Fuel Trims Crazy???

[PG01]

Check this out james
http://www.easterncatalytic.com/education/tech-tips/fuel-trim-can-be-a-valuable-diagnostic-tool/

 

[Scottydoggs]

both up and down stream o2's talk to each other. the down stream one can set a cat code some times with high flow cats, not sure if thats a common thing on these engines tho. but it can mess with the fueling.

 

[Justin]

Can the Lower O2 Sensors affect the Trims? Hmm... I have new Upper Sensors sitting on my work bench and was putting off buying the Lowers unless I had a really really really good reason to buy them. Spent too much already.

Yes, the cats are quite large, Magnaflow 51578: https://www.magnaflow.com/products?partnumber=51578

Fuel trims are exclusively driven by the front O2 sensors.

Short trims don't really tell much, as they are a direct representation of the ECM's immediate compensation for O2 feedback. O2 reports rich, ECM responds with a short term trim negative and vice versa. Any average short term trim that is positive or negative will result in that average being shifted to the long term trim so that the short term averages out to zero.

As mentioned above, what is your ethanol content looking like, or commanded AFR? Do you have a cold air intake?

 

[swathdiver]

Fuel trims are exclusively driven by the front O2 sensors.

As mentioned above, what is your ethanol content looking like, or commanded AFR? Do you have a cold air intake?

That's what I thought. Ethanol content during that Snapshot was 3% and the motor was burning 93 octane gasoline. Stock airbox and newly installed stock paper high capacity AC Delco filter.

Have since switch over to E85 and was getting about 14-16 on the highway (instant) and the 11.4 on the AVG setting for all combined driving, mostly city and idling. The engine can more closely meet its mileage ratings on E85 than gasoline. Which is puzzling to me.

 

[Justin]

Got it. You have an AutoCal, right? Do you mind collecting a log file for us? Very negative trims at low airflow are not uncommon on 2007+ models due to MAF calibration in some cases, high fuel pressure in others.

 

[swathdiver]

Not at all Justin. Does it matter if the motor is burning gasoline or E85? It's nearing the end of a tank of E85.

Commanded and actual fuel pressure has always been 43.5 psi. At WOT to 70 mph on the night of the Snapshot the fuel pressure hit 42psi for an instant and remained at 43 psi at a 70% duty cycle.

Is there an easy explanation to explain why this gets normal fuel economy on E85 and reduced fuel economy on 93 octane gasoline in relation to the published ratings?

 

[Justin]

Not at all Justin. Does it matter if the motor is burning gasoline or E85? It's nearing the end of a tank of E85.

Commanded and actual fuel pressure has always been 43.5 psi. At WOT to 70 mph on the night of the Snapshot the fuel pressure hit 42psi for an instant and remained at 43 psi at a 70% duty cycle.

Is there an easy explanation for normal fuel economy on E85 and reduced fuel economy on 93 octane gasoline in relation to the published ratings?

Nope, it won't matter, I just need to know what fuel you're using so that I can take that into consideration.

Without data, I can't really comment on mileage being lower with 93 octane.

 

[swathdiver]

Ok, will try to get a log recorded tonight and send it through normal channels.

 

[Justin]

Ok, will try to get a log recorded tonight and send it through normal channels.

Sounds good.

 

[swathdiver]

Sent!