Rough Idle - Suspect Throttle System

[Justinj360]

Hello everyone. I'm looking for some help on a weird rough idle issue I've been having. This started pretty recently during my move from MT to GA. At first I thought it was the transmission starting to go but now I think it's something much smaller than that, like the throttle system.

2004 GMC Yukon SLT 5.3L 230k miles.

The symptoms:
-Very rough idle even after trying battery reset for several days.
-Symptoms started gradually during the ~2500 mile trip.
-Haven't had a check engine light, code, or pending code.
-I got a flashing check engine light during one drive when trying to accelerate hard.
-I've used 2 scanners to read the throttle position sensor percentage. Both read a min of 20% throttle with engine running or off.

What I've done so far:
-Replaced throttle actuator assembly on throttle body.
-Replaced throttle position sensor on throttle pedal.

I've seen no changes in the throttle readings or the truck's behavior after replacing those parts. I'm not sure if I need to do a relearn procedure or something like that. Maybe it's not the throttle position at all but I don't know what else would cause the throttle position sensor to give a reading that far off. I read that it's supposed to be at 0 or very near it.

 

[Scottydoggs]

have tried cleaning the tb? carbon builds up and the butterfly tends to not close right. its the back side that gets dirty, air tube off looking in will look ok, open the blade, then you'll see the build up.

 

[Rocket Man]

Could be a clogged cat also. Might be restricted enough that when it’s under heavy load and you hit the throttle it can breathe well enough so it does this. I have had it happen to me.

 

[Doubeleive]

flashing check engine light is a misfire
when was the last tune-up? i.e. plugs, wires, coil packs

 

[Rocket Man]

flashing check engine light is a misfire
when was the last tune-up? i.e. plugs, wires, coil packs

Actually i thought it means damage to the catalyst can happen due to how bad it is. My cats were clogged enough that it caused massive random misfires but only if I mashed the throttle at first. Then if it sat for awhile I could drive it hard until they got hot and clogged up again. But I never got O2 sensor codes.

 

[Justinj360]

have tried cleaning the tb? carbon builds up and the butterfly tends to not close right. its the back side that gets dirty, air tube off looking in will look ok, open the blade, then you'll see the build up.

No I haven't yet. I had the intake tube off recently and it looked fine from the front so I didn't look into it more. Do you think carbon build up could make it stay open 20%?

Could be a clogged cat also. Might be restricted enough that when it’s under heavy load and you hit the throttle it can breathe well enough so it does this. I have had it happen to me.

I'm thinking it isn't this because the cats are only a few years old but who knows. I did have an O2 sensor recently that would give me the same low voltage code pretty often though. I replaced it about 5 or 6 months ago and I haven't seen the code since.

flashing check engine light is a misfire
when was the last tune-up? i.e. plugs, wires, coil packs

Yeah I looked into the flashing CEL and it sounds bad. Couldn't the engine think it has a misfire if something in the throttle system is off though? I had a remanufactured engine swapped in about 25k miles ago. At the same time I had the shop install new ACDelco spark plugs and wires. The coils are stock as far as I know. Is there a trouble code for coils?

 

[Scottydoggs]

flashing cel is only from two things really. one is misfires, the other is bad cats causing the misfires.

if you have not done a tune up or have no idea when it was last done. try plug and wires.

id still clean the tb up, if its never been done, its a mess back there. taking it off will let you clean it real easy, but if you dont want to, you can use a old tooth brush some brake cleaner or carb and choke cleaner scrub it up good on the inner walls and the back side of the blade and wipe it down with a rag best you can.

some times these tb blades dont like to be forced open, you can put a stick between the seat and gas pedal to hold it wot if you dont have a helper on hand to use their foot, key to run, engine off.

 

[Doubeleive]

a couple of things would be helpful a scanner and a laser thermometer the laser you can get at harbor freight pretty reasonable I think I paid $25 for mine and it works for general purpose, the thermometer you could use to check the cat temps and see if they are getting excessively hot or not. the scanner you could use to see what history codes are stored and possibly read live data when it is flashing. scanner prices and abilities are all over the spectrum, just depends on what you want to spend, you can get cheap bluetooth ones online and use a phone app if money is tight.

if the coils are original at 230k one or more could be going bad it's not uncommon for at least some of them to start going out at that mileage, but before you just start throwing money at it I would see what a scanner shows and check the cat temps

 

[Justinj360]

flashing cel is only from two things really. one is misfires, the other is bad cats causing the misfires.

if you have not done a tune up or have no idea when it was last done. try plug and wires.

id still clean the tb up, if its never been done, its a mess back there. taking it off will let you clean it real easy, but if you dont want to, you can use a old tooth brush some brake cleaner or carb and choke cleaner scrub it up good on the inner walls and the back side of the blade and wipe it down with a rag best you can.

some times these tb blades dont like to be forced open, you can put a stick between the seat and gas pedal to hold it wot if you dont have a helper on hand to use their foot, key to run, engine off.

I did have new ACDelco plugs and wires installed about 25k miles ago. I don't think I've ever really cleaned up the TB so I'll try and do that tonight if I have time. I'm hesitant about doing it though because I've read you can screw up the readings and might need to relearn it. It looks like I'll trying turning the key to run and jamming something against the throttle pedal to open up the butterfly. If there's a better or safer way to do it let me know.

a couple of things would be helpful a scanner and a laser thermometer the laser you can get at harbor freight pretty reasonable I think I paid $25 for mine and it works for general purpose, the thermometer you could use to check the cat temps and see if they are getting excessively hot or not. the scanner you could use to see what history codes are stored and possibly read live data when it is flashing. scanner prices and abilities are all over the spectrum, just depends on what you want to spend, you can get cheap bluetooth ones online and use a phone app if money is tight.

if the coils are original at 230k one or more could be going bad it's not uncommon for at least some of them to start going out at that mileage, but before you just start throwing money at it I would see what a scanner shows and check the cat temps

I do have a pretty capable Foxwell scanner that I've been using for this. It's able to run system checks and things like that but I haven't had the engine running long enough (Or smooth enough) for it to run the check on the emissions and cat systems so it just says incomplete for those. I haven't really been driving my truck since I moved here because I work from home now. Also I don't want to damage anything.
Should a bad coil throw a code? Right now I don't have any codes or stored codes but that could be because I've unhooked the battery already for a while and another time the battery was just completely dead from inactivity. I have it on a trickle charger now. I'm not sure if I can see live data for the coils... What should I look for to see that info? Are those laser thermometers from Harbor Freight pretty accurate? What temp should the cats be at in general?

 

[Justinj360]

Actually... Now I think about it, I did have the throttle body off about 2 years ago while I was working on the old engine. I managed to take some photos of the back side of the throttle body too. In the last photo you can see how much I cleaned it up but I didn't get everything because I didn't want to screw it up. I didn't open the butterfly. Do you think it looks bad?