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Toyota Repair/engine bucking issue

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QUESTION: 1985 Toyota 4Runner 4 cylinder efi  The engine will buck/slight backfire  when the a/c is on.  Turn the a/c off and the bucking goes away,  runs normally.  I have tried it at various speeds and with the a/c fan at various speeds.  I have an aftermarket electric fan installed.  All connections look good. All vacuum hoses are in good shape and attacked.

Thanks

ANSWER: When the a/c is on there is extra load on the engine which may cause weak ignition parts to fail, I would recommend doing a basic tune up using new plugs, wires ,dist. cap and rotor

---------- FOLLOW-UP ----------

QUESTION: All that you mention has been done within the past 4 months.  I have checked the plugs, they look fine.  I have checked the dizzy cap and rotor,  fine.  The plug wires and coil wire al look fine,  no corrosion.

Two things to to note:  It runs fine at idle with a/c on,  no hesitation or missing. Only starts bucking when in gear and rolling.  Secondly,  when it starts bucking I hit the a/c off button and it stops bucking immediately.

Any other suggestions?

Answer
It's possible that the a/c compressor is the problem, it may be starting to seize up or the a/c clutch may be going out, either of these can draw too much current, you may also consider checking the alternator output, most autoparts stores like Autozone will check the charging system for you at no charge.  

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