Rough Idle and A/F Ratio

[B94Sport]

This one has me puzzled, maybe it is supposed to work this way or not, I don't know...
I have recently enountered a rough idle that causes a more noticeable vibration than normal when I am sitting at a stop light. I have an A/F Ratio gauge, and while driving, the truck tends to run slightly rich. When I pull to a stop light the A/F ratio will slowly dip until it becomes stoichiometrically perfect. At the same time that it reaches this point, the rough idle and vibration start. Press the brake and rev the engine to run richer, and the vibration goes away. Should I be concerned or is this a side effect of cold weather/oxygenated fuel?

 

[Alec]

Well your truck should start leaning out a bit (safely) while coasting or slowing down, it helps your economy and emissions. But if you start experiencing problems, then your truck might just need it to be a little richer. Hmmm, you might wanna try going to mid-grade fuel and see if that helps. If not, you might need to get a little more fuel to the motor.

It could just be clogged injectors, failing O2, or failing fuel pressure regulator also. It just sounds like you need a little more burn at idle.

 

[jimmiecakes]

Going back to my carburettor days, stoichiometric is a bit leaner than "lean best idle". Instead of worrying about running mixtures at this point, you need to investigate the tangible symptoms that you have.

Without experiencing what you consider rough idle and without experiencing the characterisitcs of the rough idle, it can be hard to set you on anything less than a basics on up diagnostic.

Once you're sure of the ignition system performance and good (even) compression, it's time to work up through the usual suspects.... dirty MAF, goo in the throttle flange, vacuum leaks and such.

At the same time, I really don't trust a lot of these add-on gewgaws - most of them are designed to impress you whilst parting you from your hard earned.

 

[B94Sport]

I replaced both O2 sensors, the FPR, and the TPS around this time last year... I also have a superchip, so I already run 93 octane fuel.

 

[Alec]

Hmmm, then I'd try cleaning up the intake system. You could remove the chip and see if that makes a difference. It could also be not enough spark, as was mentioned.