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MOT emissions readout - interpretation?
Posted: 28 Jun 2006, 11:59
by cumbriankeith
Just put the camper in for MOT. It failed for the first time in the 6 years I've had it, dammit!! But only for a couple of small rusty bits which I can patch up easily this evening and re-present tomorrow, hopefully...
But I digress - I've been trying to get my newly rebuilt and run-in engine in a good state of tune re mixture and timing etc. The MOT has obviously provided me with a printout of the detailed emissions and I'm thinking this is a good opportunity to tweak the mixture or whatever a bit more.
CO is 2.008 %vol (must be 4.5 or less)
HC is 344 ppm vol (must be 1200 or less)
CO2 is 7.75 %vol
O2 is 9.34 %vol
Lambda is 1.536
Am I right in thinking this is still a bit rich? I have a Gunson Pro CO meter (2ndhand) but I'm not sure if it's working right or if I'm using it right. Is the % thing the same?
Also I can't get the idle speed right - it seems too low ie 700 ish - adjusting the idle screw doesn't seem to do anything (stock Solexes BTW)
Any ideas??
Posted: 28 Jun 2006, 19:48
by kevtherev
The exhaust gas oxygen sensor (EGO or O2), or lambda sensor, is the key sensor in the engine fuel control feedback loop. The computer uses the O2 sensor's input to balance the fuel mixture, leaning the mixture when the sensor reads rich and richening the mixture when the sensor reads lean.
Lambda sensors produces a voltage signal that recognises the amount of unburned oxygen in the exhaust. An oxygen sensor is essentially a battery that generates its own voltage. When hot (at least 250 degrees c.), the zirconium dioxide element in the sensor's tip produces a voltage that varies according to the amount of oxygen in the exhaust compared to the ambient oxygen level in the outside air. The greater the difference, the higher the sensor's output voltage.
Sensor output ranges from 0.2 Volts (lean) to 0.8 Volts (rich). A perfectly balanced or "stoichiometric" fuel mixture of 14.7 parts of air to 1 part of fuel gives an average reading of around 0.45 Volts.
The lambda sensor's output voltage doesn't remain constant, however. It flip-flops back and forth from rich to lean. Every time the voltage reverses itself and goes from high to low or vice versa, it's called a "cross count." A good O2 sensor on a injection system should fluctuate from rich to lean about 1 per second. If the number of cross counts is lower than this, it tells you the O2 sensor is getting sluggish and needs to be replaced.
Most lambda sensors will cycle from rich to lean in about 50 to 100 milliseconds, and from lean to rich in 75 to 150 milliseconds. This is referred to as the "transition" time. If the O2 sensor is taking significantly longer to reverse readings, this too is an indication that it is getting sluggish and may need to be replaced.
Taken from picotech.com
Posted: 28 Jun 2006, 20:23
by cumbriankeith
That's all very interesting, Kev... And thanks for responding but I don't know what it means that is relevant to carbs rather than Fuel Injection.
The Lambda reading is 1.536 - what units I have no idea!
Posted: 28 Jun 2006, 20:36
by philippacman
hi keith, your emissions look spot on to me .
2% co fine thats burnt fuel
344 hc fine thats unburnt fuel
dont worry about the rest of it ,they are for vehicles with cats
hope this help
Posted: 28 Jun 2006, 20:46
by andysimpson
Something wrong there, may be an exhaust leak or its too weak. A perfect lambda reading is 1, anything above one means its to weak or analyser is picking up fresh air from a leaking exhaust. Lambda of 1.5 is pi$$ weak which also shows in the high oxygen content. Carbon dioxide is low also showing it ain't running as it should.
What engine is it? sounds like air cooled with the solex bit.
Weaker than a weakling
Posted: 28 Jun 2006, 21:20
by cumbriankeith
Andy - yes it is a 2 litre Aircooled CU engine with stock solexes (which were new only about 30k miles ago so shouldn't have worn spindles yet - should they?) It was rebuilt by Laurie Pettitt a few months ago and is stock but for a slightly torquier cam and solid lifters - valve clearances checked and adjusted to 6 thou last weekend.
Sounds like I should richen it up a bit then... best on central idling circuit or on basic mixture screws? May be this will give me a better speed of idle as well.
Just had a look on Si Baxter's fine site and came across
this page again. Talks thro it all most helpfully!
Re: Weaker than a weakling
Posted: 28 Jun 2006, 21:39
by andysimpson
cumbriankeith wrote:Andy - yes it is a 2 litre Aircooled CU engine with stock solexes (which were new only about 30k miles ago so shouldn't have worn spindles yet - should they?) It was rebuilt by Laurie Pettitt a few months ago and is stock but for a slightly torquier cam and solid lifters - valve clearances checked and adjusted to 6 thou last weekend.
Sounds like I should richen it up a bit then... best on central idling circuit or on basic mixture screws? May be this will give me a better speed of idle as well.
Just had a look on Si Baxter's fine site and came across
this page again. Talks thro it all most helpfully!
Carbs maybe out of balance, one may be rich and one weak, they can wear at different rates so go out of balance. A non standard cam could also cause some strange readings.
Posted: 28 Jun 2006, 22:07
by Cate
The CO and HC levels are healthy for a good running van on carbs however the CO2 level should reach 12-13% on a correctly running engine and the O2 level ideally should be below 2%, I think there may be an air leak on the exhaust side eg manifold, gaskets or silencers/pipes, the lambda figure is showing that there is excessive O2 in the exhaust due to the leak, I presume that the vehicle is on carbs and is not fitted with an O2 sensor so it is running open loop.
More on emissions
Posted: 29 Jun 2006, 07:18
by cumbriankeith
Si Baxter has been good enough to email me direct and I share his thoughts on all this for anyone else's benefit...
"How do, saw your thingy on 80-90 but I've logged myself out of there so couldn't reply.
Your emissions seem, well the lambda indcates it's running weak, above 1 is lean, below 1 is rich, O2 which should be between 1 and 2 ish.
Co2 is low, that should be about 14% again give or take.
I would clean and gap your plugs, make sure the timing is right and recheck.
I don't like just using the CO meter alone as you don't get the full picture, the CO is about right ish on the print out but all the other are all to "cock", if you were just to measure the CO with your CO meter then you would think things were okay, but they are clearly not.
Check for worn throttle spindles, air leaks, loose rubbers on the balance and servo pipes, have a spray around with WD or something and see if you can hear a change of engine note when it's sprayed at certain points.
Remember the left hand carb does the idle, and idle mixture and it is distributed by those awful pipes.
Make sure the vac advance diaphram isn't split and the piping is correct to it as it could pull the timing off if piped wrong and upset everything.
I would have thought if your timing was miles off you would see higher HC's but you're not. Also, underlying problem may result in poor idle..."
Cheers Si - I hope you don't mind me sharing your knowledge.
Posted: 01 Jul 2006, 11:06
by Simon Baxter
philippacman wrote:hi keith, your emissions look spot on to me .
2% co fine thats burnt fuel
344 hc fine thats unburnt fuel
dont worry about the rest of it ,they are for vehicles with cats
hope this help
All the readings are relevent and aren't just for vehicles with catalytic converters, each gas holds it's own clue as to what is wrong with the engine, it's only that the MOT only checks those 2 gasses, but you shouldn't disregard what the others are telling you.
I have never used a CO meter alone as it tells you very little other than what one gas is doing, tuning an engine is getting the best comprimise of all the gasses and only when you see what all of the readings are can you make adjustment to the engine.
As you say, the CO and HC's are around a bout right but the rest aren't, I would start on that one by checking the ignition timing and air leaks with it being a CU engine.