Volvo Maintenance Hints for 7xx/9xx - Bill Garland's Nuclear ...

also noticed a bare temp-sending wire, which he cleaned and re-taped (but we don't
suspect that has to do with this failure.) Per past list advice, I've suggested
❍ examining all vacuum lines
❍ rechecking fuel-pump relay and socket for cold-solder joint/overheating
socket
❍ in-tank pump and screen
❍ the power stage connectors for corrosion
❍ radio suppression relay
❍ general condition of wiring harness (the car lives in the very heavily
salted road conditions of northern Ohio)
[Diagnostics and Suggested Fixes:] Below you'll find several procedures [Composite
from Steve McChesney and others]. Hope they will help.
Fix 1: Clean the Idle Air Control ("IAC") Valve. See above for procedure.
Fix 2: Rebuild Idle Air Control Valve: Save $150 by fixing IAC instead of
replacing it. (Note: the IAC on newer LH 2.4 cannot be dissassembled but it
can be cleaned.) Also note that Regina and Bosch IACs are NOT
interchangeable if you are considering a replacement.
Symptoms:
■ No fast idle at start up.
■ Grounding CIS (test point) does not have any effect on idle.
(grounding the test point should disable air control valve (IAC)
■ OR: The idle ('87 745ti) shoots up to about 2500 RPM. No
amount of cooling off, throttle blipping, or general search for
vacuum leaks would bring it down. I checked all the hoses, fuel
pressure, mass air sensor, and coolant temp sensor. Everything
was fine. And the car ran fine too, just a high idle.
This is a common problem on cars that use this Bosch system, including Volvos
and BMWs. It's especially bad on turbo cars, and cars that use crankcase air
for idle makeup because of the oil that comes along with the air.
Solution: clean the slip ring inside the IAC The IAC is a metal can beneath the
intake manifold, with two 1" hoses that feed extra intake air around the throttle
body to control idle RPM, and reacts to loads like A/C or heavy electrical
(alternator) loads. The "valve" has three pins, the center is a constant 12V
supply, and the outside pins are pulsed by the ECM to ground, to either open or
close a circular shutter -- looks something like a revolving door. (Except mind
would only open, and close halfway.)