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Re: engine light on
The best thing to do is have the computer memory read out. I think it has detected a problem, recorded in the computer, and put the engine light on to tell you it did that. Codes in the computer will point the mechanic to the cause.
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When you add an accesory like fixing the A/C unit now that it comes on,it puts more demand for the engine to idle higher to sustain a idle,your solution is to trouble shoot the idle control,either do a throttle body cleaning or a new idle control may be needed.
a better question is why is idle so high? could be a vaccum leak or other issue. Adjusting the idle circuit is not the answer. Since the check engine light did come on perhaps you can retrieve a trouble code...maybe even a bad tps
You will have to get the trouble codes read from your car, to know exactly what is causing the check engine light to illuminate and the rough idle. Most major auto parts stores will read the codes and give you a report printed out, for free.
Code 46 is indeed the trouble code for the Idle Speed Controller which is that big black box thingy on your throttle body. A lot of Chilton-type manual books omit that code because the ISC isn't found on pre 94 Metros. The ISC is a computer controlled auto-adjuster for the engine idle.
An important note: code 46 is often caused by misadjusted distributor timing. As you probably know, timing affects the idle, and if improper timing moves the idle beyond the ISC's adjustment range, the check engine light will come on and diagnostics will return code 46.