2006 TB LS, 4.2L, with rough idle

[dmtaurus]

Have a 2006 TB with the 4.2L engine. It has 120,000 miles, and has started to run rough at idle after a few minutes. The symptoms are:
1. Somewhat rough idle after thorough warm-up. Idle is at about 650 rpm.
2. When cold, and idling at about 650 rpm in gear, it runs smooth.
3. There is no misfiring at idle or when driven on the road.
4. There are no stalling problems.
5. It has good power.
6. After extended highway speed driving for several hours, I noticed when I stopped and allowed the engine to idle that it took several minutes before the rough idle would begin. Idle would be at about 650 rpm.

The engine maintenance history is:
1. Oil changes every 3k~4K since new with 5W-30 Valvoline natural oil and AC filter.
2. Spark plugs changed at 60,000 and 119,000 using AC spark plugs as listed in the owners manual.
3. Throttle body was removed and cleaned with CRC throttle body cleaner at 30,000, 60,000, 90,000, and 120,000.
4. The intake manifold was removed at 106,000 and the manifold gaskets, the valve cover gasket, and the fuel injectors were replaced. By the way, the manifold bolts were loose from the factory and I torque tightened them per the specification listed in the Haynes manual.
5. The spark plug coil tubes were replaced at 106,000.
6. The variable valve timing solenoid was replaced at 99,500 as a precaution against the oil screens over the ports loosening at traveling through the oil circuit. All screens were intact on the old VVTS.
7. Thermostat was replaced at 106,000.
8. Alternator was replaced at 102,000.
9. MAF sensor has been removed and cleaned over the years at regular intervals with the last cleaning occurring last summer.
10. Installed new rubber plug on intake manifold port to replace age-cracked plug.
11. Inspected the brake booster valve and brake vacuum hose and both appear to be good and functional.

Any hints as to what might be causing the rough idle? I have not checked fuel pressure or the exhaust air injection system. I'm not sure if the air injection system would contribute to this problem. Also, I don't see an underhood sticker for vacuum hose routings, or I would have checked them too.

 

[jsheahawk]

Does anyone know if the 2006 was subject to the warped head/rough idle issue? I know my 2002 is. It's rough at idle and will throw a misfire code if I let it sit long enough. It goes away above idle. People have posted the bulletin before.

 

[mrrsm]

@dmtaurus ... Your Original Post should stand as the "Very Best Way to Solicit Mechanical Information and Assistance at GMT Nation" ...for all time. You're a thoughtful, careful Mechanic that doesn't run around with his "Hair on Fire" and your approaches to figuring things out get Straight "A"s. It is a pleasure to read this and see how organized your thinking is... I mean it...

This might sound a bit crazy... but my "Mechanical Spirit Guide" here... @Mooseman ...might suggest that if what you are feeling as a "Rough Idle" includes vibrations felt in the body, steering wheel and brake pedal... then perhaps a look at your Motor Mounts for any not-so-obvious greasy and sticky leakage from underneath them... may point out your culprit.

The below link is a recent post covering the subject and provides images and part numbers of the only acceptable replacements if this turns out to solve the problem--> AC-Delco OEM Motor Mounts:

http://gmtnation.com/forums/threads...replacement-passenger-side.15720/#post-506654

@jsheahawk ... If the Aluminum Head on this Atlas engine should ever warp... or if the Rubberized intermediary in between the layers of the S/S MLS Head Gasket were to erode and fail... quite a few other symptoms like escaping steam from vaporizing coolant or White Smoke leaving the Tail Pipe and/or Oil and Coolant mixing in the lower Oil Pan and turning the the Motor Oil into Brown Pudding would probably be present.

Conversely... you might also observe Oil Droplets floating in your Coolant just under the Radiator Cap. And if any of these symptoms were to be present... the situation would never improve once that happens from having the engine become "warm and swollen" ...so to speak.

