How long must the engine/car run before the computer will throw a error on running in open loop?

 

On an OBDI vehicle the coolant temp sensor is the main sensor that dictates when the ECM will go into closed loop. If the thermostat is stuck open, or is opening too soon the engine will run cooler longer. That results in the ECM staying in open loop longer. It may not throw a code because the ECM is only going off of what the coolant temp sensor is telling it, that is, if the data to the ECM seems logical it won't throw a code.

 

How are you monitoring open/closed loop?

The best/easiest/cheapest is to use the ELM 327 bluetooth obd adapter and android phone with 'TORQUE PRO' app. Less than $30.

 

1993 not odbii.

 

No it is OBDI as stated by Bustedback.

Also. I believe the 02 sensor must see 600F before it functions.

 

If the engine fails to reach operating temperature, it usually sets a code 17. >http://wiki.answers.com/Q/What_is_di..._jeep_cherokee

I've seen that a few times when people install a lower temp thermostat than the specified 195 one in an attempt to cure an overheating condition. That results in the coolant temp sensor sending the signal to the PCM, which determines the engine is taking too long to reach operating temperature. It keeps the fuel mixture and ignition timing set accordingly, and goes into closed loop mode.

It should set the code 17, but no check engine light is usually shown.