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I have a 1999 toyota tacoma 70,000/mi. While idling, at random intervals, truck surges forward. Have to stay on the brake. Cannot drive in winter. Took to Toyota dealer and other mechanics… described problem and that A/C seems to be on all the time. No one has been able to fix. My brother disconnected the A/C and surging has stopped, but I need a better fix that will allow my using AC.

Assuming A/C works when it was not disconnected, something is wrong with the electrical there. Get the dealer / mechanic to troubleshoot that circuitry.

Kind regards and good luck,

Has anyone checked the mass airflow sensor (MAF), idle air control valve (IAC), and looked for a vacuum leak?

If your car has automatic climate control, the control unit may have gotten flaky. If not, can you answer as to whether the light that indicates the A/C is on lights up when the A/C remains on?

Surging has numerous potential causes, as mentioned above. Besides the above, if anybody has changed the idle adjustment screw (if you car has one) to correct an idle speed problem, that could be the culprit. That adjustment has be be within a narrow range otherwise it confuses the ECM. And vacuum leaks, any unmetered air getting into the intake manifold can cause this. Like a leak in the brake booster diaphragm.

One clue to consider though in your particular case is the link w/the AC. On my Corolla anyway, according to my service manual, when the AC is on, the idle speed is increased via a solenoid-activated gadget that opens up an air passage and allows more air into the intake manifold. The ECM activates it I think when the AC is on. Why? B/c the AC compressor loads the engine, so the idle speed is increased to prevent stalling. In your case there may be something wrong w/all that and it is increasing the idle speed when doing so isn’t required. That’s probably where I’d start.