Subject: 1990 Montero IAC and Stalling
Question:

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Followup To
Thank you for your response, Anthony. All good suggestions. I tested my fuel pressure regulator, and gas cap, both are fine. I inspected my ECU and found it to be flawless. By back-pinning the ECU, I foud that of the four pins which are supposed to fluctuate +/- 3v, at idle, one seems to be always on at + 3v. This does not explain, however, why the previous ECU and IAC had the same problems. Can you think of anything else that would cause this recurring pattern? As for the stalling, I don't see how it could be related to this. I have driven, with this IAC, for some 5,000 miles without a problem. The stalling is recent, sudden, and always after restart. Something seems to be giving out. The items that you'd suggested tested okay, can you offer any other ideas as to what would cause stalling after restart? Thanks again, Anthony.
-TMD
Answer: Toby,
It's really tough to guess why you'd be measuring a +3V signal on a pin that is suppose to fluctuate. It may not be the IAC or ECU that causing it to stay high. The ECU may be getting a signal from another sensor that is causing it to stay high.

As far as your stalling, I still have an idea for that. Check your oxygen sensor and your catalytic converter. The normal aging process will eventually cause the oxygen sensor to fail. However, the sensor may also fail prematurely if it becomes contaminated with lead from leaded gasoline, phosphorus from excessive oil consumption or silicone from internal coolant leaks or using silicone sprays or gasket sealers on the engine. Environmental factors such as road splash, salt, oil and dirt can also cause a sensor to fail, as can mechanical stress or mishandling.

A dead sensor will prevent the onboard computer from making the necessary air/fuel corrections, causing the air/fuel mixture to run rich in the "open loop" mode of operation, resulting in much higher fuel consumption and emissions.

An additional consequence of any oxygen sensor failure may be damage to the catalytic converter. A rich operating condition causes the converter to run hotter than normal. If the converter gets hot enough, the catalyst substrate inside may actually melt forming a partial or complete blockage. The result can be a drastic drop in highway performance or stalling because of a buildup of backpressure in the exhaust system. Have those items checked.
Good luck!