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Rough idle after warm up

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I have a 2008 lgt with 100k. Not on the road yet (milking my 5 speed OB) I run it periodically just to keep the juices flowing. I notice that once it's warmed up it starts to shake a bit when it's just sitting there. Prior knowledge would lead me to think that it's spark plug wires/ something to do with ignition. I have not changed the timing belt yet but it's number one on my list. The car is completely stock. Also I would like to know what some of the first things that should be modified are, not necessarily for performance as much as to safeguard the car. All help will be greatly apreciated!
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Another rough idle / misfire.....these cars really have a problem with this....


Top common causes:

- Vacuum leaks

- Upper intake manifold gaskets (your 08 should already have the better ones though)

- Injectors

- Coil packs

- Plugs


And the really bad causes (but more unlikely):

- Burnt valve

- Cracked ringland

- Leaky headgasket


Coil packs sit directly on the plugs, no wires. You need to determine which cylinder is misfiring / roughness.


Plan on getting an AP or a TACTRIX any time soon? Would help troubleshooting a lot, at least until the misfire gets bad enough to throw a CEL.

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Mine was doing the same thing...except it would get cylinder 2 misfire on idle- cold or hot. Outside temperature would make it worse or better- worse in summer, less so in temps around or below 50 degrees ambient.


My rough idle and misfire are mostly gone, but I had a lot of things done at once (for other reasons) so I do not know exactly which was causing my issues.


Recent changes on mine:

- Valve lash adjustment

- Timing belt replacement

- New intercooler (original leaking)

- New intercooler to intake coupler (shot)

- New valve cover cross pipe (hard line, was corroded, and not seen until engine removal)

- New upper intake manifold gaskets (replace my old model ones with your model year part #)

- New Iridium one-step-colder plugs (went stage 2 and catless up pipe)


So I had a lot of intake related leaks. After 100 miles this morning I had a slight misfire feel starting to come back- but it was not enough to register on my AP monitoring cylinder roughness. If it comes back full force, I will be looking into my fuel system- mainly injectors and pump.


If you suspect injector, might be worth a shot to pick up some fuel system cleaner/additive- there is a thread about which is recommended- and see if that helps clear up the rough idle. Might not see an immediate improvement, but if it is semi-clogged injector it might make a difference and tell you the injectors need attention (replace/rebuild).

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Wow. Very helpful! I plan on having the Tbelt done and at the same time, having the car Gone through mechanically. But maybe I will end up buying an AP first to find out the problem and maybe save some money on unnecessary things. At the moment, I have sta-bil marine formula in the tank. Haven't noticed a difference yet but then again it's only been a week.
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