I am working on VE tables and a rich condition at Idle. Getting closer when the car isn't down for a mechanical issue. In reviewing the data log, came across the engine switching from Closed Loop to Open Loop while is was driving at 50-60 mph. I was in the process of slowing down, according the the vehicle speed. I didn't notice anything physical while driving, just appears in the CSV file data log. I have viewed all the entries above and below the event and see nothing other than the CCP disengaging (which I think is automatic when you go to Open loop. The Check engine light came on several samples later. The engine returned to Closed loop and the Check engine light went out later in the drive.

The Rich Flag was on from Cold Engine Open Loop Idle when I started the engine.

1227730 Computer
$8d
AUJP V6
502 BB Chevy TP injection

CSV attached. Goes into open loop at sample 10236. Check Engine light at 10274. Returns to Closed loop at 10447. Light goes out at 10515.

Maybe someone can see something or suggest a cause.

 

I don't really see anything crazy. TPS voltage seems a little high. At 0 throttle it's .75v, a 1989 MAF car wants .61v or less, don't know if yours is different. Shop manual says cars may enter open loop after extended idle, but you were cruising so that doesn't make sense.

Intermittent issues are hard, I'd be suspicious of a momentary break in an electrical connection. Maybe check all wiring. Can you get the issue to reproduce?

 

Could be a sign of a dying O2 sensor. I just recently had that happen, mainly in idling situations but also intermittently at cruise. New O2s, no signal drop.

 

Of course, you never know, but I put in a new NB O2 when I put in the new short block. Suspect even new ones can be faulty. Maybe throw in another for good sake. I have the WB O2 as well. Kormet, I am a speed density set up so, no MAF.

 

Honestly, I don't know, but when I saw the title of your thread, I was thinking "Open Loop" ??? These cars??? Haven't studied what Open Loop means for a while but I'm using an educated guess our cars don't have a true open loop to closed loop switch over, which is what I thought when the upstream O2 sensors (or is it downstream) heat up enough to become reliable sensors, and then air/fuel mixture can be precisely adjusted. But hey, maybe I'm wrong on that, just throwing it out there. So maybe to start with, what does Open Loop / Closed loop mean for our cars? Don't our cars always use the VE tables (or what ever it is) where as a modern day car only uses the VE tables during open loop??

 

I did another long logging drive yesterday (after replacing the PS pump). Again, the car dropped out of Closed Loop, into Open Loop, doing about 60 mph on reasonably level road. In looking at the CSV log file, there is no detectable reason for it to drop out. When it dropped to Open, the BLM Fine was locked at 128, the BLM Main was locked at 118 and the INT was locked at 128. The NBO2 was bouncing around 700-800mv before going into Open Loop. MAP was 38-41 KPA (I am at 7500 feet ASL)

I would have expected BLM Main to lock at 128 in Open Loop as well.

Several samples after dropping into Open loop. NBO2 was now between 800-900mv. The "Rich" flag was set. I think that might be related to the Open loop vs Coolant Temp bumped up the fuel.

Half a Minute later, it went back into Closed Loop and remained there the balance of the test. For about twenty samples before going back to Closed Loop the NBO2 dropped below 800 and ten samples before returning to Closed loop NBO2 dropped down to between 500-200mv. The Rich flag went off after returning to Closed Loop

Speed was basically constant the whole event.

I am not sure what the potential causes for dropping out of Closed loop could be. Faulty ECU?

Don

 

Sounds to me that the O2 signal was not swinging, which would send the ECM into Open Loop. Then the O2 began registering lean/rich again and so ECM went back into C/L. Have had that happen to me a number of times which then gets remedied by a replacement of the O2s.

 

I ordered a new NBO2 today. Not in stock in this town.

My O2 swing point and boundaries seem high compared to what I have read on this forum.

Don

 

My LT5 is modified w cams, and I adjust the O2 swing points principally for idle. Otherwise everything else is stock.

 

O2 Cross Counts before, during and after the Open Loop event. Going 60 mph, level road.

Don

 

was it the ecu?
I needed to bother you about an older post also if you get time.. and i have a partial s&p harness if you can use it. Lmk

 

I don't think the ECU is going to cause itself to drop out of closed loop.

Most likely going to be a faulty O2 sensor, or intermittent wiring connection, or maybe a non-heated O2 sensor cooling off at low-MAP and/or low-RPM operation when mounted in exhaust headers (particularly in the collector of full length headers).

 

Agree.