2012 impala 3.6 ls flex idle fluctuation
 

i have a problem with the idle rpm, it fluctuates between 400 and 1100 rpm, once warmed up. 150,000 miles and has apparently missed a number of oil changes and has substantial sludge. Removed valve covers to clean pcv system and replaced valve cover and intake gaskets. In tank pump and high pressure pump pressures fine, vacuum gauge reads 16 inches steady when cold but starts to fluctuate from 13 to 17 inches when warm, vacuum fluctuation is in line with rpm flux. Have checked extensively for vacuum leaks but don't find any. Next step is cyl leak down but heard there is a possibility that the cam timing system could be at fault. Anybody have any experience with this issue? BTW- Only occurs at idle, much more pronounced in neutral, over the course of several hours problem will lessen or go away periodically. Drove car and drives fine and does not lack power. No codes, other than high and low idle codes on occasion, but that is result of problem not the cause. Engine data does say commanded air/fuel equivalence ratio, which should be between 10-20 is at 1. As far as I know air/fuel mixture problem could account for the 13-17 inch swing in vacuum.

best way to find a vacuum leak....viewing the long term and short term fuel trim in closed loop operation at idle and 2500 rpm......post your results.

Thanks for the heads up, will check in the morning and post results.

One more thing I neglected to mention, car came in a week and a half ago and coded for TPS malfunction and had the flux issue. Replaced throttle body and it solved the problem. Came back 4 days later surging again.

after throttle body cleaning or replacement a idle relearn should be performed. Not saying that is the problem.


what was the tps dtc and has it reset?

Yeah, did idle relearn, no help. I don't recall the exact code but it was a TPS fault code, reset fault code after replacement, the only codes I have gotten since, and only once at that, are idle speed high and idle speed high, which are a result of the fpux.

Sorry , that's idle speed low and idle speed high, p0506 and p0507, if I recall correctly.

hey Tech2, checked out fuel trims and here's what I got: ST FT1 and 2 both read .98 at idle and 1.01 at 2500 rpm. LT FT 1 and 2 both read .91 at idle and .91 at 2500 rpm, all with engine warm and in closed loop. Commanded a/f equivalence ratio still reads 1.0 warmand .08 cold.

Hey Tech, just wanted to let you know what I found built cyl lek down tool, #1 cyl valve intake valves laking. Pulling heads now. Thanks for your help, I apperciate it.

what did you do to determine that? that would cause a rought idle...but no misfire?