CO / HC Emissions Too Low?

[Zad]

A quick question for MoT tester types: Having miraculously got my 1991 Sierra through it's MoT without any work (or bribery), I was looking at the emissions pass ticket today. It read:

CO: 0.16%vol (max 3.50%vol)
HC: 159ppm vol (max 1200ppm vol)

Remember this is 1991 J reg with 128k on the clock, so without a catalyst and O2 sensor, hence running open loop.

I couple of years ago it was struggling to beat the CO (until I told the tester where the adjustment potentiometer was), but now I am wondering if the CO and HC might be an indication of running a bit lean? The idle speed is slightly high (maybe 100-200rpm) so I intend to look around for a potential air leak. I don't have access to an injection non-catalyst car to compare numbers.

It is no great worry, just wondering if it is worth spending an hour or so checking things over. The possibility is that the engine is running absolutely fine, with emissions lower than a cat equipped car. Not something I was expecting in a 20 year old motor.

[oakdale]

You can't have a HC level that's too low, the lower the better.

The CO's are a bit low, but the low Hc level proves that it's running fine at that level, so I'd leave it as it is.

Edited by oakdale on Monday 15th August 21:28

[stevieturbo]

Unless you have an actual running problem, then clearly there is nothing wrong.

[stevieturbo]

Unless you have an actual running problem, then clearly there is nothing wrong.

[Zad]

Thanks guys, I just wanted a bit of a "sanity check" to make sure I wasn't missing something