2001 Cadillac Seville SLS Q&A

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

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. -
Answer 1
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 1
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 2
it might be related to the vac leak repair that 1st -