MOT emissions results - very low CO%

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bustrucket
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MOT emissions results - very low CO%

Post by bustrucket »

Hi all

I got an emissions check as part of the MOT which I wasn't expecting, but hey ho. Anyway it appears to show a very low CO of 0.15% which is about 10x less than the manual says it should be (1.5%) assuming it was done on a warm engine at idle. Engine is standard 1.9 DG on carb.

The engine does run hot according to the gauge. Timing seems fine and no air leaks that I can find.

I know this isn't a lot of info to go on but based on these results does this look like it could be a problem?

Thanks

David

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Re: MOT emissions results - very low CO%

Post by TONYT25T25 »

Mine were CO 0.23% and HC 639 at last MOT as a comparison, passed so quite happy. Same engine with carb
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bustrucket
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Re: MOT emissions results - very low CO%

Post by bustrucket »

Fair enough, I expected it to be closer to 1.5% but maybe I'm just reading too much into it!

Thanks

David
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Re: MOT emissions results - very low CO%

Post by TONYT25T25 »

Worth keeping post alive for comments from other members and their readings, plus pos or neg comments regarding the ideal or best achievable settings.
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ajsimmo
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Re: MOT emissions results - very low CO%

Post by ajsimmo »

That's a very clean and healthy engine. I'd be delighted with that result. It indicates near complete combustion of fuel at idle, and if your CO2 figure was available I bet it would be very high (a good thing, as more of the available hydrocarbons have split and the carbon's combined with more oxygen - a better burn). This suggests good compressions, very little oil burn, excellent fuel atomisation from your carb, correct timing etc. If it was overly lean, your engine would suffer a less complete burn resulting in higher HC emissions.
Of course this test is performed at idle (or possibly fast idle for HCs), and does not indicate lean running (or otherwise) under load.

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bustrucket
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Re: MOT emissions results - very low CO%

Post by bustrucket »

ajsimmo wrote: 20 Jul 2021, 12:33 That's a very clean and healthy engine. I'd be delighted with that result. 
I rebuilt the engine and the carb myself, so that's good to hear! It doesn't seem have any other symptoms of a lean mix under load that I can tell apart from the high temps, but I realise that could be down to other things.

Thanks

David
 
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Re: MOT emissions results - very low CO%

Post by pbar »

Very interesting thread this. I too have a 1.9 DG (T25 1985)

I wouldn't just assume that such a low emission result reflects a super clean/efficent engine though, it may not.

LIke you, mine is also very low. I have a good relationship with my MOT tester, he's an old school mechanic and very helpful, I always ask for the CO readouts and we always discuss them. A couple of years ago mine was similar to yours (can't remember my exact readings without checking) at nought point something. He said that this reading is too low and could result in the engine running way too hot (which is something you have mentioned) and that I should richen/open up the mixture a little. I think he said we are actually allowed a reading of 4, (it will only fail if it's too high, never too low) so nought point something is incredibly low.

My van has always seemed to run ok (no sign of being too lean) but anyway when I got home I opened the mixture screw a small amount, a quarter of a turn. I didn't go back as I was happy to leave it at that.

A year later we discussed it again at the MOT test, and although it was a little higher due to opening the carb a little, it was still at nought point something, his advice was to open it up more, so again when I got home I opened it up another quarter of a turn. And I think I'll leave it at that now.

Why don't you increase the mixture a half turn or more (make sure you 'memorise' its position so you can put it back the same) and try a couple of journeys to see if it runs any cooler.

I wouldn't take too much notice of the book figure of 1.5, that may have been the reading to aim for back in the day but fuel has changed since then and I guess that makes a difference too.

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Re: MOT emissions results - very low CO%

Post by TONYT25T25 »

I bought a Gunsons Gastester from Halfords years ago, obviously not as efficient as ones at the MOT centres, and only measures CO2 I believe, but it gives a digital reading as the mixture screw on the vehicle is adjusted.  I find it quite useful if I am suspicious of a too weak or too rich a mixture.
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Re: MOT emissions results - very low CO%

Post by keynsham1 »

Low values can be as bad as high ones. Actually you are unlikely to damage your engine with high values (The environment wouldn't be so lucky of course!) but low values suggest running weak which causes overheating, over-running and pre-ignition (pinking) amongst other things. These are old designs and I suspect that all settings really should be in the region of where the manufacturers data suggests to get the best from your engine.

Of course if you are worried about the emissions you can always convert it to electric for about £40k! :D :D :D . But that, of course is for another post I suspect!

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Re: MOT emissions results - very low CO%

Post by TONYT25T25 »

So based on what has already been said, what would the optimum/best readings be for a good engine or to keep an engine healthy and be within the MOT pass criteria.
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Re: MOT emissions results - very low CO%

Post by ajsimmo »

Based on the initial question of low CO reading at MOT, there seem to be a lot of references to lean running causing hotter running, engine damage etc. Please remember that the MOT emissions test is done at idle, and idling a bit lean won't cause overheating, pinking or damage as it's not under load. The worst that might happen is it misfires a bit, runs a bit lumpy, or stalls.
A Pierburg (and almost every other) carb has fixed jets (and therefore fixed mixture) for driving, and the idle mixture setting have little or no bearing on mixture under load or at higher RPM.

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Re: MOT emissions results - very low CO%

Post by keynsham1 »

Good points ajsimmo. I am more used to SU and Stromberg carburettors which only have the single jet. If it is weak at tickover, then it is weak everywhere!!

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