GM LL8 slight miss at idle

[Falcon_LS]

Vortec 4200 L6 in my Envoy, about 68,900 miles as of now. Idle isn't exactly rough, but you notice a slight vibration in the engine at idle that wasn't there before. Also, if you're coasting to a stop with the transmission in neutral (regardless of speed) and have the brakes applied, as soon as you stop, idle jumps to ~1,100 rpm and then comes back down. If the AC compressor is running, idle roughens up momentarily as soon as the clutch kicks in and when it stops. Performance is not affected. No check engine light or any codes stored in the PCM.

Couple of notes:

- Engine was flushed with LM Pro-Line and switched to M1 0W-40 exactly 4,000 miles ago - no drop in oil level, but the oil is pitch black. Oil life is at 61%. I'm running a Wix 51522XP series synthetic filter, and am considering a simple drain and refill with M1 0W-40 without replacing the filter. Then when it gets to 1% or so, change it again with the filter.

- Throttle body was cleaned 4,000 miles ago with the oil change, and a new air filter was installed.

- Spark plugs (Autolite APP-5263), coil boots (Denso 671-6069) were replaced 13,400 miles ago.

- Coolant (Dexcool) was flushed and a new thermostat (OE GM) was installed 13,400 miles ago. I monitor coolant temperature with my ScanGauge-II and since the weather dramatically cooled down, it's anywhere between 190-198°F. Back in the summer 216°F was pretty common though, and the fan doesn't go on full blast until 240°F anyway.

- Brand new blueprinted Delphi injectors with stainless steel basket filters installed 11,750 miles ago.

- Engine and transmission mounts replaced with OE GM parts 7,500 miles ago.

- Battery (Deka 778MF) installed 8,500 miles ago. Alternator's working properly, and there appear to be no electrical problems.

I run either 3M Max Strength Fuel System Cleaner or Techron every 3,000 miles. It got Techron when I changed the oil, and got it's dose of 3M 1,000 miles ago. I had a bottle of Berryman B-12 Chemtool Injector Cleaner I poured in the tank about 150 miles ago as well, but I doubt it's anything to do with the fuel system.

I've tried checking for vacuum leaks, but the only vacuum line I can find is the one going to the brake booster, which is flawless.

With the oil colored pitch black, the only thing that comes to mind are the VVT sensors for the exhaust valves, which also control EGR. Not entirely sure how I can go about checking those though.

 

[95busa]

Tps voltage fluctuations? Fuel filter?

 

[Falcon_LS]

No inline fuel filter, GM discontinued that in starting MY2005. With electronic throttle, I'll have to check to see if it has a TPS. Thanks!

 

[The_Eric]

With black oil, are you getting fuel dilution? What about fuel trims when this happens?

 

[Falcon_LS]

The oil turned black after I ran the 3M Max Strength stuff. I checked the fuel trims about a month ago with a Tech-II, when I was considering replacing the oxygen sensors. They were fine.

I'm thinking this is either going to be the CMP or the CPAS.

 

[tomcat27]

Have you cleaned the IAC valve?

 

[Durango]

I'm no mechanic but if your engine idle is missing slightly then I'd have your mechanic check the electronic engine module. Than might be if not part of your cause assuming everything is running well as your seem to describe.

Durango

 

[OVERKILL]

Is the fuel that bad there that you need to run that much injector cleaner through brand new injectors?

WOW!

I'd take a look at your trims and make sure they look OK.

Also misfire count.

 

[Cardenio327]

Originally Posted By: Falcon_LS
Also, if you're coasting to a stop with the transmission in neutral (regardless of speed) and have the brakes applied, as soon as you stop, idle jumps to ~1,100 rpm and then comes back down.


Nothing to worry about when it does this, the engine is just reacting to a condition in which it wasn't programmed to operate within. Vehicles aren't designed with coasting in neutral with the engine running as a normal operating condition, in fact in the US there is a federal highway standard that prohibits vehicle movement without a gear engaged.

If you are trying to hypermile by coasting in neutral, you are actually wasting more gasoline than by coasting in gear. When in neutral the engine must use fuel to maintain idle speed, additionally, idle speed is one of the most inefficient speed/load conditions an engine operates at. When coasting in gear fuel flow is cut way back, and in some cases shut off entirely.

 

[strat81]

The guys at GMTNation.com insist on nothing but AC Delco plugs. YMMV.