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high idle: '86 740 Turbo

Mr. V

Well-known member
Jul 5, 2004
Portland, Oregon metro
After fixing a no-start I now have a high idle.

Stays 2000 to 3000 RPM through warm up.

Idle comes down when I clamp the air hose to the IAC.

I cleaned the throttle body and adjusted the TPS and the idle screw per spec.

Problem occurs with both a used and a new IAC.

Suggestions as to cause and fix?
I think I found the problem, but I am a bit baffled.

It appears unwanted air is getting into the engine, but for the life of me I cannot name or explain the purpose of the part which seems to have failed.

It is what appears to be a vacuum operated valve (vacuum line to intake manifold) and is mounted on the top, front passenger side of the engine.

Held by a bracket which has three bolts to affix it to the head.

The valve has two fairly fat hoses going to and from it; one attaches to the fat air intake hose, before it gets to the turbo, and the other attaches to the fat hose from the turbo to the intercooler.

None of my manuals or prior experience with turbobricks can help me name this thing or even describe its purpose.

WTF is it???
If it's this thing it's the compressor bypass valve.

I verified that my CBV will not hold vacuum.

Looks like a Forge 008 would be my best replacement?

Meanwhile, would there be any likely problem if I simply don't use the valve by plugging up the two hoses that attach to it?

I don't have one in my '82 244 Turbo, and it has a Garrett turbo, so I suspect it isn't critical?
You'll be ok to drive with the hoses plugged. But I wouldn't do any abrupt letting off the boost if you can avoid it. I installed a Forge valve on my 242 turbo using the pipe from a 740. I've had the plastic bosch valve fail and it causes a massive bypass of intake air at idle and lowered the vacuum as if you were opening the throttle. Low boost happened as well. My Forge valve has lasted for many years at this time.

On a 240 turbo there is the small spring loaded valve in the throttle plate. But it's not adequate to the task.
The problem with the idle was not fixed by removing the CBV.

When cold, it starts immediately and revs between 2500 and 3000.

If I "blip" the throttle it comes down, briefly, to 1500 to 2000 for a second then climbs back up again.

I swapped a used TPS which ohms out OK, but maybe it and the original TPS are bad?

Still no solution.

The TPS, IAC and coolant temp sensor all test OK, and checking the wiring with a VOM showed nothing wrong.

Spraying with carb cleaner I find no air leaks.

I disrconnected the wiring harness to the IAC and there was litt
I disconnected the IAC and there was little change in the high idle.

Suspecting that air was getting through the IAC, I used a 9V battery attached to two wires and put one lead on the center of three prongs on the IAC, and the other on an outer one; this presumably caused the open IAC valve to close, blocking the passage of air through it.

Success of a sort: idle came way down, dying: I used the black knurled knob in the throttle body to set the idle to around a grand.

So now it idles OK, but the IAC wiring is not hooked up.

Not sure what to do next as all the wiring and auto parts test OK: guess I'll put it in the hands of a more experienced tech.