I've had my 2003 MC SS (NA 3800) for about a month (has 108,200 all original miles), and I've noticed a number of things going on:

1. Climate control panel won't change vents (default mode)
2. P0171 check engine light
3. Rough idle
4. Audible noise at high engine speeds (misfire?)

I'm led to believe that these are all related, can't seem to find anywhere on the forum that gives me a fix for my specific issues. Maybe an air leak somewhere in the intake manifold? Can someone with knowledge or experience help point me in the right direction? Thanks much!

 

Misfire would be a flashing CEL

P0171 means you are running lean, possible o2 sensor failure

Rough Idle and Climate control is probably a vacuum leak somewhere. Maybe spray some soapy water / starter fluid around and see if you can find it.

Audible noise could be a wheel bearing starting to go

 

If I am not mistaken, the HVAC controls on an '03 Monte are vacuum driven (I know on the '04 it is NOT vac driven). If so, that could be linked to the engine code and the rough idle issues.

I'd go with the starting fluid method. Start the car and mist starting fluid around ALL vac connections/lines, listen for the engine idle to change. If you find a spot it changes, inspect that area you sprayed for a problem with a vac line.

 

Personally, I'd use carb cleaner over starting fluid, but yes, squirt it around the engine, especially around the intake to find an idle change (idle should increase) then you know you've got a leak.

 

I've read vacuum issues can throw a P0171, my vacuum reading fluctuates from 18.7 to 19.5 at idle though. It's in the healthy range, but it's bouncing back and forth with my rough idle.

Fuel trims are 20.3 and 16.4 on the lean side (short-term and long-term, respectively). Yes, my climate control is vacuum-powered.

I have both carb cleaner and starter fluid. Where exactly on the intake should I spray?

 

Mist the spray (so no use of the straw on the can). Mist it around any vacuum lines (most if not all are in the front) and seals, around injectors, base of the black plastic upper intake, the brake boost line connection on the back of the upper intake, connections and sensors on the throttle body.

LISTEN for the idle to change (even slight). If you found a spot that when sprayed the idle changes, spary a couple more times (if it's a leak, you can repeat the scenario). Visually inspect that region (you may need to turn off the car at that point). Could have some dry rotting hoses for example (look for cracking on rubber connections).

 

Alright, found the culprit. There is a T where the brake booster line connects at the UIM. I guess I overlooked it before. I put my finger over it and could hear the engine running differently - I'm going to assume better! Also, fuel trim readings are nowhere above 3% either way now. Vacuum reading is also now steady at 20.8. I cleared the CEL and hopefully it doesn't come back.

But my HVAC still isn't working right.. Can anyone tell me what line is supposed to be connected here?

EDIT: http://www.grandprixforums.net/vacuu...ose-59455.html
I'll keep looking..

 

I found a vacant hose tucked away in the plastic conduit (see bottom of photo https://montecarloforum.com/forum/ge...19/#post475068 ). It's a black hose that wraps around and goes right in through the firewall. I plugged it into the T connector and voila- HVAC controls are functional! I still have no clue as to why the previous owner would have tucked it away.. My engine also still runs optimally (or at the best it's been since I bought it, on May 19 this year). Thanks all for your help!

 

Sweet! Easy fix