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Old 10-09-2017, 02:13 AM   #1
Mr. V
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Default Sudden max revs: B21FT

I was driving tonight in my '82 244 Turbo with a B21FT when for no reason, while driving at a slow speed and gradually accelerating the motor immediately and unexpectedly raced to max rpm.

I shut it down, pulled over, started it up: same thing.

Waited a couple minutes, tried again: same.

The throttle cable seems fine, not kinked, in normal closed position with engine shut off.

WTF?

I suspect the throttle must be stuck wide open, but I wasn't on the throttle much at all when it happened.

Oh well, I'll try to sort it out tomorrow in the light of day.

A new one on me; any ideas as to what it might be?
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Old 10-09-2017, 02:41 AM   #2
MasterBlaster
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I vote for big vacuum leak.
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Old 10-09-2017, 11:49 AM   #3
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Check brake booster hose.
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Old 10-09-2017, 11:58 AM   #4
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Vote #3 for vac leak. That spring returns the throttle plate to closed. You'd have to have something wedged in there or some real black magic fvuckery to make that happen.

Brake booster hose or the brake booster check valve are my guesses.
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Old 10-09-2017, 07:09 PM   #5
smeha
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Theoretically, would not he have problem stopping then?
Brake pedal soft?
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Old 10-09-2017, 07:53 PM   #6
PromiseRing
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This happened to me one time. But on a 2.4 car that doesn't act quite the same as kjet.

My issue was one of those little clips on the intercooler couplers had come off and got lodged in the throttle plate. It eventually found its way holding cylinder 4 exhaust valve wide open..

Probably not your porblem, but it might be worth checking that nothing is stopping the throttle plate from fully closing.
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Old 10-09-2017, 08:06 PM   #7
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I've encountered this problem with 2 other 240T's... Both times the issue was the CIS (Constant Idle System) ecu gone bad.

It resulted in a very high idle (upwards of 2k), and the only way to bring it down was to pinch off the IAC air hose.

No real good way to test other than a known good "Leerllaufregler" That appears to be the name of the part/ecu that controls the IAC. It's the smaller of the 2 ecu's in the passenger footwell, and has 2 connectors.

HTH-
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Old 10-09-2017, 08:26 PM   #8
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OK, problem discovered and solved.

The idle control switch on the throttle body had loosened a bit, such that its dangling little flat lever piece would sag too far.

This caused the outside cam (hooked to the throttle shaft) to bind to it when it rotated, preventing rpm's to go down.

Loosened and repositioned the switch, works fine now.
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Old 10-09-2017, 08:50 PM   #9
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Good find. Always glad when it's an easy fix.
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Old 10-10-2017, 02:07 AM   #10
John242Ti
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That also would cause it to "idle" at 3200 rpm or so, even if it hadn't hung up on the throttle lever on the shaft. When the switch opens, the CIS motor is told to open up all the way. Dealt with this on my 245's old engine, when the throttle switch somehow had gone out of adjustment on the B21F intake manifold I'd installed.

-J
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