P38 CO Emissions higher at low idle - advice please?

Hi Guys,

I replaced the whole exhaust system on my P38 4.0 2002 including cats and 02 sensors recently as the old one had numerous leaks causing a fail in emissions. Before going to my MOT in a few weeks I got it hooked up to an emissions machine and it passed ... just.

At low idle it was exactly on the limit for CO of 0.2 but on the faster idle the result dropped to 0.15 against a limit of 0.3. I am assuming as the limit rises by 50% for the faster idle the result should be higher rather than significantly lower? Also I am a bit nervous with the low idle result being right on the edge.

At this stage I haven't changed plugs, coils etc but would like to understand if firstly this is fairly normal or not and if not what might be the root cause. Any thoughts, advice, experience massively appreciated as always

Cheers

First of all have you just tried doing the good old Italian tune up

Hmm.. could be the o2 sensors are old and sluggish, getting a bit better when they warm up with a few revs.
Was the engine and exhaust fully hot?

Rubberknees said:

First of all have you just tried doing the good old Italian tune up

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In this case it might help get everything nice and hot, blow any soot off the sensors

kermit_rr said:

In this case it might help get everything nice and hot, blow any soot off the sensors

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Always have done it on all my motors especially before a MOT

Rubberknees said:

First of all have you just tried doing the good old Italian tune up

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Thanks @Rubberknees. Will defo do prior to the MOT and a bottle of cataclean but would like to try and understand why the results seem the wrong way round to me !

kermit_rr said:

Hmm.. could be the o2 sensors are old and sluggish, getting a bit better when they warm up with a few revs.
Was the engine and exhaust fully hot?

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Thanks as always @kermit_rr. Should have mentioned in the original post (now edited) I replaced both 02 sensors with the cats. Not sure how hot it was as dropped it at garage and picked up later !!

AndyF07 said:

Thanks @Rubberknees. Will defo do prior to the MOT and a bottle of cataclean but would like to try and understand why the results seem the wrong way round to me !

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Not sure whether that stuff works or not. Didn't make any difference in my case.
Low air flow at low idle, plus lower exhaust temp, if the sensor isn't working at 100% it could be cool. O2 sensors don't work if they aren't at the right temperature

AndyF07 said:

Thanks as always @kermit_rr. Should have mentioned in the original post (now edited) I replaced both 02 sensors with the cats. Not sure how hot it was as dropped it at garage and picked up later !!

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Why replaced? Are you sure the heater circuit is working?

kermit_rr said:

Why replaced? Are you sure the heater circuit is working?

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They were 22 years old and pretty much welded in to the old pipes so thought new ones were the way to go. Not getting any error codes but I have a lynx eco diagnostics which hopefully will allow me to test / check .... job for tomorrow

When we're the plugs last replaced?
Also the air filter condition?
Any misfires?

Might be worth checking the throttle body is clean and the butterfly moves freely. Oil pressure will rise will idle and that might make the rings seal a little better too.

Mark Piercy said:

When we're the plugs last replaced?
Also the air filter condition?
Any misfires?

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Thanks Mark. No misfires at all and runs smoothly. The plugs are pretty old so I will replace them as a matter of course but if they are worn out would it not also cause the same issue at higher revs?

AndyF07 said:

Thanks Mark. No misfires at all and runs smoothly. The plugs are pretty old so I will replace them as a matter of course but if they are worn out would it not also cause the same issue at higher revs?

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It would depend on the condition of the plugs, good plugs burn fuel better

Has it run enough time for the fuel trims to adjust?

AllanWorms said:

Has it run enough time for the fuel trims to adjust?

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We only use for short trips so that is a good point. How many miles would approximately be needed ?

I don't know how long it takes, there are official 'drive cycles' that should be done once problems have been fixed, this is for disco2, but likely same similar for the p38
https://www.justanswer.com/car/677vj-landrover-discovery-ii-long-proper-complete.html