger similar to the ones used on a P-car motor but is directed directly into the carburetor. Simple and efficient but the catch is that the motor needs to be running to generate sufficient heat to melt the ice. Volumes have been written on this problem in aircraft.
That said I doubt that is what happened to you. I would guess that some water froze near the end of the throttle cable.
regards,
JNeteler
The ice forms downstream from the butterfly and basically constricts the intake passage. The cure is carburetor heat to melt the ice. The heat comes from a heat exchanger similar to the ones used on a P-car motor but is directed directly into the carburetor. Simple and efficient but the catch is that the motor needs to be running to generate sufficient heat to melt the ice. Volumes have been written on this problem in aircraft.
That said I doubt that is what happened to you. I would guess that some water froze near the end of the throttle cable.
regards,
JNeteler
Track Day
Joined: Jul 2007
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From: Pewsey, Wiltshire, UK
While carb ice has killed lots of people in aircraft, there it only applies to carburated engines.
I'm a newbie here - lurking research ! - but I thought the 964 was fuel injected ? Or does the ISV have a fuel/air mixture passing through it which may, at partial settings, cause the idle speed to be radically higher ?
I'm a newbie here - lurking research ! - but I thought the 964 was fuel injected ? Or does the ISV have a fuel/air mixture passing through it which may, at partial settings, cause the idle speed to be radically higher ?
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Nordschleife Master
Joined: Sep 2004
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From: not in HRM
Throttle ice and carb ice are not same same .
"Fuel ice forms at and downstream from the point that fuel is introduced when the moisture content of the air freezes as a result of the cooling caused by vaporization. It generally occurs between 40-80°F, but may occur at even higher temperatures. It can occur whenever the relative humidity is more than 50%. "
"Fuel ice forms at and downstream from the point that fuel is introduced when the moisture content of the air freezes as a result of the cooling caused by vaporization. It generally occurs between 40-80°F, but may occur at even higher temperatures. It can occur whenever the relative humidity is more than 50%. "
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Joined: Jan 2007
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From: Mercer Island, WA USA
I like the ice on throttle cable theory now (carb ice, while something I've always feared in my rental Cessna, shouldn't be at issue here) as it helps explain why, mechanically, this would stick, but stopping for a bit and letting ambient engine heat clear it would fix the trouble. Who knows where the water originated from. The car had been sitting for almost ~2 weeks, albeit indoors.
"What are you doing, Dave?"