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Greetings Brickboarders,
I have not posted in a long time because until now, I have been able to figure out everything on my beloved 744 using the superb FAQ on this site. Frankly, not much has gone wrong in recent years, until now that is. This is a rather long post in order to provide as much detail as I can. Hope someone can offer a solution.
Car: 1991 Volvo 744, normally aspirated, Bosch LH2.4, 199,000 miles
Short summary: Fast idle problem in Park, but only after the engine has fully warmed up and car has been driven. New IAC valve (one year old), clean TB, and new, properly adjusted TPS. ECT sensor resistance vs. temp is exactly according to spec (measured last Feb.) Year-old Volvo (Bosch) AMM. No OBD codes.
Start car and allow to reach normal operating temperature (NOT). Measured input voltages to IAC valve by back probing connector
1. In Park, AC off = 4.4 volts
2. In Park, AC on = 4.9 volts
3. When electric fan kicks in, voltage increases to 5.0 volts
4. Now in Drive, AC, electric fan on = 5.4 volts
5. In Drive, AC off = 5.0 volts
5. Returned to Park, AC off = 4.4 volts
Throughout these tests, the idle speed is normal, and remains normal afterwards for a long time (10 minutes or so). Hard to tell exactly from the dash tach, but it is 700-800 rpm. Tentative conclusion: The ECU is receiving appropriate input signals and applying appropriate voltages to IAC valve.
Next, pinch off inlet hose to IAC valve. Voltage increase to 7+ volts, and idle drops significantly. Tentative conclusion: No major vacuum leaks.
Now drive car. Backed out of garage and drove around the block (literally, very little driving). Pulled back into garage. After 30 seconds to a minute, the IAC input voltage drops to 2.6 volts and the idle increases to 1100-1200 rpm.
Turn off car, wait 10 seconds, restart. Normal idle and 4.4 volts, but in less than one minute, the voltage decreases to 2.6 volts and idle increases to 1100-1200 rpm.
I am stumped...my guess is a faulty idle circuit in the ECU. Perhaps it thinks the car is still in gear after being driven and put back in Park. Can't find an ECU input related to this from the Bentley 240 Service Manual for the LH2.4 system.
Any ideas?
Thank you in advance,
Jim
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Jim Nau '91 740 178K
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