Hi MGW welcome to the TM forum ....I'm well used with the P4SS problems having owned one for two and a half years.
The first and cheapest fix is to replace the spark plugs ,they will no doubt be the originals which have not been changed or checked.
Mondeo or MX5 2.0 .the plug gap should be 130 mm they were reduced to 90 mm for some reason this will reduce hydro carbons and may get you through the MOT without any further rectification .
However the ecu set up makes the car run too rich and the injectors are too big causing the car to stutter cough and hesitate at low speed and junctions .
It was impossible to keep a steady throttle reading for the MOT between 3000 and 3500 rpm .
Williams in conjunction with Omex have come up with a solution but I have not seen any power out put figures or prices.
My car was down to 184 bhp ex factory and ran miserably ,so bad I had to call at the factory on the way home to complain about the fumes and poor performance .
I had no help from the factory they set up the throttle bodies and told me it was a race car so that's how they are .
I found a guy who said he he could fix the problem and he did the car is now as smooth as silk gets a full 204 bhp low emissions ,easy mot pass ,no back fires ,easy starting and when the VVTiming kicks in sounds amazing.
The down side is I had to pay for it but it transformed the car .
When did you buy the car ? As of 1st OCTOBER THE CONSUMER RIGHTS ACT puts the onus on the dealer to sort out the problem .
PM if you would like any more info .
