Re: New MAF
If this helps in any way, it might at least save you some money. I also had the 4000rpm flat spot on my 166 2.5 V6, and read on here it was likely to be the MAF. So, on the basis that I could not make it any worse, I removed the air filter housing and took the MAF sensor unit off the intake pipework, using long nosed pliers to grip the 2 screws. I cleaned all the exposed metal bits with nail varnish remover and cotton buds and put the whole lot back togther again. I then reset the ECU, and hey presto - flat spot has completely gone and she revs right round the dial without a stutter. Don't know whether this will last, but it has certainly cured the problem for now.
However, before you rush off and try it, I must advise that the red injector light came on after I did this and has been on since - I assume because the system recognises that something has been disturbed and therefore needs resetting, but it may be something more sinister resulting from my actions.