MY 1984 Volvo 245 Turbo had very high revs (over 2500) after the turbo was replaced. One mechanic thinks it is a damaged wiring harness. I read on this website that you can test if it the Idle Air Value by pinching off the hose going to the IAC. I did this and it cuts the idle greatly. I want to know what damage I will do to my car if I just clamp the hose and solve the high revs in this way?

The same thing happened to my turbowagon once. I used a ball valve in line with the hose and adjusted for a warm idle of about 1K rpm. It didn't like to idle on cold mornings and I eventually traced the problem down to some shorted wires at the firewall like Jessie said to look for. So I ran with the valve for about a week but was able to electric tape up the bad wires and it has been fine ever since.
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Volvo Farmer:

20 Volvos '58-'91

It won't hurt the car to pinch the IAC off, but it won't idle on cold days till it warms up. It will keep stalling for about 5 minutes. I drove mine this way for about a year. You just have to keep your foot on the gas at idle for about 5 minutes on cold days. I got very good at using my right foot for the brake and gas pedals at the same time.

Check to make sure its temp sensor wire is connected. If its off the control unit will think the engine is really cold and set the idle high. The wire harnes does degrade on the turbos. Mine was really bad, including the IAC wires. The grey plug on the exaust side of the firewall is pretty much all wires for the IAC, and my wires disintigrated right around that connector (heat from the turbo)

Check for vac leaks, set the Base idle, You should determine the cause. Is the mechanic versed in Volvo b21fti engines they are easy to test. get Bentleys manual. good luck
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john,, 1985, 245:Ti, 251k mi..so.california,,,1974, 145, 230k mi. ..