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What could give a vehicle fitful revolutions?

Dear Neto,

I recently took my 2003 Toyota fielder for an engine overhaul after it overheated and blew its head gasket. Now the vehicle doesn’t idle well. The revolutions are never constant. It plays between 700rpm and 1000rpm. It has been checked several times but the problem still persists. What is causing it to do so?

Newman.

Dear Newman,

Your problem is known as engine hunting. I would recommend that you have the throttle plate and the idle air control valve (IACV) cleaned of any carbon deposits.

Usually, cleaning a sticking idle air control valve will solve this problem particularly if the engine hesitates and shuts off. A thorough check  up should be done on your intake system for vacuum leaks.

Hunting happens mostly when the engine runs too rich so the engine management computer tries to adjust the air-fuel ratio.

The engine then runs too lean and the revs get low and the computer again tries to inject more fuel and the revs go up. It is a cyclic process that can be corrected by identifying the source of the unmetered air or leakage. Make sure the air filter is clean and the air flow sensor is well connected.