Diagnosing an intermittent stalling problem can be a challenge, especially if the engine only stalls occasionally and won't stall for you (only your customer). And if there are no pending codes, hard codes or history codes in the vehicle's computer to give you some direction, you may find yourself guessing at a diagnosis.

Every engine needs three things to run and idle smoothly without stalling: a correct air/fuel ratio, sufficient idle speed for the idle load, and a good spark. If any of these is lacking, the engine may stall.

Cold stalling problems are the most common because the engine needs a richer fuel mixture to maintain idle speed until it warms up. Intermittent cold stalling problems are almost always fuel-related.

On older, carbureted engines, cold stalling (and hard starting) is most often due to an automatic choke that is sticking, misadjusted or broken. The engine also may be leaking vacuum around the base of the carburetor, vacuum hoses or the EGR valve. Other problems may include a faulty heat riser valve (stuck open) that prevents the intake manifold from warming up, or a defective thermostat that prevents the engine from warming up quickly or reaching normal operating temperature. Any of these things can upset the air/fuel mixture and prevent the engine from idling normally until it warms up.

On fuel injected engines, cold stalling also can be caused by conditions that upset the air/fue...