When you cold start your Blazer the fuel system is in open loop for a period of time. I know that during this time the ECM ignores the O2 sensors. Which sensors are being used at that point. I have read conflicting opinions such as coolant temp.

Thanks,

George

 

I believe its the 2 pre cat sensors

 

Perhaps my OP was not clear. I am trying to understand the set of sensors used for open loop fuel calculations and then the set of sensors used for closed loop fuel calculations to help me with diagnosis. For instance I assume that the MAF Is used in both but O2 sensors are only used for closed loop. What about ECT, MAP, TPS etc.?

Thanks,

George

 

I understand your question. When the engine is cold the loop is open and the pcm ignores O2 sensor data and uses data from temp. and other sensors to control fuel mix. When the engine warms up it switches to closed loop status and the pcm uses O2 sensor data to control fuel mixture which is much more accurate and efficient. How many things need to happen before it switches from open to closed? Temp. and what else?

The only answer I've found is "several". Maybe there's another way to help. Are you trying to diagnose a fuel trim problem?

 

I have rough idle and hesitation on initial acceleration. No codes. Fuel system pressures and CMP retard all good. Fuel trims actually became normal after CMP retard work but idle got worse. That means that to the extent that this fuel, the ECM thinks its delivering the right ratio. Plugs wires cap rotor almost new.

I am also looking at ignition, Cat converter, MAF and other sensors. I will be cleaning the throttle body next.

 

you know a cap that had crossfiring will leave a corbon trail. Causing a misfire. If its a the from before the cpm retard fix the cap could be bad again

 

I don't know which sensors the Blazer uses. But I think it would use the TPS sensor (it has to know what the driver is trying to accomplish), the MAF, I has to know the amount of air being brought in), possibly the Vacuum sensor (to know how much load the engine is providing and adjust the timing accordingly) , The CAS and cam sensor (to know when to fire which cylinder) and the coolant temperature sensor (to know when to reduce fuel enrichment for cold engine running). I think the O2 sensors, once the engine runs closed circuit, may only adjust the predetermined fuel requirement from the fuel table cell determined by the other sensors. If there are no faults showing up on a scanner, most likely the problem exists in an area that is not monitored. Abnormal sensor info usually sets up fault code.

At least that is what I think happens.