Facelift - Chasing LTFT Trims

I think you have that backward. The downstream sensor monitors the effectiveness of the catalytic converter, while the upstream sensor is reading exhaust O2 and sends the voltage signal to the ECU, which then regulates the air/fuel mix via changes in injector pulse width. SD Faircloth

I thought the same at the time when I’ve read Veekey’s post, but as I don’t know these engines, I doubdet my self…
And at 60.000 miles they must be way past their life span limit.

From a training manual, “Four oxygen sensors are installed on the exhaust system, two upstream and two downstream of the primary catalysts. The two downstream sensors are used by the ECM for closed loop fuel metering correction. The upstream sensors are used for OBD catalyst monitoring.”

I know it’s weird. But that’s how the AJ16 engine reads the oxygen sensors.

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Are the upstream sensors before or after the the catalysts?
They can’t be monitoring the catalysts if they are before them.

From the same manual, "The efficiency of the primary catalytic converters is monitored and any deterioration in efficiency is flagged as a fault by the ECM. Monitoring for catalyst efficiency is achieved by sampling both the incoming and outgoing exhaust at the primary catalysts. Two oxygen sensors are positioned in each exhaust downpipe assembly — one upstream of the primary catalyst and one downstream of the primary catalyst.

By comparing the voltage swings of each set of sensors, the ECM can detect when catalyst efficiency drops off. An “area difference” technique is used to compare successive oxygen sensor swing measurements."

I got down to 0.8% on the trims, and then something popped me back up to 2.4%

Not sure what may have caused that. I was running low, but not out of fuel. Monitoring to see if I come back down again. I don’t know why low fuel would have affected anything.

Any dirt/restriction before the fuel pump would have a more severe effect when the fuel level is low as you loose the additional pressure from the fuel in the tank and the pump could start cavitating.
Fill it up and see if it changes.

If not, I would be considering changing the upstream 0² sensors, still not convinced that they are not the ones that dictate fuel trimming.

I drive the car everyday, so yes, the tank has been refilled. I’m at 3.1% LTFT, currently.

I reread my notes and all 4 oxygen sensors were in fact replaced at the same time.

I did seal the crack in the exhaust manifold Thermosteel.

I did notice some evidence of oil getting past the camcover seal. I cleaned the area up and will monitor.

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Veekay, I’m not sure if this was addressed but the AJ6/16 engines use conventional spark plug numbering.

yes…I even have the little sticker on my air filter indicating which one is which. That post was just clarifying that I indeed do have two separate (exhaust) banks.

OK. After patching the hairline crack in my exhaust manifold AND replacing my air filter AND unplugging an replugging my TPS while engine was running, has now brought my LTFT to:

negative, -0.8

That’s right, I went from a +3.1 LTFT to a -0.8 LTFT on both banks!


Persistance always wins Veekay!

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Not sure whether it was sealing up the hairline crack on one of the two exhaust manifolds…or something much more interesting.

Was it because I unplugged, then replugged in the TPS, while engine running. The idle went higher when I unplugged it, no doubt because it went into open loop. When I plugged it back in seconds later, idle returned to the lower speed. I wonder if that has any bearing on my situation.

I will wait and see if idle drifts away in the future…

As always, thanks for the help and advice here.