I have a 1998 version 4 sti, if i put my foot flat on the accelerator as it gets to 4000rpm-5000rpm it splutters after 5000rpm its ok again. But if i take it easy on the accelerator and don't put it flat to the floor it will go right upto 7000rpm with no issues can anyone tell me what it could be. It is not over boosting as it doesn't cut the fuel. Any help would be much appreciated the cel doesn't come on at all when it happens.

 

I had the same symptoms with faulty HT leads, it only missed/cut on wide throttle, but fine on small throttle inputs. Mine occured after I'd just done a massive upgrade - I ended up swapping back fuel pump, turbo, ECU etc, only to find it was the HT leads! Try the easy stuff first!

 

Leads or MAF issue at a guess.
What boost pressure is the car making ?

 

If it's like a hesitant splutter then I'm aiming that you have an igntion issue. What plugs are in? When were they changed. Same with the HT leads and coil/coilpacks (age dependant).

I say as I've just had my Scoob remapped and had similar issues after the map driving home. Your first instinct is to assume it's the mappers fault - incorrect most the time. With my car it had gained 65bhp and 100lbft on the map, so a dormant issue which wasn't noticeable on standard boost (within either the HT leads or coil) suddenly became the biggest ball ache going. I'd put my foot down in different circumstances and come full boost around 1.5-1.6bar I'd get a double/triple misfire. Putting a working coil/leads from a similar car in similar state of tune seemed to solve the issue.

To recap. Try the ignition system first with known working s/h parts just to elliminate them. Then look at sensors etc.

But you will get a dozen different opinions on here, it's how forums work.

Best of luck chap.