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I have a Chevrolet 2008 Suburban 1500 with the V8 5.3L engine. It has been stumbling at idle intermittently. Sometimes it starts fine, sometimes you need to step on the gas to get it to start, sometimes it will start fine but then all of a sudden stumble and be struggling for survival as if it can’t regulate idle speed properly. It seemed like the problem was getting worse before I change the spark plugs and wires (they were sooty), but changing them did not get rid of it. I’ve also changed the ECT sensor. Compression test was good. Oil was changed recently. Fuel injectors were changed recently. Fuel pump was changed recently, but apparently “it was for the next tier up, not actually for this Suburban” (I was not the one who worked on it). I believe the problem subsided after all those replacements were made. Air filter is good.

The throttle body, and that end of the air intake was notably very dirty, with black rubby stuff that possibly came from this mystery hose connecting to the air intake from behind the right valve cover.enter image description here It seems to be putting a black oily liquid into the air and take on occasion.

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  • Somebody left an answer that seemed correct, but when I came back to mark it as such, it was gone. They said this was basically the other side of the PCV system.
    – TheLabCat
    Nov 26, 2023 at 20:47

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The inside of the throttle body needs to be spotless clean and shiny bright for the car to idle properly. Get yourself some throttle body cleaner and keep at it until the cleaning cloth shows no further residue. Your engine should then idle properly.

Whatever the issue with the PCV system, you need to fix it so your throttle body will stay clean.

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  • Is throttle body cleaner the same as carburetor cleaner?
    – TheLabCat
    Nov 27, 2023 at 22:26
  • @TheLabCat Not necessarily. If all it says on the label is "carb cleaner" then you shouldn't use it on a throttle body. If it says something like "carb and throttle body" then it's OK. "Throttle body" needs to be on the label or you shouldn't use it.
    – MTA
    Nov 27, 2023 at 22:43
  • It said “carburetor and air intake cleaner,” and seemed to dissolve the gunk well enough. I’ve already done it, and it seemed to help. So, is it good?
    – TheLabCat
    Nov 28, 2023 at 0:52
  • @TheLabCat If it's clean, it's good. For future reference, if "throttle body" is on the label, the solvent is designed not to soften or damage any plastics that are commonly found in a throttle body. Carbs may not have any plastics and they can stand more aggressive solvents.
    – MTA
    Nov 28, 2023 at 1:50
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    @TheLabCat Right, and an actuator attached to the side, maybe a sensor. Not sure what it looks like on your engine, but on a 2008 perhaps not much plastic at all inside. No worries, your idle is good now I assume?
    – MTA
    Nov 28, 2023 at 19:41

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