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740 Fast Idle Problem 700

posted by  jimnau on  Sat Jun 27 14:43 CST 2015
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last visit: Sun Jul 6 09:52 CST 2014
[RELATED]

Thank you, Chris.

Yes, I have disconnected and capped the vacuum line to the brake booster. No change, i.e., the engine still idles high. Yes, the car has an EGR. Hadn't thought of this, but I am not sure the EGR affects idle. I will look into it.

I have also checked the AMM and TPS according to the suggestions in the FAQ:

AMM
1. Good ground. Continuity between connector terminal 1 and intake manifold ground.
2. Measured 2.5 volts at idle between terminal 3 and ground. Seems OK according to the FAQ.

TPS
1. Disconnected the TPS, no change in the fast idle. Threw OBD code, of course.
2. Jumped each outside connector terminal to inside ground terminal. No change in fast idle in either case.

I remain stumped, but I am gravitating towards a faulty idle circuit in the ECU. I read somewhere in the FAQ that the 556 ECU is not a good one. That is the unit I have, and it is 24 years old.

Jim
--
Jim Nau '91 740 178K






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