Over-fuelling/ECU Reset (2ZZ)

[Alex Knight]

Hi all,

I came across an issue recently when driving my 2ZZ MR2.

At the last Bedford trackday, I had fuel surge/starvation on a long, high-G corner with the car showing just under 1/4 tank of fuel.
I got the car back onto a straight, and the car was fine again - probably just low fuel and high G.

I drover the car on the road for the first time (I think) since that trackday, and the car felt a little flat on WOT. Felt like about 15-20% reduction in power. Revs were taking ages to climb, but it engaged lift and made all the right noises - just very slowly.
It was also belching out lots of smoke from the exhaust. It hasn't used any oil at all, and the coolant level is fine, so I can only presume that it was over-fuelling?

I have been getting a very intermittent P0136 ECU code, which I believe is related to the post-CAT O2 sensor (please correct me if I'm wrong!), which had a resistor wired into the circuit in around 2012, which may be slowly dying.

I pulled over, and checked ECU codes - there were none. I reset the ECU by pulling the 15A fuse, and went out for another drive.

All was well, pulling like a train again, no smoke from the exhaust, no issues at all. Very strange indeed. It's obviously ECU related, as for some reason, it thought that masses more fuel was required, but I've no idea why? Nothing has changed on the car, with the exception of the fact that I thought I'd try some Redex injector cleaner (but I cannot remember if this was before or after the last trackday). Could this be a factor?

I guess my question is: Has anyone experienced this before?

'Tis puzzling me no end.

As a precaution, I am going to spray the MAF sensor and throttle body, change the plugs (NGK BKR6EIX) and clean the lift OCV filter, but I suspect these things are probably not related.

Any help/advice welcome!

Thanks,

Alex.

[shnazzle]

I can see this turning into another "post cat sensor doesn't affect fueling"..... 

Replace your sensor mate. Its 35 quid for a universal denso. 
Takes 5 mins to snip the connector off your existing sensor and crimp the new sensor onto it.
Crimping gear even comes with the sensor

[Alex Knight]

I can see this turning into another "post cat sensor doesn't affect fueling".....

Replace your sensor mate. Its 35 quid for a universal denso.
Takes 5 mins to snip the connector off your existing sensor and crimp the new sensor onto it.
Crimping gear even comes with the sensor

To be fair, I've not had a post-cat sensor in for 7 years, since 2012. I've got a blanking bung where the sensor screws in, and a resistor wired into that circuit. I really don't think I need to replace a sensor that I don't even have.

[shnazzle]

I can see this turning into another "post cat sensor doesn't affect fueling".....

Replace your sensor mate. Its 35 quid for a universal denso.
Takes 5 mins to snip the connector off your existing sensor and crimp the new sensor onto it.
Crimping gear even comes with the sensor

To be fair, I've not had a post-cat sensor in for 7 years, since 2012. I've got a blanking bung where the sensor screws in, and a resistor wired into that circuit. I really don't think I need to replace a sensor that I don't even have.

Ah! That definitely changes things! Definitely wouldn't replace it then.

Sorry had no idea you were without sensor.
Nope not a clue then Watching

[Alex Knight]

I can see this turning into another "post cat sensor doesn't affect fueling".....

Replace your sensor mate. Its 35 quid for a universal denso.
Takes 5 mins to snip the connector off your existing sensor and crimp the new sensor onto it.
Crimping gear even comes with the sensor

To be fair, I've not had a post-cat sensor in for 7 years, since 2012. I've got a blanking bung where the sensor screws in, and a resistor wired into that circuit. I really don't think I need to replace a sensor that I don't even have.

Ah! That definitely changes things! Definitely wouldn't replace it then.

Sorry had no idea you were without sensor.
Nope not a clue then Watching

You may actually be on to something though, you know.

If the resistor is dying, which throws an intermittent code, then this is possibly affecting fuelling.

I have a BT OBDII dongle. Is there any way I can check the LTF trims with that?

[shnazzle]

I can see this turning into another "post cat sensor doesn't affect fueling".....

Replace your sensor mate. Its 35 quid for a universal denso.
Takes 5 mins to snip the connector off your existing sensor and crimp the new sensor onto it.
Crimping gear even comes with the sensor

To be fair, I've not had a post-cat sensor in for 7 years, since 2012. I've got a blanking bung where the sensor screws in, and a resistor wired into that circuit. I really don't think I need to replace a sensor that I don't even have.

Ah! That definitely changes things! Definitely wouldn't replace it then.

Sorry had no idea you were without sensor.
Nope not a clue then Watching

You may actually be on to something though, you know.

If the resistor is dying, which throws an intermittent code, then this is possibly affecting fuelling.

I have a BT OBDII dongle. Is there any way I can check the LTF trims with that?

It's entirely possible. Low voltage = lean condition. Lean condition = dump fuel in. 

If it has been doing this for a while with long term and short term fuel trims it is possible for it to add up to 35% more fuel. But at that point it would throw errors. 

You can indeed easily check the STFT and LTFT using a Bluetooth obd2 reader and any car diagnostic app. Torque Pro being the most popular. Assuming you're on Android.

It would be the most drastic effect from a post-cat sensor I've seen, as generally it's only used to tweak the fueling a bit after the other sensors have done their thing.

[Flibby]

Hows your mpg? As Shnazzle says over fuelling is easy to check for with the STFT going 20+ would give you some issues. The resistor could be shorting out based on the accumulation of heat given when it happened?