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240: too high idle at start up

Mr. V

Active member
Joined
Jul 5, 2004
Location
Portland, Oregon metro
1982 244 Turbo (K-Jet)

Car had no issues until one morning I started it up and a few seconds after it started the revs went up to around 2500: stayed there for a few minutes, then would come down to normal.

Revs would not come down if I tap the accelerator pedal (as they might if the car had a chokeL as a K-Jet there is no choke).

Throttle switch seems OK: it clicks when I slightly open the throttle.

No obvious air leaks.

Any ideas as to what the cause might be?
 
It checks out fine with a VOM.

Oh well, I figure it's a C.I.S. issue: I was hoping for an easy fix.

Time to run elecrical tests per the blue book to see if the problem is with the IAC unit itself or its electronic brain / control box.
 
You may need to clean the throttle body if it's keeping the throttle switch from making contact reliably.

There's a temp sensor in the block to raise the idle when it's cold and the A/C will also raise the idle but neither should raise it that high.
 
I ran the test per blue book, using two jumper wires on the black box while engine warm: idle raced, so IAC unit itself should be OK.

I removed the old black box and substituted a spare black box: now to wait for it to cool down and restart; if the problem is gone then it was the black box; otherwise back to square one.

Thanks for weighing in.
 
The switch is adjusted correctly and clicks when I begin to open the throttle and checks out OK with a VOM.

I cleaned the throttle body recently, so I can rule that out.

When idling high I can get it to stall by clamping down on the idle air valve hose.

Could the valve be bad and allowing too much air to go through at cold start?

What should the correct cold idle speed be?
 
The greenbook doesn't specify a cold idle speed. It does say that the idle is raised based on coolant temp and that a high idle could be caused by a faulty or disconnected sensor.

It's the sensor in position 6 at the back of the block.

SensorChart.jpg
 
If the sensor is disconnected, you will have it racing at 2500-3200 rpm. Essentially, if the box isn't receiving a temperature signal, it thinks the car is at -460 deg F or zero Kelvin, so it will basically short out the four wires, causing the engine to run at a high rpm. It will, however, not drop down to a normal idle, if the sensor is disconnected. It's the two pin sensor on the block, above the starter.

I have seen a 1982 244GL get stuck at 3200 rpm when cold, but then it finally dropped after a minute or so. Suspecting that car may've had a sticky idle air valve, since tapping on the valve a few times did seem to make it drop. Might be worth pulling the valve on your car, holding it up to a light source, then swivel it back and forth to see if the internal air slide opens and closes.
 
I'll pull the intake manifold and check it out: no other easy way to do it.

I wondered how a bad temp sensor could lead to a high RPM and the above explanation clarified things for me.

Thanks fellow' brickers!
 
I pulled the intake manifold to allow easy access and verified that the coolant temp sensor is bad.

When checked with a VOM it is way out of spec, nearly dead.

It's only a year or two old, an aftermarket sensor from IPD: this is twice now in the past couple years that a nearly new aftermarket IPD-sourced coolant temp sensor has failed (the other in my 740 Turbo).

I am fitting a used / spare Bosch sensor I had on hand, that should do the trick.

Thanks.
 
Fini:

Put it back together, fired it up, purrs like a kitten with a used Bosch sensor installed.

Lesson learned by this clusterf%&k: avoid aftermarket sensors.
 
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I've even had bad Bosch sensors, right out of the bag. Bought a new LH-Jet temp sensor for my father's old '84 245. Swapped it in, and the computer ignored both the oxygen sensor and the air mass meter. Plug and part number on both sensors matched. Eventually, after the car blew its head gasket, I replaced the head with a B23FT head, and the car actually ran properly, with that head's original sensor.
 
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