Failed Mot on emissions - natural idle CO only. Help!

[RobWai]

Hello all,

Hoping for a bit of help here - never had to diagnose emissions problems before.

The Micra is a 2001 K11 1.0 coilpack version. Had it tested at two different garages with the following results (permitted ranges in brackets)

GARAGE 1
Fast idle CO - 0.06 (0.20) PASS
Fast idle HC - 20 (200) PASS
Fast idle Lambda - 1.014 (0.970 - 1.030) PASS
Natural idle - 1.36 (0.30) FAIL

GARAGE 2
Fast idle CO - 0.063 (0.200) PASS
Fast idle HC - 50 (200) PASS
Fast idle Lambda - 1.004 (0.970 - 1.030) PASS
Natural idle - 5.430 (0.300) FAIL

Between the two tests I tried a few things including cleaning the idle control valve, Italian tune-up, new plugs, etc, but the second set of results are worse. The first garage wanted to replace both O2 sensors and the catalytic convertor, but at a cost more than the car's worth!

Note at no point did the engine management light come on (apart from for 1 minute after I first drove with the new spark plugs in).

I got an OBD scanner (OBDLink SX to link to a laptop) which gave only one fault for STORED fault codes - P0120, indicating an incorrect voltage from the Throttle Position Sensor. However, on clearing the code and running the engine again, I couldn't get the code to come back again. On checking the resistance on the TPS with the sensor disconnected I'm getting 2Kohm regardless of accelerator pedal position. But no further fault code.

As the emissions are fine on the fast idle, I'm inclined to disregard the suggestion to replace O2 sensors and catalytic convertor and concentrate on the TPS and/or throttle body. Right or wrong?

Advice, please!

Regards,
Rob

 

[frank]

i would agree with you rob, best to take some live readings from the lambda,s at idle

 

[RobWai]

i would agree with you rob, best to take some live readings from the lambda,s at idle

Thanks for the reply. Any pointers on how to interpret the readings I get on the OBD scanner / OBDWiz software? It's just voltage readings, doesn't translate to CO percentages.

 

[frank]

just voltage would suffice rob, they should be bouncing between .4 and .6v roughly (the 1st one more markedly )

 

[RobWai]

Thanks for the figures Frank.

I got the engine properly warm and looked at the voltages on ODBWiz - they don't look much like you quoted!

At idle of 1000rpm (affected by testing or my fiddling?, usually idles at 600rpm) I get 0.8v and 0.9v on sensors 1 and 2 respectively.
At 3000rpm standing still I get 0.13v and 0.09v

 

[SuperUno]

Did you have your lights or heater fan on? This will increase idle speed...

 

[RobWai]

Nope, didn't have lights, heater fan or any other power-draining stuff. Should add that the 1000rpm idle was after sitting there at 3000rpm for a little while to get the O2 sensor voltages as per Frank's suggestion

 

[frank]

Thanks for the figures Frank.

I got the engine properly warm and looked at the voltages on ODBWiz - they don't look much like you quoted!

At idle of 1000rpm (affected by testing or my fiddling?, usually idles at 600rpm) I get 0.8v and 0.9v on sensors 1 and 2 respectively.
At 3000rpm standing still I get 0.13v and 0.09v

here,s mine when cruising mate

 

[RobWai]

Thanks Frank. Not sure what to make of all this though! Guess I need to do some Googling to work out what the voltages mean.

 

[Agent_Sm1th]

Here's what the lambda sensors should look like.

 

[RobWai]

Ta. Think I may be reading the wrong screen in OBDWiz, as it's just giving maximums, not telling me about the normal expected fluctuations on sensor 1 as per the first graph above.

Will recheck tomorrow.

 

[Agent_Sm1th]

I use the torque app.

 

[frank]

this link says that the 2nd lambda should be near zero eh ? https://www.picoauto.com/library/automotive-guided-tests/zirconia-pre-post-cat
and the fluctuating voltage test will depend on the refresh rate of the meter being used

 

[Agent_Sm1th]

That 2nd reading is a bit misleading as it's usually around 0.5 steady line.

 

[frank]

personally, i would expect the 2nd lambda to show a low/steady voltage (when the car was new) and i would expect on a 15 yr old cat for it to show a lower fluctuation (straighter line) mirror image of the 1st sensor

 

[frank]

like this

 

[Agent_Sm1th]

That 2nd lambda shows around 0.5 to 0.6 which is where mine usually is.

 

[RobWai]

THANKYOU gentlemen! This is good stuff - think I understand what I'm seeing from the O2 sensors now.

Armed with the information you've provided, I think I've fixed the problem by swapping out the throttle body with MAF sensor and TPS.

On the OBD scanner software I was reading the wrong screen - what I should have looked at was a graph of the O2 sensor voltages which properly shows the change between high and low values over the space of a few seconds. Doesn't read often enough to get graphs as good as yours, but enough to work with.

Here's my understanding - please challenge if any of this looks wrong. All tests were done with the car parked, not while driving.

With a properly warm engine at 3000rpm the upstream O2 sensor was reading 0.2v - 08.v, while the downstream was reading 0.7v -0.8v. From Frank's last diagram, this is consistent with a properly working cat (and presumably working O2 sensors). The downstream sensor voltage is higher than what you get, but I'm not worried there, as the car DID pass the fast idle test at this RPM. Also these figures are close to Agent Sm1th's graph - where was that from?

From Frank's link to www.PicoAuto.com: "An ECM will only control the fuelling in 'closed loop' when the appropriate conditions allow, which is normally during idle, light load and cruise operations. When the vehicle accelerates, the ECM allows overfuelling and ignores the lambda signals. This also happens during initial warm-up."

From that I take it the O2 upstream sensor voltage at idle on a warm engine should go up and down in a similar way to the same engine running at 3000rpm

But I wasn't getting that - the engine was idling at 640rpm and giving a steady 0.8v and 0.9v on the upstream and downstream sensors respectively.

Conclusion - although all components are old, the cat and O2 sensors are still OK, but something was preventing the ECU going into closed loop, so it was ignoring O2 sensor output and fuelling rich as if the engine was cold.

How's my logic so far?

 

[frank]

the k11 will usually only openloop in 1st and 2nd gears rob (unless you disconnect the speed sensor wire) and yes (when cold ) the ecu tuns in coldstart enrichment mode if the coolant temp sensor is functioning ok

 

[RobWai]

Hello again Frank. All figures here were with the car parked. Maybe why my readings are so different to yours? I checked the coolant temperature sensor at the start of this exercise and found nothing wrong with it - reading the right resistance at hot and cold.

As it is, after I swapped the throttle body, the car actually ran worse on the first trip out (ECU learning about the new MAF?), but when I put the OBD scanner on it I was getting the expected voltage fluctuations at idle - 0.2v - 0.8v on the upstream sensor, approx 0.8v steady on the downstream. Tried taking the car out a second time, and this time it was fine. Also, some problems it sometimes has when attempting to rev hard had gone away.

Job done?

 

[frank]

yes calibrating (self learn) if the 1st lambda is fluctuating ok at idle it should pass the emission test now eh rob

 

[RobWai]

Here's hoping!

Thanks for the education gentlemen! Thought I was wasting my time with the throttle body swap - until I saw what the O2 sensors were doing afterwards.

It'll be a while before I get a chance to take it in for retest, but I'll report back on the result

 

[Agent_Sm1th]

Hi Rob. The graphs were a screenshot from the torque app which is from the Google play store. It's a couple of quid.

 

[RobWai]

Got the Mot done - PASS, no work required at all.

THANKYOU again gentlemen!

 

[Agent_Sm1th]

Glad it's sorted.