when tinkering with the car i unplugged the iac and the revs steadily increased to an acceptable level,i'd previously been suffering idle problems,this cured it for about a day then the old problems returned.Can i check the iac and how?

I just replaced the IAC on my 85 740 Turbo. In my case it would stick sometimes causing the idle to run too high. Hitting it with a screwdriver handle would make it move back to the correct idle. After I put the new one it, I could turn the air conditioner on and off at idle. It would quickly bump the idle RPM to 900, and within 10 seconds or so after turning it off the RPM would be back down to 750 or so. I think that is a pretty decent test to see if it is working. It has to turn quite smoothly because any stickyness will affect RPM. Cleaning my old one did not work. Of course you need to be sure you have taken care of any vaccuum leaks first, but see what it does with the A/C turned on and off.

Another problem is the idle may be set incorrectly...you have to ground a wire on the rhs of the car (check your manual) and turn the big black knob on the throttle body to the correct idle..your car may have the idle speed burned into the ecu in which case there is no knob, no setting...the ecu does it for you with a functioning iac valve.