Running lean in open loop
Thread Starter
Racer
Joined: Sep 2005
Posts: 304
Likes: 1
From: Carson City Nevada
Running lean in open loop
What would cause a stock 88 to run lean in open loop? The O2 reads around 180mv's with 0 cross counts and the block learn is down to 118. As soon as it switches to closed loop, the O2 starts reading between 400 and 700mv with tons of cross counts and the block learn starts coming back up. The temp looks like it is ok. Could it be a weak O2 sensor?
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
The O2 sensor is not used in open loop. The ECM waits for the O2 sensor output to reach a threshold voltage (I think it's .45 volts) and then switches to closed loop mode.
The ECM uses the BLM table as the basis to determine fuel injector pulse width. The BLMs are NOT updated in open loop mode, so you're stuck with whatever values were stored there when the system was in closed loop mode (when the BLMs ARE updated). You're seeing this when the ECM switches to closed loop and the O2 cross counts start.
I don't understand the BLM of 118. The BLM table (16 cells) is updated by the ECM during closed loop mode and will retain those values in RAM when the engine is shut off. The only way I can see that you could get such low BLM values at startup is if the RAM lost power somehow while the ignition was off. The default BLM values are 120 (on my car -- '86 Corvette), so maybe that's what is happening. Are you disconnecting the battery when you don't drive it? That will cause this.
There is a fusible link and connector that supplies power to the ECM while the ignition is off to keep the RAM alive. The RAM stores the BLM table values and the error codes. This connection goes directly to the ECM -- there is no fuse. Check this connector (item 2) and the wiring associated with it to make sure it's OK:
The ECM uses the BLM table as the basis to determine fuel injector pulse width. The BLMs are NOT updated in open loop mode, so you're stuck with whatever values were stored there when the system was in closed loop mode (when the BLMs ARE updated). You're seeing this when the ECM switches to closed loop and the O2 cross counts start.
I don't understand the BLM of 118. The BLM table (16 cells) is updated by the ECM during closed loop mode and will retain those values in RAM when the engine is shut off. The only way I can see that you could get such low BLM values at startup is if the RAM lost power somehow while the ignition was off. The default BLM values are 120 (on my car -- '86 Corvette), so maybe that's what is happening. Are you disconnecting the battery when you don't drive it? That will cause this.
There is a fusible link and connector that supplies power to the ECM while the ignition is off to keep the RAM alive. The RAM stores the BLM table values and the error codes. This connection goes directly to the ECM -- there is no fuse. Check this connector (item 2) and the wiring associated with it to make sure it's OK:
Thread Starter
Racer
Joined: Sep 2005
Posts: 304
Likes: 1
From: Carson City Nevada
Well Cliff, I think you nailed it. I cleaned the connection and and all is well. I also put a new O2 sensor in it. Starts up with a BLM of 131 on cell 0(idle) and purrs like a kitten. The O2 is moving between 170 and 750 with tons of cross counts right at start up.
Thanks for the help.
Thanks for the help.
