: Surging/Stalling When Warm, Lean Codes -- Not Plenum



nesls
03-06-10, 09:49 PM
I have a 2001 SLS and the engine intermittently surges or stalls and throws P0171 and P0174 (lean) codes. However, this behavior only sets in after the car has fully warmed up -- usually after 12-15 minutes of driving.

Originally, the idle was also rough. I consulted cadillacforums, and found information on the infamous plenum. Using a telescoping mirror, I located a crack on the back side of the plenum (not on the bottom, as is most common.) I had it replaced at a local garage along with the manifold gaskets. As I drove away I noticed that the idle seemed smoother, but the temperature dependent surging/stalling returned once the car warmed up, accompanied by the same lean codes as before.

Any ideas?

Ranger
03-06-10, 10:49 PM
Check the grommet that the PCV tube mounts in. It may be loose and cause a bad seal (vacuum leak). Also check the vacuum hoses as well as the manifold over pressurization valve at the front of the manifold in the "tunnel" behind the P/S pump for a vacuum leak.

stoveguyy
03-07-10, 12:11 PM
you had new gaskets and plenum replaced. could be a warped manifold. buick V6 motors have issues with manifolds. yes it could be a loose hose but you would hope the shop might catch any fitting that seemed loose or maybe cracked.

Submariner409
03-07-10, 04:24 PM
Very difficult to warp a composite glass/plastic manifold, and the gaskets are silicone flex pieces torqued to 7.5 ft/lb.

Stall/surge when warm - after engine controls go to open loop. Air/fuel mixture fault.

Remove the beauty cover and trace down every plastic and rubber vacuum line from/to the intake manifold passenger end recess and throttlebody.

Double and triple check the clamps and connectors at the MAF and throttlebody, as well as the intake resonator thingy: a leak in any of those connections will set lean codes, and that MAF is ultra-sensitive to leaks. (Have you tried to modify the MAF ??? That's a NO-NO.)

Consider this mod to make things easier to get to, as well as air filter changes.

If checking for leaks doesn't work, check your codes and consider a junkyard MAF as a double-check.

nesls
03-08-10, 10:45 AM
Thanks for the replies. I will double/triple check the prescribed hoses, clamps, etc.

Submariner, I guarantee you that I definitely have not modified the MAF. That would be way over my head!

stoveguyy
03-08-10, 05:08 PM
i get the same codes on my car. replaced intake. new gaskets. checked all hoses. been thinking of trying a boneyard MAF also. i get 0174 about 90% of time. i did get both 0171 and 0174 one time. the PCV valve lets in a metered amount of air. would a loose grommet have that much of an effect to cause a code? how about a new PCV valve?

Submariner409
03-08-10, 05:19 PM
I believe his (and my) engine has a PCV fixed orifice rather than a rattle-style PCV "valve".

The reason I threw in the "modified" MAF gig - I did a LOT of fooling with a JET Performance mechanically modified MAF in my 2002, including 2 round trips to Florida in which I ran 1 MAF down, the other back; using the "modified" MAF lost a tad of gas mileage and set all sorts of O2 sensor codes as well as a SES light. It made no difference whatsoever in quite a few stopwatch-timed 1/4 mile runs.

Here's a comparison of the two - I still have the JET. It's been on the shelf for over a year. As new. ???

Ranger
03-08-10, 06:30 PM
i get the same codes on my car. replaced intake. new gaskets. checked all hoses. been thinking of trying a boneyard MAF also. i get 0174 about 90% of time. i did get both 0171 and 0174 one time. the PCV valve lets in a metered amount of air. would a loose grommet have that much of an effect to cause a code? how about a new PCV valve?
I think it might. I seem to recall something along those lines in a past thread.