This must be magic ?

Ever since I have had the car it has been marginal on the emissions test and this year, even with a completely new exhaust system it only just scraped through.   At the same time I have always felt that something wasn't working properly.   Although she idled OK there was a bit of hunting.   When I pulled away she occasionally hiccupped, when I put my foot down there was sometimes a bit of hesitation before the revs picked up, and she would regularly have a small uncommanded surge for no apparent reason (lamda sensor switching mixtures?).  Finally, there was lots of popping and banging on the overrun.

But up till now I have lived with that, as with a non-programmable ECU and the fuel injection I didn't think anything could be done or adjusted.  I just assumed some of the sensors were a bit iffy.

But while I was asking for help on the Robin Hood forum regarding my catalytic converter Nigel Evans (n.r.evans) told me of a trick a mechanic had recommended to him when he was trying to pass the emissions test, which I don't recall reading on the forum before, and that was to block off one of the 2 exhaust outlets.   So, willing to try anything I shoved the remains of an old tea towel up the exhaust.

 What an amazing transformation.  She now idles perfectly, and there is no hesitation during acceleration.  The power (apparently more than before) comes on smoothly when I put my foot down, and there are no more unexplained surges.   She is now working as you would expect.   There is also no more popping and banging on the overrun (which is a pity as I quite liked it )

So what is the explanation for this dramatic improvement?   Nigel's mechanic reckons that when the system was designed there was no attempt made to match all the exhaust components, they were just picked because they fitted together in the right space available.  In particular he thinks the lamda sensor needs some back pressure before it will work correctly and with the exhaust as designed there just wasn't enough.  Blocking one of the outlets is just enough to provide the necessary back pressure to make the sensor work.    Obviously I can't prove it but it seems very logical?

I haven't had a chance to check if it has improved the emissions yet but based on Nigel's experience I am very confident.   Meanwhile I am enjoying a much improved performance.   I just need to find a more professional way of blocking it than an old tea towel.