I have a '91 Formula 350 and when I initially start it the idle is high (1000-1100 RPM), when I drop it into gear the idle drops to about 600 RPM. It only seems to do this when the engine is cold. After it has been driven it seems to be okay. It almost feels like the car thinks it cold outside and is increasing the idle speed? Any help would be greatly appreciated. Thanks.

 

Chris, does the idle drop down to normal after a while(with trans in park); if so, then I'd say that it's a normal thing. One reason why this happens is so that the engine gets up to operating temperature faster. The faster an engine gets to it's operating temp, the sooner it can better burn off emissions. If the idle stays at 1000or so rpms, then you're probably looking at a problem. Several things can cause a high idle, one example is a leak in the induction.

 

That thing happend to me too.my bird's engine RPM got higher and higher.then later it got to a point where i just put in park and let it idle at 45MPH. Then one morning the engine would not go below 1000 at all.Then the engine would run rough and shake the whole dam car. I asked a mechanic an it was AIR control valve.I have told this to many people with the same simptoms. They all worked. Look at your Throttle Position Sensor(TPS),Exhaust Gas Recirclation(EGR--->can be checked by pushing in the diaphram,and of course the evil Idle Air Control(IAC)valve (that looks like a joy stick).
That is your problem...have you also changed your MAP sensor lately? Just part of a master tune-up. Other then the idle when it is cold ...that is normal.You maybe looking at a faulty sensor.

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It is not how you stand by your car,but how you race it.

 

It's all part of the programming within the PROM. There is a table that sets the Idle Speed vs the Coolant Temperature. As the car warms up (higher coolant temperature), the idle speed decreases.

Also, having your engine in park/neutral will add 100 extra rpm (over drive) and there is a small increase if you have the AC on.

If you want to change the idle speed, you must adjust the table in the prom.

 

My idle does eventually drop down to approximately 600 RPM in drive. It must just be the "cold" mode that the car is in. If it gets worse I will have to start checking sensor, air leaks, etc..Thanks for the information guys.

Chris

 

Yes, once the coolant gets up to operating temperature it will go to around 600 rpm in drive. But you CANNOT adjust the idle "mechanically". Only inside the eporm. If you attempt to "cludge" it with a mechanical method, you cause problems.