Mk5 GTI POWERLOSS??

[Ben-MK5GTI]

Hi guys, can anyone help, I recently brought a mk5 and when I brought it the thermostat was stuck open so the engine was running a cold temp. About 50-60.
I drove it like this for a week and the car felt really quick being my first GTI.
I've just got it back from the garage after having a new thermostat put on. It's running a perfect temp at 90 but feels way down on power now from when I dropped it off.
Would an open thermostat make it run quicker?
And this is infact how the car should be. Or could something be wrong?

[GreigC]

It could be a coincidence that fixing your thermostat something else has broken. Start with scanning the car for codes. One of the most common parts for low performance is the diverter valve they are known to tear.

[imprezzaworks]

Something will have been left off. My stat needing replacing and there was no change in performance.

[pudding]

Agreed, there should be no change, if not slightly better actually as running too rich hurts performance, not to mention mpg.  It's like leaving the 'choke' on permanently.

Something else was disturbed by the sounds of it.  Maybe they removed one of the boost pipes to get at the thermostat and didn't fit it back properly.

Looks like a clean example by the way    Tidy ones are getting scarce now due to age, rust and neglect.

[Ben-MK5GTI]

Thanks for the comments guys!
After investigating the 3 electrical connectors from the wiring loom he had to unplug to do the job he managed to snap every single one so they was vibrating loose and not making a proper connection, not sure what every connector does but I've had to cable tie and glue them in place temporarily until new ones come by post! This seems to have cured it.