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?

 

[Scottydoggs]

its dirty in the pics there, and that was 2 years ago, so its more dirty now.

but thats not the cause if the flashing cel. thats gonna cause hard starts poor idle.

spark plugs and wires do or a bad coil or two. or mouse chewed injector harness wire.

being you have a scanner, see if you can make the cel flash again, then scan it truck running, dont turn it off. once turned off a pending code will clear, see what if any thing you come up with. a p0300 is my guess. random misfire code.

bad maf can also do some screwy things as well. sometimes they dont set a code right away. maf will also mess with the trans not shifting right, or feeling right.

 

[Doubeleive]

might be easier to try a back pressure test to rule the cat's in or out, I don't remember off hand what the cat temps should be, your scanner may show the the cat temps while you are driving I think mine were showing 700-800 deg on my tech2 data while driving on the freeway (live data), I think when checking them with a laser you want to see if one end of the cat has a lot of difference in temp than the other end.

 

[Justinj360]

its dirty in the pics there, and that was 2 years ago, so its more dirty now.

but thats not the cause if the flashing cel. thats gonna cause hard starts poor idle.

spark plugs and wires do or a bad coil or two. or mouse chewed injector harness wire.

being you have a scanner, see if you can make the cel flash again, then scan it truck running, dont turn it off. once turned off a pending code will clear, see what if any thing you come up with. a p0300 is my guess. random misfire code.

bad maf can also do some screwy things as well. sometimes they dont set a code right away. maf will also mess with the trans not shifting right, or feeling right.

Alright I'll eliminate one thing at a time. I'll start off with cleaning the throttle body tonight to see if that helps. I might try cleaning the MAF sensor after that too. The real mystery to me here is why the throttle position sensor would be reading a minimum of 20% so that's why I'm starting with that. Could there really be that much carbon build up holding it that far open?

 

[Scottydoggs]

ive noticed my ss camaros commanded tp% is high as well. might be normal.

 

[HiHoeSilver]

Alright I'll eliminate one thing at a time. I'll start off with cleaning the throttle body tonight to see if that helps. I might try cleaning the MAF sensor after that too. The real mystery to me here is why the throttle position sensor would be reading a minimum of 20% so that's why I'm starting with that. Could there really be that much carbon build up holding it that far open?

Mine was running at 13%. I cleaned her up good, and it came down to 2.7%

 

[Justinj360]

Alright some interesting things happened today. I cleaned the TB and the MAF sensor last night and started it up today. It fired right up like nothing was ever wrong with it and like it hadn't just been sitting around for a few months. I have no idea what the actual fix was.
I took it for a test drive around the neighborhood and within the first 5 minutes I got a "tighten fuel cap" message. I've never gotten that message before and I always make sure the fuel cap clicks. I stopped and tightened it and I haven't gotten the message again. The message cleared itself before I had a chance to stop. The gas cap was also replaced with an ACDelco one a few years ago because the plastic string broke. I also had the fuel pump replaced by a shop a couple of years ago because it died.

The truck still isn't running at 100% but it is much better than before. At times when I tried to accelerate fairly hard it felt like there was something holding it back still. Almost like it wasn't getting enough fuel or something like that. 45 was about the top speed I got up to because I just stayed fairly close to home. I have been noticing lately that the engine and trans temps get warm faster than they used to but not up to any excessive temps. The engine doesn't exceed operating temperature.

There still aren't any stored or pending codes. I left my scanner plugged in while I took a couple test drives but I didn't record the information. According to the scanner, there still needs to be a test run on the catalyst and evap systems. I'm not sure how often that occurs. I also noticed that when the truck is idling the TP sensor drops down to around 7 percent but goes back to 20% when you turn it off. Seems odd to me.

 

[Scottydoggs]

sounds like the cats went bad on ya. engines are quite gutless when the exhaust cant get out.

you could pull the 2 up stream o2 sensors and drive it, if its got more power with two exhaust leaks the cats are bad for sure.

 

[wildcatgoal]

This sounds like a clogged cat-verter to me...

You can diagnose by looking at expected vs. actual O2 sensor readings in combination with expected vs. actual laser temp readings before, at, and behind the cat.

Look up South Main Auto on YouTube -- he diagnoses this stuff all the time.

 

[Rocket Man]

I suspected the cats when I first heard the symptoms. I’m curious if that’s the problem after all.

 

[Justinj360]

Alright I was finally able to do some more diagnosing today. I've been crazy busy at work for the last few days so sorry about the lack of updates.

I went to Harbor Freight and bought a laser thermometer. While I ran there I also left my scanner plugged in and set to record the whole data set. When the engine was at operating temperature I tried 2 medium-throttle runs from stops. Those both gave me the flashing CEL again and I had the same symptoms as the first time I saw the flashing CEL - Lack of power, feels like a misfire or not enough fuel, and jerky etc.). It then immediately gave me a pending code for P0308, cylinder 8 misfire. There was never a stored code though. After those pulls I babied it the rest of the way home and when I had to stop for a few stoplights, it would intermittently idle rough for a second. Before those pulls, it idled fine at stops.

When I got home I left the truck running and used my new thermometer on the cats. There wasn't a very obvious difference in their temps but I'm also not sure how big of a difference there should be between them if one went bad. I've listed the temps below. I don't remember the temps just before the cats but they were a little higher than the temps listed below and slightly higher on the drivers side again. I don't think the temps before the cats exceeded 500*. Same story after the cats but slightly cooler.
Cat temps:
drivers: ~450*
passenger:~415*

A quick search online tells me that cylinder 8 is closest to the firewall on the passenger side. Before I knew where number 8 was I was thinking maybe there was a correlation between the "misfiring" cylinder and which side of the exhaust was hotter but maybe that wasn't the case because they're on opposite sides.
Also, I suppose the cats being bad could possibly be the problem because I had the infamous cracked heads from Castech... That's why I had the engine swapped. I looked up the causes of cats failing and one was due to leaking coolant. I have no idea how long my heads were cracked on the old engine but this newer Magnaflow exhaust was coupled to the old engine for at least 3 years.

What do you guys think so far? I'm getting tempted to just take it to a shop soon and have the pros diagnose it.