Mot Emissions Readings...what Do They Indicate?!

[lagonda 42]

Just after rebuilding the engine I had readings at the MoT of CO: 0.972 & HC: 141. That was in 2007 at 163000 miles. This year, they are 1.01 & 161 respectively. The car is a 1989 1.6 CTi, mileage 187000, but I rebuilt the engine at 163000. As always I turned the idle speed up to around 1050-1100rpm as I was told a higher idle means less chance of failure on emissions. I know these figures are well below the limits for failure....but what do they actually mean....is the mixture about right, or too weak?

 

Out of interest in 2000, at 138000, the previous owner had readings of 2.69 and 78 respectively.

[James_R 3]

Hydrocarbons: this class is made up of unburned or partially burned fuel, and is a major contributor to urban smog, as well as being toxic. They can cause liver damage and even cancer. The regulations regarding hydrocarbons vary according to the engine regulated, as well as the jurisdiction. In some cases, "non-methane hydrocarbons" are regulated, while in other cases, "total hydrocarbons" are regulated. Technology for one application (to meet a non-methane hydrocarbon standard) may not be suitable for use in an application that has to meet a total hydrocarbon standard. Methane is not toxic, but is more difficult to break down in a catalytic converter, so in effect a "non-methane hydrocarbon" standard can be considered to be looser. Since methane is a greenhouse gas, interest is rising in how to eliminate emissions of it.

Carbon monoxide (CO): a product of incomplete combustion, carbon monoxide reduces the blood's ability to carry oxygen and is dangerous to people with heart disease.

[j_turnell 137 3 Cars]

Your hydrocarbons is very good usually see it around 2-300 mark. Your CO maybe a touch low, i always used to set them between 1.5-2.00 and always saw good results, you may be leaning out higher up the rev range. If you have access to a CO machine, get it back on and turn the 5mm allen key in the afm clockwise to richen, anticlockwise to lean. May well need to set the idle again after this, and make sure the TPS is clicking when moving the throttle.

[lagonda 42]

Unfortunately no access to a CO machine...I did buy a Gunsons Gastester many years ago, but found the readings so inconsistent I returned it to Gunsons for a full refund. So it seems my rebuild is good...good-oh!

 

So...can any one say how much I need to turn the allen screw to get the mixture about right?

[j_turnell 137 3 Cars]

Unfortunately not, you could do it the old fashioned way and go for a long drive and take the allen key with you, just do it 1/4 turn at a time and see how it goes, may feel bit better higher up the revs, then soon as it starts to bog down lower revs, or throttle response isnt as sharp lean it off a touch.

Edited October 10, 2009 by j_turnell

[lagonda 42]

Many thanks, will give it a try...hopefully it'll also clear very slight pinking.