Service engine light keep popping up with error code P0171 and P0174. on 2001 Cadillac Seville SLS

Repleaced MAF, PVC, air filter and checked for vacuum leak and found none. This weekend I am going to replace the Plenum boot between the intake manifild and the throttle control to make sure there is no leak. When I raised the engine RPM pass 4000 I can feel one valve missing during the firing sequence. Appreciate your help on this.

by in Lees Summit, MO on October 17, 2012
1 answer 2 comments
ANSWER by on October 17, 2012
this code can be very tricky to diag and repair at times. you most likely have a vac leak and on the 4.6l the boot does crack between thottle body and intake.when you remove it look on the bottom for small cracks or holes. if its ok i rec you get a smoke test which will show any vac leaks you have. ps repl the boot since it will be off and is suppect to leak at times
COMMENT by on October 17, 2012
Thank yoo so much for your quick response. I did have the mechanic smoke test for vacuum leak and found one. I will check the Plenum boot. I have one more question, when I raise the engine speed rpm while the car is parked I can feel one cylinder speed hesitation. Do I need to chagne the park plugs or could it be a clogged injector. I did use the Chevron fuel injector cleaner and still the same proble. Thanks in advance.
COMMENT by on October 17, 2012
it might be related to the vac leak repair that 1st