For the 3rd year running my 1.8 16v has struggled to pass its MOT emissions test. 2 years ago it had new cat and lambda, last year another lambda and bottle of 'magic potion' (forte).
This year it has failed again, despite doing a 320 mile motorway run the day before with V power and redex in the tank.
The strange thing is it is running as well as it ever has and that 320 mile run was the most economical it has ever been, so hard to believe there is anything fundamentally wrong with it??
Emission results are as follows:
fast idle hc 219 (max acceptable 200)
fast idle co 0.48 (max acceptable 0.30)
fast idle lambda 1.056 (acceptable range 0.97-1.03)
2nd fast idle hc 206
2nd fast idle co 0.43
2nd fast idle lambda 1.058
natural idle co 0.26 (max 0.5 - PASSED that!)
So pretty close to a 'just pass' but they had it reving for ages to try and get there but it never did...
Ever since I got the car the exhaust has smelt odd, sort of a bit like butane gas??
There is no smoke at all at the exhaust. Also unlike most Twinsparks mine hardly uses any oil...rarely need to top up between oil changes (always check though!)
Decided to change plugs...they are now fitted. In process did a cold compression test which gave very odd (and low) numbers:
cyl 1 -110psi
cyl 2 -80psi ??
cyl 3 -110psi
cyl 4 -60psi ???!!
At this point thought I must have at least a couple of burnt valves, but carried on other tests anyway...
Ran up and checked lambda sensor output, seems okay. took for quick spin, drives just like always does ie fine.
Hot compression test immediately after:
cyl 1 -170psi
cyl 2 -170psi
cyl 3 -165psi
cyl 4 -175psi
So pretty good after all...?
Took cam cover off and all cam lobes look in good order, nothing obviously wrong...
A bit stumped now.
I have just got one of those code reader jobbies and will try that later, but think it will be full of spurious stuff due to all the messing about I have done trying to work this out.
Anyone been here before and know what to do next?
Cheers for any help,