This is on my friends 01 3.8 firebird. I have been fighting an issue of detionation that I almost have fixed. But today while driving with him having the scanner hooked I noticed something...

The PCM is swicing back and forth between open and closed loop. After about one minte it entered closed loop as normal. Then while driving part throttle it wouold enter open loop for a random amount of time.

My questions are.. I know that under WOT the pcm will enter Power enrichment mode, but will the computer switch to open loop or maintain closed loop operation and just go to the pre programed fuel maps for PE mode?

also on decel it would go into open loop for a long time. I know that decel has a different cell but I was not aware that it would also hit open loop then??

And the last thing. Is there any reason that under normal driving it should be hitting open loop. I am talking on acell and not WOT at random times.

Thanks a lot everyone



EDIT: I should also note that he is running a 160 thermo witch is causing problems with the computer and throwing a P0128. Everyone esle that I talk to is crying about the thermo causing the problem. But to my knowlage the PCM bases closed loop operation by a timer in the PCM based on imput from the ECT to compare and will set a "too long to closed loop" code. The sensor by way of teh heater will reach conductive temps within a minute of startup and bypass the timer and alow closed loop operation for it. That coupled with that fact that it was still running at 210 degrees and switching back to open loop leads be to beleve that the thermo is not the issue

 

anyone have any thoughts? I would really like a confirmation that it should not be entering open loop as well as to confirm my thoughts as to the cause

 

What is P0128? I couldn't find it in the service manual. Or did you mean p0125, Engine Coolant Temp (ECT) Excessive time in closed loop fuel control. Could the problem going in and out of closed loop unexpectedly be related to the coolant temp sensor. Im sure you did but did you monitor the coolant temp sensor reading through out a complete warmup cycle noting the voltage and temp. reading. Also is he seeing knock retard since you said it was a detionation problem.

 

the temp sensor is fine. The code was in history from an issue that the 01-02 V6's have when you add a 160 thermo. it stands for P0128
Coolant Thermostat Malfunction

The computer thinks that the thermo is stuck open. But even with that code not running at the time of the test, the 02 going into cloded loop from the heaters, and the ECT sensor reading well over closed loop operation it's still switching

 

IIRC the obd2 still does open loop when in WOT...

you might want to consider emailing charlesbeyer@netzero.net

He is one of the owners of digitalhorsepowerinc and they have one of the best tuners out there for v6 3.8 engines....

As to your questions:
again: they run in open loop(drive) when WOT, so its still OPEN loop

I've seen in my LT1 that with heavy decel it will default to open loop also... and the injectors would default to 0 pulsewidth...

The thermostat will not give any problems at all.. so forget the thermostat

There is no other reason while normal driving it will default to open loop unless there is an O2 sensor problem... My suggestion.. try to change the O2 sensor as they are one of the most common faults on these cars.

 

Yeah. I can understand open loop in decell sine it;s pulling all fuel anyway. but I just wanted to confirm on PE mode weather it was open or closed. I belive that you will enter PE mode weather your in open or closed loop but I didn;t think that it would switch to open loop in WOT situations. But I do know that there should be no reason that it should be switching under normal driving situations.


And yes. The Thermo is a mechanical part and has zero effect on the computer. There must be a ratonality test for ECT voltage VS time that is used to test for an open thermo. But still that should have no effect on loop status. Since the 02's will even over ride the internal closed loop timer if they reach temp before the timer sets closed loop. The semi strange thing is. When the SES light sets for the thermo code it also disables the temp gauge. I am going to remomend to him that he replaces the 160 with a 180. The only reason that i had him do a 160 and made a manual fan switch was to keep temps down as much as we could to prevent some of teh detonation.


My recomendation would be for him to replace both upstream 02's and the thermo with a 180 and take it from there. But so far since the last time I worked on his car it ran 100% perfect except for the open loop issue.



If anyone has more insite I would really welcome it.

 

Check with the scanner if you see a decent switching (even on open loop)on the O2s to see if they are working or not

After than that... do not change the thermostat.. it is not needed.... i would suggest check any existing codes as they should be pretty thorough....

 

the thermo change will be for peice of mind. The only reason I will sugest it to him is do to the disabling of the temp gauge. It will go out for a day or so and then come back. The 180 will not set this code. Besides with the manual fan swich that I made for him he can keep it plenty cool for at teh track (where he is right now)

The 02's were switching very heathly. They responded as normal. I could see no visable fault with them. fuel trims look normal as well. The only indecation that there was a problem was the open/closed loop monitor. Everything else was in range with the exception of the TPS (the newer 3.8's have a motor actuated throttle blade with an assembly that contains 2 TP sensors) It is confusing untill you learn how the system works cause at idle TPS volts read 0. but anywho.


any more thoughts highlander?