P38A 4.6 FAILED MOT due to CO emissions

O2 sensors, MAF and fuel flow are all interlinked.

MAF measures the Air flow in, the O2 Sensors measure what is being put out, the fuel flow is adjusted accordingly...

rewmer said:

I'm worried by the maf reading. My maths make that 151kg / hr !!!
Heres some known good readings
Idle 680rpm - 22Kg
1200rpm - 32.5Kg
2000rpm - 55Kg
3000rpm - 92Kg

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Here they only measure idle emissions...

rewmer said:

But then again, checking my maths its 15.2 kg hr - which is still out of scope on tickover.

No, im right first time 151 kg hr.
42 * 60 = 2520 grams a minute
2520 * 60 = 151200 grams an hour
151200 / 1000 = 151.2 kg / hr

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That screenshot was at partial load.
But then again, I don't know how reliable is an obd wifi dongle and a smartphone app...

what are the o2 sensors reading.

MAF is just one reading..

ukadamwest said:

what are the o2 sensors reading.

MAF is just one reading..

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I would have to take the readings again.
What other values would help and on what conditions? As I said, here they only test idle.
I've sent the car to the workshop this morning anyway and told them to get it passed. If it passes, they'll fix the AC, an axle boot and a washer pump, which I had in the car for the past 2 weeks and didn't had time to fix and check on the SRS light always on and Airbag Fault message in the instrument cluster. I've checked already the sockets for both seat sensor and steering wheel but the fault is still there. Maybe it only needs to be deleted.

If it doesn't pass, I'm selling it... and move on.

The fuel trims are controlled by computer.. so you can't adjust them.

Like i said before, the O2 reading and the MAF interplay with the fuel flow. So would need to see the O2 sensor readings as well as the MAF, if the HC is high it's running rich.. burning too much fuel for the amount of air going in. You've put on a new Bosch MAF so it's unlikely that. So this would lead me to the output end. If the O2 sensors are old and cocked up, they may not be getting good readings and leading the fuel flows to be enriched.. resulting in a high HC reading on the emissions test. Its possible that a leaky exhaust manifold would do this too as gasses would be escaping from the manifold, leading the O2 sensors to read weak gasses and enrich the mixture.

How about this one...?
The guys at the workshop threw in a used MAF sensor from another 4.6, loosen the petrol tank cap and got it to pass!

Tilo said:

How about this one...?
The guys at the workshop threw in a used MAF sensor from another 4.6, loosen the petrol tank cap and got it to pass!

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more likely he threw the probe up the exhaust of someone elses motor

No, I went with him. The testing center is not far from the workshop. Before we went he swapped the MAF from one of his P38's, loosen the petrol tank cap (you could hear the pressure release) and sprayed the throttle body with some tune up/cleaning ACDelco stuff. We drove over there (5min drive), queued in for the test (engine always running) for another 5 min. Just before testing, he gave it a couple of revs.
Passed with less 0.4 than the limit.

So.. your MAF is faulty.. You sure its a genuine bosch and not a chinese copy ?

Tilo said:

Is this anyhow useful (or even reliable)?

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Love the intake temperature !

ukadamwest said:

So.. your MAF is faulty.. You sure its a genuine bosch and not a chinese copy ?

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Bosch genuine.
What he says is that the O2 sensors must have a lot of carbon so the mixture is not quite right. With a new MAF and old O2 sensors the readings are unbalanced.
Think of it as buying only one new shoe. It won't give you a balanced walk. You new either both used shoes or both new.

ukadamwest said:

Love the intake temperature !

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That wasn't a bad day...

what did i say !!

ukadamwest said:

If the O2 sensors are old and cocked up, they may not be getting good readings and leading the fuel flows to be enriched.. resulting in a high HC reading on the emissions test.

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Tilo said:

That wasn't a bad day...

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Yep.. I've just been approached about a job in Riyadh..

Readings with new MAF and old O2 sensors...

Oh 50 deg.. Time for a hoodie !

have u checked for any fault codes on the ecu with ur dongle l check mine regularly mine failed checked the ecu codes reset bingo mine is on lpg aswell but if the old maf has thrown some faults they maybe still there only a thought . l use the torque app and a blue tooth adaptor only a thought

Just to close the topic...

Spark plugs and leads cut it.
After changing that, CO emissions went down by 1.0 units.
In the end, replaced air filter, air mass meter, did a couple of intake cleaning treatments, spark plugs and leads.
Problem fixed (for now...)