I had a simalar problem once,couldn't hold a steady RPM it would idle fine just touch the accelerator and you were at full throttle,stopped for fuel and about couldn't get the cap off there was so much vacuum in the tank,it was causing the problem the cap wasn't venting had to get a new tank and cap,it sucked the tank in so bad it ran out of fuel at half a tank.About 2 months later same problem but this time it was the IP.

 

yea mine lets off a bunch of air when i unscrew the cap...can i drill a hole in the cap and try to fix it that away?

 

I wouldn't just buy another cap.

 

make sure its a diesel cap though. parts guys usually wont know the difference. so ask and make sure it is

 

I bought mine at the dealer ship