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Bad Idle and Hard Hot Start Problem 700 1989

posted by  BobbyC on  Fri Aug 16 11:33 CST 2013
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last visit: Tue Oct 29 08:31 CST 2013
[RELATED]

1989 740GL non-turbo, auto trans, 110k total miles

A few weeks ago I transported the car to our AZ condo. It was running like a Swiss watch, but started to get hard to start when hot, now requiring half throttle to start it after being driven. Cold start is still instantaneous.

Then it started stumbling when put into, or taken out of gear. At that point, once it regained its idle speed, it idled smoothly.

Now the idle speed varies even in neutral, or in drive while stopped. It still runs fine on the highway, but my wife (who is AZ using the car) is freaking about having it strand her. Unfortunately, I am in TX 1000mi away, and because of surgery can't get out there to look at it.

I've done a LOT of reading (FAQs, Forum posts), and am guessing it's probably the Idle Air Control valve (idle stabilization circuit), or remotely, a bad engine temp sensor (under #3 intake runner), but can't be sure, of course. Another possibility is the TPS, throttle position sensor not switching to idle mode.

We see that it is SUPPOSED to have a start injector, but can't find any installed. Anyone know for sure if there IS one on the LH2.4?

The engine doesn't act like it's an ignition or injector problem, as it never has a "hard" miss even under load.

Our son is there and is willing to do some troubleshooting if I can get him data.

From what I understand from extensive reading (and see), the car has the Bosch LH 2.4 Jetronic system, with a TWO terminal IAC. I read the resistance of the IAC motor should be 14-18ohms. Not sure if applying 12v to the terminals momentarily to check operation is permissible. Son already flushed the IAC w/ brake cleaner, but read that it could take several flushes w/ carb cleaner, though he says he can reach in and the vane appears free. Because of age, we're reluctant to pinch off the IAC hose tightly as a diagnostic procedure, but maybe we could plug the ends or something.

The car has a "Check Engine" light installed, but while it has occasionally started coming on during start (it didn't use to), it always extinguishes quickly in a second or so and doesn't leave a trace.

Any personal experiences with such symptoms, or suggestions would be most helpful.

Thanks,
Bob






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