Damn it - unusual idle
 

See my signature for work performed on rig. Currently at 112K miles. Solid running truck.

Today, I noticed a rough idle at stop lights. Can actually feel it upon stopping, then seems to normalize and steady out after 15-20 seconds. If truck is put into park, the idle smooths out immediately.

It's nothing spectacular - it's just that the truck typically runs so smooth. Any ideas on what I might check? Always run 93 octane, by the way.

DRW

do you have an ultragauge? I have the same exact problem. I have a very slight exhaust leak at the manifold flange to the y-pipe, and an o2 that goes flat once it warms up. I'm assuming once I get the manifold bolts tightened up and the new o2s I should have mine fixed. mine is very mild, almost unnoticeable, but when your the driver you know it shouldn't be there.

Do you have any codes, when was it last tuned along with 8 mm silicone wires and have you ever cleaned your t/body?

Sorry to thread jack but from gaining tons of experience trying to track down my "manifold downpipe flange" leak I needed to share this: I found the factory weld had ceacked at the Y-pipe flange. Tried welding it didn't hold for more than 2months before it started leaking again. Ended up replacing the entire Y-pipe assembly $$.

MAF could be dirty, throttle body could be gunked up, injectors could be gunked up, plugs, wires (as stated above). Low octane fuel. I always run 93.
175,000 miles and kickin; the sumbitch purs like a kitten.

We need more history to help properly diagnose.

Use a scanner and look at the O2 sensors at idle.
I bet an O2 is defective - or will be so soon.

Wouldn't waste time doing that unless he has codes which he hasn't mentioned so far.

Thanks
 

I'll read the codes this weekend and post. The plugs and wires have less than 10,000 miles on them. Haven't cleaned throttle body lately, though I run a can of sea foam through the gas probably every fourth fill up (and I always use 93 octane). The O2 sensors were all replaced within the past 6000 miles or so.

If you have a live code scanner - take it along with you on a drive.
Look at the temperature of the engine.
Look at the fuel maps
Look at the O2 sensors.
You have nothing to lose except a little time doing this.

The clue here is you are going from load to no load at a stop light and the idle
gets rough... Then smooths out.

OK what if you do this..
When you are coming to a stop light, put it into Neutral when you are going about 10 miles an hour and keep it in neutral.

Does it still stumble?

---
Or, if you are parked after a trip -
engine is still running.
Put it in D.
Put the brake on with the left foot firmly.
With the right foot - take the engine to 1,200 RPM or so and let it off..
Does it still stumble at idle?

Wondering if the speed sensor is causing an issue....

Code read
 

Finally read codes:

P1175
P1172

Pending code:

P0103 Mass or volume air flow circuit high input

Perusing the forum, it looks like 1175 and 1172 are variously attributed to O2 sensors, MAF sensor, and/or vacuum lines. O2 sensors are Bosch and relatively new. I run a standard paper air filter. The erratic idle that prompted me to post disappeared as suddenly as it came - not sure how to explain that. I cleared the codes - I'll see if they come back anytime soon.

I did use my iPhone to take a couple of gauge readings that might be pertinent:


at idle at 2200 rpm
MAF 7.14 g/s 24 g/s
Fuel trim SB1 -12 +4
Fuel trim SB2 -15 +4
Fuel trim LB1 0 0
Fuel trim LB2 +1 +1

Any suggestions?