Fighting a lean condition when coolant temp is below 150 F, like when the motor is cold. Idle's about 17:1-19:1, and goes very lean and stumbles everytime you shift gears, but eventuallyg catches up and runs decent till the next gear change. Once warm, no issues.

OK, so I'm thinking front 02 sensor going bad, needs to be replaced. So I datalog a couple cold starts with the front 02 unplugged just to verify before I spend the money. As expected, the 02 sensor feedback in evoscan is at 100 when I cold start and sure enough, idle is very lean. But I wait..... I watch, and sure enough the 02 sensor feedback in evoscan starts to elevate from 100 up to about 160 before I stop datalogging, and all the while a/f goes from 18:1 down to 12:1 and the idle smooths out a ton.

How does 02 sensor feedback change with the 02 sensor unplugged? And is this still indicative of a bad front 02 sensor?

 

Not sure what O2 sensor feedback in EvoScan is...is that one of the trims? I wish they were labeled correctly.

Anyway, it sounds like you need to either scale your injectors correctly or richen up the open loop fuel maps. The reason you are having the issue when cold is because the car runs in open loop until coolant temps reach a certain value. Then, it switches to closed loop and uses the fuel trims.


Eric

 

Exactly, that's what I was thinking. Hence, unplug the 02 sensor and see what the open loop fueling is doing at idle and not allow it to go into closed loop. That's the kicker, when the motor is cold, it's extremely lean based on the open loop map, but as the motor heats up, it goes rich. This is with absolutely no changes to the open loop fuel maps. And yes, I checked the MAF, the output is fairly stable around 50 Hz at 1000 rpm during this whole exercise.

So I started looking for trigger criteria.... because it was like a switch from lean to rich when it happened. Datalogging with EvoScan and a Utec showed that a changed occured when the 02 Feedback trim came off "100", which it stayed at while it's lean, and doesn't move at all. From initial start up (bone cold) to the time the car just starts to goe rich, 02 Feedback Trim is a constant "100". Then finally it elevates.

Per malibu, the 02 feedback trim is the short term fuel correction from the front 02 sensor. But how does it vary when it's unplugged?

Here's the code from the data file in Evoscan:

Display="Oxygen Feedback Trim" LogReference="O2FeedbackTrim" RequestID="0F" Eval="0.78125x" Unit="%" MetricEval="" MetricUnit="" ResponseBytes="1" GaugeMin="0" GaugeMax="200" ChartMin="0" ChartMax="200" Notes=""/>

I'm convinced I need a new front 02 sensor, I'm just trying to understand how the "trim" can vary when it's unplugged. It doesn't really vary though, it's a steady climb towards 200 once it finally budges off 100, but I just haven't datalogged long enough to let it get there. I'll do that tomorrow. But I'd be willing to bet it's gravitating towards a "failed" position of sorts when the sensor is unplugged. And anyone who is disconnecting their front 02 to adjust open loop fuel trims at idle is adjusting them with maximum enrichment conditions of "200" coming off the 02 trim and not "100" as previously thought.

late edit to add: I'm in Houston, temp outside is 55 degrees. My minimum coolant temp. for closed loop is set at 7 C (stock value), which is 45F. So, I should be going immediately into closed loop.

 

If he is talking about the "oxygen feedback trim" it is actually the STFT.

 

Coolant temp does affect compensation in open loop..

In order to test this properly, you have to set your Coolant Temp for Closed loop high enough so it never goes into closed loop (which you probably have done)

If its lean in while in open loop, it can be one of several things..

Airflow / Baro / Air temp at is reading very low at cold start, this can result in a very lean condition that goes away, Look at your MAF frequency at idle, Air temp, and Baro values.. plot those values as coolant temp goes from cold to running temp while in open loop..

Check for leaks that are happening while cold..

A normal MAF requency at idle is roughly 30-40hz

Your wideband should read 14.7 to 14.5 at idle in open loop, a bit richer when cold, however your idle speed should rise if its running lean until coolant temp rises.

Injector scaling is a tricky thing, generally you scale the injectors based on fuel trims, once you get your trims in check for cruise and idle, everything is fine at normal temps..

You definitely need to look for other issues, but at idle, excessive recirculation can lower the MAF reading, among other things.

Once your sure everything is working right, then you can make minor adjustments to open loop fueling at idle and low RPM, I'll bet your not hitting cells your expecting since vacuum at cold start, and vacuum when the engine is up to temp can be different enough to fall into different cells.

 

In your case, check the EGR system also, as this may also be giving you trouble..