 

[jsheahawk]

@jsheahawk ... If the Aluminum Head on this Atlas engine should ever warp... or if the Rubberized intermediary in between the layers of the S/S MLS Head Gasket were to erode and fail... quite a few other symptoms like escaping steam from vaporizing coolant or White Smoke leaving the Tail Pipe and/or Oil and Coolant mixing in the lower Oil Pan and turning the the Motor Oil into Brown Pudding would probably be present.

Conversely... you might also observe Oil Droplets floating in your Coolant just under the Radiator Cap. And if any of these symptoms were to be present... the situation would never improve once that happens from having the engine become "warm and swollen" ...so to speak.

I know that a warped head or busted head gasket is a disaster, but I'm talking about the misfire at idle issue. There's a bulletin out there for it; someone will pipe up with it hopefully. It goes away as soon as you give it any throttle. I've had it for years. It isn't a huge issue, and it's costly to fix.

Re-reading the OP, he has no misfires at idle. I withdraw all of this and fade quietly into the shadows!

 

[Mooseman]

Never heard of such a bulletin. Only one similar to this would be for cylinders dropping into the block but all of those would be gone by now.

 

[dmtaurus]

MRRSM, first, thank you much for the compliment. And second, I will check the link and look at the motor mounts as you described.

 

[gmcman]

Rev it up to 800-1000 rpm and see if it smoothes out. I agree the motor mounts will make it feel rough and will be most pronounced at 600-650 rpm.

 

[dmtaurus]

I just tested for excessive engine torque twist at the mounts while in forward gear and reverse gear and holding the brake. The engine twisted very little in either direction in both instances. I inspected the motor mounts and felt all around them on both the top and underside for a greasy, sticky substance and both were dry all around. Even though I don't see any tears or deterioration of the rubber, are the mounts capable of still being worn out? As far as idling, yes, rev the engine from idle up several hundred rpm and it smooths out.

 

[AzTruckGuy]

Have you checked the coils themselves? I have 98k miles and I had a slight vibration at idle thinking it was the motor mounts. I did a fuel pressure check and a coil came up bad, without giving a code. I'm replacing all the coils today and then seeing if I have a vibration still at idel, prob will since being under it looks like the drivers side motor mount looks shot. But don't rule out the coils.

Did you turn the AC on to check the motor mounts while you idle? They tend to vibrate more..

Update I just replaced all my coils and that vibration that I thought were motor mounts at idle seems to have disappeared, even when I turn on the AC the vibration just isn't there as it was before. I have 98k miles and those original coils just looked shot, I wouldn't be surprised if my mileage goes up some

 

[dmtaurus]

That is a good suggestion. I think Haynes goes into how to check them; I'll have to check. Did you perform the test with a VOM, or some other type of test meter? I'm somewhat surprised that a misfire code didn't recorded by the PCM; could a coil misfire be intermittent and not occurring enough to be recorded?

 

[AzTruckGuy]

That is a good suggestion. I think Haynes goes into how to check them; I'll have to check. Did you perform the test with a VOM, or some other type of test meter? I'm somewhat surprised that a misfire code didn't recorded by the PCM; could a coil misfire be intermittent and not occurring enough to be recorded?

I'm not surprised at all, its a GM product. But they are coils and these parts do tend to go out, since they get the most use. There was already 2 coils replaced, which meant the coils were going out. My issue was a another coil on its way out but still firing, so the PCM just wasn't reading it. I feel the PCM, since GM tuned it to be forgiving and give the best drive is flawed. The other problem with the coil going out, I mistook the vibration as a motor mount issue, my motor mounts are shot BTW, but I just read into as it being just the motor mount and not something like the coil too. I've learned that the Trailblazer is a vehicle you can keep throwing parts at it and it will still come up with something else being the issue....I would recommend complete coil replacement at or before 100K or sooner if one goes, no point in just replacing one. I had to break my tabs to get 2 off, the plastic seemed fused to the connector.