P0420 code and white on manifolds...

[dcwright02]

vehicle is an 04 grand prix, 3.8L S/C... runs good and well maintained but i have a cat efficiency below threshold code... ummm, don't know what cylinder but it's front passenger side on the exhaust manifold is turning white and i can't get rid of the code... think the cat is bad but i'm not sure what would cause my exhaust manifold to turn white? running lean maybe? too hot? gas mileage has dropped to 16-17 as well... thats down from 20-22... vehicle has no mods. any help? thx!

 

[Thinkmoto]

I have the same car, year etc. My front manifold is the same way, I found this out while rebuilding my motor. These cars have piss poor exhaust flow(which everyone will agree with). You have options headers, manifolds or downpipes to improve exhaust flow. Even in a stock application I'd recommend it as my ex's car fell victim to chipped pistons, which ultimately comes down to the heat built up by the s/c,poor exhaust flow and tight ring gaps. How do your plugs look? Check for smashed plugs. Your cat could be plugging up explaining the code. Might want to replace it with a 3" downpipe/cat combo.

 

[CTKGP09]

if you are staying stock you can just buy a downpipe, but if you want to mod even a little i say get headers instead of paying for a used cat. flow is unmatched

 

[dcwright02]

plugs should be fine as they were just replaced last summer with stock acdelco's (8 bucks a piece!!)... i clean the fuel system regularly including maf cleanings and throttle plate cleanings... wires are new so it's not tune up related... new state mandates emmisions, would a highflow cat and o2 emulator pass I/M? or should i just replace stock? i don't race but the extra power would be nice... monies an issue and junk yards are scarce in AK...

 

[SilvaMan61]

what he meant buy the plugs is that if theres enough back pressure and heat to cause the mannifold to turn white than theres probably enough to cause a piston to smash a plug.... so Id check man better to know now than later when the pisons start to chip.............

 

[Thinkmoto]

YES^^^^ also the O2 emulators don't work on the 04+ cars. I gutted my cat and got the PO420 so I'm trying the spark plug no fouler trick to see if that'll keep the PO420 away.

 

[dcwright02]

well, i'm goin for the cat... can't go performance else i won't pass the damn state emmisions... (plz make an emmulator work for the 04+!! lol) i'll definititely check those plugs too but the white is not too bad yet either (maybe a half inch down the pipe from the head if that). thanks for all the advice!

 

[blueguy]

The emulators work...it is the fact there is something different about the fueling in the PCM with the '02+ cars. You have to run a rear o2 to get the tuning correct...or else it'll richen up really bad. That was the issue I ran into.

 

[BwolfGT]

If that cat is REAL bad that could be causing the manifold to glow.

 

[Thinkmoto]

The no-fouler did the trick on my ex's car. There's at least 100 miles on it thus far and no codes. We don't have emissions here so I'm not worried about her running it catless.