: 2000 DeVille Northstar Problem

05-14-08, 11:26 AM
Originally I had a PO122 code - Throttle/Pedal sensor switch "A" low voltage. That has disappeared and now I have PO101, PO171, PO174 codes, a slightly irregular idling and last night it suddenly raced (in park) and quit. Two subsequent start attempts were the same but after about three minutes it started normally and had a smooth idle. Tell me what is going on and what do I do about it? does this have anything to do with air filters as I also have a relay clicking loudly under the dash as the A/C starts up. the clicking stops and the A/C runs normally although at one time it spewed tiny bits of foam through the vents but that has quit.

Alan L. Caldwell
Major USAF Ret.

05-14-08, 12:52 PM
with ese codes you might want to look in the area of the mass air flow, the air intake hoses are tight and proper connection with the maf, be sure the hose on the throttle body is tight and the air filter box is all secure

05-14-08, 09:10 PM
Also look for a vacuum leak in the rubber coupler between the TB and the manifold. It's called the plenum on the Northstar. They seem to be a rather common failure. It's generally on the underside where it is not visible. Spray the area with some Brakleen and listen for an RPM increase.

05-15-08, 02:48 PM
Thanks, it WAS the plenum. It's fixed. No SES light and car runs great. I still have the clicking noise apparently from the A/C vent door solenoid. Someone replied to that fact but the question is how do I change it? Can I get to it without dismantling the whole dash? I'm not even sure exactly where it is. Help!!!


05-15-08, 03:04 PM
Thanks, it WAS the plenum. It's fixed. No SES light and car runs great. I still have the clicking noise apparently from the A/C vent door solenoid.....
All of your HVAC doors are motor operated.

Display your DTCs again and pay attention to "B" codes.

05-15-08, 10:57 PM
Try cycling the HVAC system through all modes of air delivery and fan speeds as well as running the temp settings up and down a few times.

There's always the chance that the problem is just a sticky door.......worth a try. Been done in here before.

05-16-08, 11:00 AM
I think the actuators are accessible from under the dash, but not positive. If you are going to R&R it yourself, I'd suggest you get your hands on a FSM by Helm Inc, not Chiltons or Haynes. Ebay is your friend.

05-19-08, 02:44 AM
The codes are all there...

Classic plenum leak.

I tought mine was a bad tank of gas.

High failure rate part. Plan on replacing. $20 part.

Logan Diagnostic LLC

06-05-08, 12:26 PM
Where do I find said, plenum hose? I too have the rough idle issue. However to ad confusion every now and again I get a blinking service engine soon if I gun it on the highway, goes away in a few seconds. Ideas?


06-05-08, 03:34 PM
SuperC, What year is your car? The connector plenum (rubber bellows) between the intake manifold and throttlebody was not used until 2000 and later. If you have one, and it's cracked, a brief squirt of choke cleaner or throttlebody cleaner down underneath it will change the idle speed/sound of the engine for a second or two.

Not a hard fix.....call Luke, Parts, at Lindsay Cadillac over there >>>>

Might as well get the injector O-ring kit and intake manifold gaskets, too. It all has to come off. Not a hard job with simple hand tools.

The manifold only gets torqued to 8-9 ft/lb. Not too tight...it's fiberglass/plastic.