Help with GMC ecm problem
Question:
Not necessarily. It is very common at idle speeds for the ECM to switch back and forth between open and closed loop.
– Hide quoted text — Show quoted text – > snip > It’s also pretty typical for some of these throttle body trucks to go > open loop at idle. > Wouldn’t this indicate a sensor problem? > — > For NG please reply to group only. > High Sierra > 89 S15 Sierra Classic 4X4 4.3L Short Box > 91 Dodge Shadow 2.5L 5 Speed > 94 Grand Caravan 3.3L
Response:
With the O2 sensor unhooked it won’t go into closed loop operation, but the disturbing thing is that the ECM didn’t set a code 13 when you did this. The ECM should have detected an open circuit in the O2 system and set the code 13 inside of 1 minute. If you’ve checked all your connections, and your O2 voltages scan within parameters I would suspect the ECM.
– Hide quoted text — Show quoted text -> Update. I was able to scan after every thing warmed up last night. I > rechecked the connections on the ECM and the Prom. I cooled the ECM in > the freezer and no problems. I had another O2 sensor laying around so I > threw that in. I unhooked the O2 sensor and let the truck run for a few > minutes and couldn’t get a trouble code to set. The system stayed in the > open loop mode until I hooked up the O2 sensor. This morning truck > started and immediately idled down to low rpm. CTS reported 62 degrees. > It was around 50 degrees outside this morning. > Very true. In this case I would tend to think that the rich condition is > probably just a fraction below the parameter of setting a hard code (45) > such as an O2 signal close to 750mV. I think the inability to scan is > probably due to the ECM going into open loop and field service mode as > indicated by the flashing light. In a case like this I would assume a code > 13, and start the diagnostics there. It would provide a basic start point > and give a logical process of elimination. This could be an ECM problem. > ? Chevrolet Certified Master Tech. > ?
Response:
snip
> It’s also pretty typical for some of these throttle body trucks to go
> open loop at idle.
Wouldn’t this indicate a sensor problem?
— For NG please reply to group only.
High Sierra
89 S15 Sierra Classic 4X4 4.3L Short Box
91 Dodge Shadow 2.5L 5 Speed
94 Grand Caravan 3.3L
Response:
Update. I was able to scan after every thing warmed up last night. I rechecked the connections on the ECM and the Prom. I cooled the ECM in the freezer and no problems. I had another O2 sensor laying around so I threw that in. I unhooked the O2 sensor and let the truck run for a few minutes and couldn’t get a trouble code to set. The system stayed in the open loop mode until I hooked up the O2 sensor. This morning truck started and immediately idled down to low rpm. CTS reported 62 degrees. It was around 50 degrees outside this morning. – Hide quoted text — Show quoted text – > Very true. In this case I would tend to think that the rich condition is > probably just a fraction below the parameter of setting a hard code (45) > such as an O2 signal close to 750mV. I think the inability to scan is > probably due to the ECM going into open loop and field service mode as > indicated by the flashing light. In a case like this I would assume a code > 13, and start the diagnostics there. It would provide a basic start point > and give a logical process of elimination. This could be an ECM problem. > ? Chevrolet Certified Master Tech. > ?
Response:
Very true. In this case I would tend to think that the rich condition is probably just a fraction below the parameter of setting a hard code (45) such as an O2 signal close to 750mV. I think the inability to scan is probably due to the ECM going into open loop and field service mode as indicated by the flashing light. In a case like this I would assume a code 13, and start the diagnostics there. It would provide a basic start point and give a logical process of elimination. This could be an ECM problem.
– Hide quoted text — Show quoted text ->There are 4 things that effect open/closed loop operation: >TPS angle >CTS signal >O2 sensor signal >ECM timer > It’s also pretty typical for some of these throttle body trucks to go > open loop at idle. > Big Daddy > ICQ#6342575 AOLinst messenger "WallyC1951" > ASE Master Auto Technician > Chevrolet Certified Master Tech.
Response:
>There are 4 things that effect open/closed loop operation: >TPS angle >CTS signal >O2 sensor signal >ECM timer
It’s also pretty typical for some of these throttle body trucks to go open loop at idle. Big Daddy ICQ#6342575 AOLinst messenger "WallyC1951" ASE Master Auto Technician Chevrolet Certified Master Tech.
Response:
I have a 1992 GMC 4×4 with the 5.7 that has been showing signs of intermittent ECM problems. Lately after driving at a fairly constant speed (highway or freeway) and when I stop at a light the emission system goes into the open loop mode and runs rich. I have an auto xray diagnostics hooked up and all parameters show fine. Just driving on residential streets and stopping the system stays in the closed loop mode. This morning the temp was around 45 degrees and when I started the truck it started fine then the rpm dropped and the check engine light started flashing extremely rapidly. The auto Xray could not get the ecm to respond to the scan command. After driving on the freeway for about 15 miles the check engine light went off and I could scan the ecm. No trouble codes were stored. any ideas? Thanks, Mark
Response:
There are 4 things that effect open/closed loop operation: TPS angle CTS signal O2 sensor signal ECM timer
– Hide quoted text — Show quoted text -> I have a 1992 GMC 4×4 with the 5.7 that has been showing signs of > intermittent ECM problems. Lately after driving at a fairly constant > speed (highway or freeway) and when I stop at a light the emission > system goes into the open loop mode and runs rich. I have an auto xray > diagnostics hooked up and all parameters show fine. Just driving on > residential streets and stopping the system stays in the closed loop > mode. This morning the temp was around 45 degrees and when I started the > truck it started fine then the rpm dropped and the check engine light > started flashing extremely rapidly. The auto Xray could not get the ecm > to respond to the scan command. After driving on the freeway for about > 15 miles the check engine light went off and I could scan the ecm. No > trouble codes were stored. any ideas? Thanks, Mark
