Lude Generation

First off, sorry it's been forever since my last post on the forum. No excuses, I'm just useless at these forum things
Anywho I digress. I've had the Type S in at Grinspeed in Leyland for various engine/clutch/suspension recon work and everything's been going fine until the MOT, and oh boy, she's throwing out some high HC numbers
It's stumped them and me as to what the heck is going on! The weirdest thing for me is the cold to warm running. She starts fine from stone cold and then when the engine starts to just head into being warm, I get a sudden increase in the Short Term Fuel Trim warning of up to 49% and mpg drops to 15 down to 10mpg! Next thing, from anywhere between 1 to 10 minutes, the STFT will drop to zero and the car will then behave for the rest of any journey I'd make Turn off the engine for 5 to 10 minutes and the damn cycle will start all over again. Obviously not good during MOT running!
It's the binary nature of the issue that's odd, like there's definitely a sensor causing the issue. I've replaced the ECT and PCV as they've been thought of as possibilities.
Other the STFT reporting high, when it does, I get no other CEL or sensor reports on the Hondash.
Work done since last year's MOT, are a custom cat back exhaust and work done to replace the cambelt/water pump.
Any ideas would be most welcome

Think I saw something similar on a MB6 civic once. Backbox was leaking for some time, we replaced it, but was still getting CEL for lambda related readings.

plugging in the cheapy blue tooth dongle, and logging the lambda output, something weird was happening. Instead of oscillating when it went into closed loop mode - it did as you describe, initially behaving, but then the fuel trim was very high, and it was over fueling and the lambda was no longer oscillating (was stuck at rich). It seemed to do this on the button after X number of minutes.

What we had to do in the end was reset the code many times, and drive/stop/shutdown/repeat many times.

After about 7 such repeats around the town - the fuel trims gradually got dragged down enough for it to work in harmony, eventually the CEL stopped coming on, and the fuel trim lowered to the point where the ECU would happily manage the fueling and give us a nice oscillating lambda reading.

I think having run around with such a blowy backbox for so long, the trims got dragged up needlessly, and these don't change back quickly enough after repair work - hence the several drives and resets needed.

Whether that is the case here I couldn't say - but it was an odd enough experience to share.

i agree, could be a faulty lambda - have you tried cleaning your egr as well? could be blocked maybe

Drax wrote: ↑

Tue Nov 24, 2020 8:45 am

I agree, could be a faulty lambda - have you tried cleaning your EGR as well? could be blocked maybe

The weird thing was, the lambda itself wasn't actually faulty. My theory is that the exhaust deteriorated over time slowly (hole got bigger) , and the ECU just kept adjusting the trims up and up to compensate.

Since the holes in the exhaust were not gradually shrunk over the same time frame, but fixed immediately by replacing the back box, the ECU is not trained to deal with sudden changes to the parameters/trims , and so over fueled willingly for 5 minutes before giving up and saying 'your lambda is probably broken'.

Without the 5quid Bluetooth dongle with data logging, we would have just had continued CEL for lambda failure. Since the logging proved oscillating readings from the lambda, we stuck at it and just kept driving and resetting and then it just stopped coming on all together once the trim had come down.

Cheap bluetooth logger plus the torque pro app really paid off that day.

excellent stuff

I'll forward this to my mechanic chap... he's truly baffled

Success
After messing us about not starting (failed igniter unit), he fitted a new Lambda Sensor. Sailed through
Cheers guys. I will try and update my profile when I get chance. It may be a while though

glad its sorted