'89 Buick Century - IAC or ECM problem???


Old 08-09-06, 10:28 AM
Thread Starter
Join Date: Jun 2002
Posts: 54
Talking '89 Buick Century - IAC or ECM problem???

I have an '89 Buick Century, AT, with a 3.3l engine that wants to stall at stops, and hesitates and "bucks" as you drive it uphill. Lower reading than normal on voltmeter and "Service Engine" indicator light comes on every so often. Sometimes I need to keep my foot on the accelerator at stops to keep the engine running. Had a similar problem twice before, one time it required replacing the IAC valve, the other time, a new ECM (computer). Id like to do the repair myself this time, but am wondering how to determine which device is causing the problem. Ive been told the ECM cannot be returned once I purchase it, not sure of the return policy on the IAC.

Removed and cleaned the IAC valve, cleaned the orifice it gets installed in, reinstalled the IAC, and still have the trouble mentioned above. Would a bad IAC cause the "bucking" and hesitation with uphill driving, or should I be focusing in on the ECM?

thanks for any suggestions.
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Old 08-09-06, 06:16 PM
davzack's Avatar
Join Date: Jun 2006
Posts: 304
The first place to start is with that Check Engine Light as the vehicle may have already identified the offending culprit...all you have to do is get the car to tell you what's broke.

You can jumper terminal A to terminal B of the diagnostic termial under the dash and get your vehicle to flash the codes. Terminal A and B are the top farthest to the right pins on the connector. Turn the engine off, jumper A to B, then turn the engine on but do not start the engine. The check engine light will flash long flashes for the first digit of the code, followed by a short pause, and then quicker flashes for the second digit. I believe each code will be flashed 3 times. Oh, Code 12 will flash first...it's just an indication that you have entered the diagnostic mode...the real codes will follow after code 12.

Here are the code descriptions:

89-91 3.3L VIN N MFI

13-Oxygen sensor voltage stays between 0.35 and 0.55 volts for 30 seconds with a throttle angle above 4% after engine had been running for at least 40 seconds.

14-Coolant temperature sensor signal indicates a temperature of over 284 F for 4 seconds after engine had been running for 10 seconds.

15-Coolant temperature sensor signal indicates a temperature colder than -38 F for 4 seconds with the l engine running.

16-ECM sees a battery voltage above 16 volts for more than 10 seconds.

21-TPS voltage was above 1.95 volts for 5 seconds when the MAP sensor signal showed a high manifold vacuum or the TPS was above 4.9 volts at any time.

22-TPS voltage was under 0.24 volts for 4 seconds when ignition was on.

24-Vehicle speed sensor signal showed less than 3 MPH for 40 seconds with engine running and transmission in high gear.

26-Quad-driver sense voltage shows a low voltage when battery voltage should be present or if the sense voltage shows battery voltage when a low voltage should be present.

27-3rd gear switch was closed or grounded for 10 seconds when vehicle was in 3rd gear, or drive gear was open when engine was first started.

28-3rd gear switch was closed or grounded for 10 seconds when vehicle was in 3rd gear, or drive gear was open when the engine was first started.

29-4th gear switch was closed or grounded for 10 seconds when vehicle was in 4th gear, or drive gear was open when the engine was first started.

31-Park/Neutral switch was open for 4 consecutive starts, or Park/Neutral switch was grounded with transmission in high gear and vehicle speed was above 45 MPH for 12 seconds.

34-MAF sensor did not give any signal for 4 seconds when engine was running.

38-Brake switch signal was not present when vehicle speed changed from above 35 MPH to 0 MPH 23 or more times.

39-TCC was commanded by the ECM when transaxle was in high gear for 15 seconds but engine speed did not indicate that the TCC was engaged.

42-The EST signal did not change when the ECM applied bypass voltage to the ignition module.

43-Knock sensor voltage was either above 3.5 volts or below 1.5 volts for 20 seconds.

44-Oxygen sensor voltage was under 0.25 volts for 4 1/2 minutes of closed loop operation.

45-Oxygen sensor voltage was over 0.75 volts for 2 minutes of closed loop operation after engine had been running for at least 1 minute.

46-Power steering switch was closed while vehicle speed was over 40 MPH for more than 25 seconds.

48-Vehicle misfired. TPS was between 0.58 and 0.93 volts, engine RPM was between 1500 and 2500 and vehicle speed was between 50 and 60 MPH for 30 seconds.

66-A/C pressure sensor voltage was either above 4.9 volts or below 0.1 volt for 25 seconds.

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