Expanding on what Brian has said, cold idle problems are often the result of improper air/fuel mixture. The cold start valve squirts extra gas into the intake manifold to “choke” the engine until it warms up. The temperature sensor gauges coolant temperature and the coolant temperature switch gradually closes the cold start valve as the engine reaches operating temperature. So you start by checking the temperature sensor and switch, as Brian mentioned. But other factors can affect mixture as well.

 

All you said about symptoms was that the idle is erratic during warm up and your father thinks the car runs sluggishly.  Try to determine if the mixture issue is too lean or too rich. You will get to the solution faster.

 

Too lean a mixture during warm up - your engine should be hard to start or will stall frequently after starting, it hesitates during off-idle operation or accelerates poorly.

 

Too rich - you should experience a stumbling or galloping engine; possibly even engine stall after a minute’s operation due to fouled spark plugs. Also, do you smell gas from the exhaust or see black smoke?

 

Immediately upon starting, blip the accelerator. What happens? Does idle speed fall below cold idle specs, then return to normal? Probably too rich a mixture. If the engine hesitates or stalls as you blip the pedal, mixture is likely too lean.

 

Lean mixture main culprits are, as Brian points out, air leaks in the rubber and a faulty coolant temperature sensor or switch. Other causes can be faulty o-rings on the injectors, clogged or faulty fuel injectors, faulty or out of adjustment idle air stabilizer valve - even a leaking or damaged valve cover gasket or front or rear crankshaft oil seal. Note that a lot of these problems will mostly disappear after the motor has warmed up and gaskets and seals seat better.

 

Too rich a mixture can be caused by leaking or sticking open injectors, the cold start system not shutting down as the motor warms up, faulty coolant temperature sensor or switch too high fuel pressure, something clogging intake air (old, dirty air filter?), weak spark, or incorrect ignition timing.

 

Bob S.