I bought a third hand 04 cooper s. I chose to replace the thermostat and accessory belt. I figured while doing this I figured I would do the 15% pulley upgrade. Everything went smooth, however, when I started the car for the first time, it was not good. The idle initially shoots up high and then the car throws the p0300 random misfire. The car is running super rough. I did not touch the ignition system during this whole job. Has anyone had this happen to them? I need to fix this fast as it is my school car and school starts in January.

 

Did you change to a shorter belt and colder plugs when you put the 15% underdrive pulley in? If you used the original length belt, its probably slipping, causing low flows and a miss.

 

The pulley came with a 1360 belt, but it would not fit even with a fully compressed tensioner. I put a 1390 on

 

I put the smaller belt on and the problem is still there

 

Did you try rebooting your ECU?

 

Yes

 

Double check the ground strap by the engine mount as well as all the wires/plugs around the radiator. I got a 0300 code and one of plug wires at the coil was loose.

 

I fixed it. The intake to supercharger seal wasn’t perfectly fit. Runs like a top now. Now to put it all back together

 

GREAT !!!!

LOVE it when t works out pretty quick !!!

 

Thats great, when they get a boost leak, that can knock the power down. I would check out both sides intercooler seals/boots.