Chasing a rough idle?

[2009TahoeLT]

Ok so my 2009 Tahoe 5.3 has been running a bit rough/choppy. It has had codes previously but have since not come back. Small and large evap leak, and misfire codes. Problem is it’s idling at about 575-600. When it idles over 600 it runs nice. When it drops into the 500s is when it runs rough but doesn’t show any misfires. Now if I rev it to 1200-1600 it misfires horrendously. But doesn’t throw a code. It also sputters when I come to a slow stop. I’ve replaced all plugs, wires. Replaced intake manifold gasket. Replaced purge valve. Fuel pump was replaced. Cleaned maf. Intake manifold pressure reads 69-70kpa. Long term fuel trim bank 1 is -11.719 and bank 2 is -10.156.
Short term fuel trim bank 1 and 2 is all over the place. Bank 1 is anywhere from -3.960 to 1.556 and bank 2 is -1.568 to 4.688. I’ve done a throttle body relearn, crank relearn, fuel trim relearn. I don’t know if any of those numbers are in a good range honestly. There’s also 2 coils that seem to be clicking loud and you can feel it. Also injectors seem to be louder than normal also. Any ideas? My buddy mentioned timing chain as a possibility but I’m not sure. I have 250k on this 5.3. Got it with 242k. Idk what has been done previously. Any insight would be amazing! Thanks in advance

 

[Doubeleive]

Ok so my 2009 Tahoe 5.3 has been running a bit rough/choppy. It has had codes previously but have since not come back. Small and large evap leak, and misfire codes. Problem is it’s idling at about 575-600. When it idles over 600 it runs nice. When it drops into the 500s is when it runs rough but doesn’t show any misfires. Now if I rev it to 1200-1600 it misfires horrendously. But doesn’t throw a code. It also sputters when I come to a slow stop. I’ve replaced all plugs, wires. Replaced intake manifold gasket. Replaced purge valve. Fuel pump was replaced. Cleaned maf. Intake manifold pressure reads 69-70kpa. Long term fuel trim bank 1 is -11.719 and bank 2 is -10.156.
Short term fuel trim bank 1 and 2 is all over the place. Bank 1 is anywhere from -3.960 to 1.556 and bank 2 is -1.568 to 4.688. I’ve done a throttle body relearn, crank relearn, fuel trim relearn. I don’t know if any of those numbers are in a good range honestly. There’s also 2 coils that seem to be clicking loud and you can feel it. Also injectors seem to be louder than normal also. Any ideas? My buddy mentioned timing chain as a possibility but I’m not sure. I have 250k on this 5.3. Got it with 242k. Idk what has been done previously. Any insight would be amazing! Thanks in advance

Have you done a back pressure test?

 

[Trey Hardy]

Ok so my 2009 Tahoe 5.3 has been running a bit rough/choppy. It has had codes previously but have since not come back. Small and large evap leak, and misfire codes. Problem is it’s idling at about 575-600. When it idles over 600 it runs nice. When it drops into the 500s is when it runs rough but doesn’t show any misfires. Now if I rev it to 1200-1600 it misfires horrendously. But doesn’t throw a code. It also sputters when I come to a slow stop. I’ve replaced all plugs, wires. Replaced intake manifold gasket. Replaced purge valve. Fuel pump was replaced. Cleaned maf. Intake manifold pressure reads 69-70kpa. Long term fuel trim bank 1 is -11.719 and bank 2 is -10.156.
Short term fuel trim bank 1 and 2 is all over the place. Bank 1 is anywhere from -3.960 to 1.556 and bank 2 is -1.568 to 4.688. I’ve done a throttle body relearn, crank relearn, fuel trim relearn. I don’t know if any of those numbers are in a good range honestly. There’s also 2 coils that seem to be clicking loud and you can feel it. Also injectors seem to be louder than normal also. Any ideas? My buddy mentioned timing chain as a possibility but I’m not sure. I have 250k on this 5.3. Got it with 242k. Idk what has been done previously. Any insight would be amazing! Thanks in advance

I’m chasing this same issue mine has 400,000 on it and I’ve done plugs wires pcv lines purge canister intake gaskets swapped a couple coil packs out and changed a 02 sensor and nothing has seemed to help

 

[2009TahoeLT]

Have you done a back pressure test?

I have not. How would I go about that one?

 

[Fless]

Use a scanner on it that can count individual cylinder misfires to see what the counts look like.

The IAC (you know, Idle Air Control) on the throttle body may need to be cleaned or replaced.

The LTFTs are a bit high -- it's learned to take away fuel so you may want to check the operating fuel pressure to see if it's high.

 

[j91z28d1]

trims removing 10% fuel from each side is a signs it's bit happy.

can your scanner read alcohol percentage?


intake leaks? exhaust leaks? missing any exhaust manifold to head bolts? they are common to be missing.


how does one do a fuel trim reset? I've seen it posted but don't know how this commonly done?

 

[Doubeleive]

I have not. How would I go about that one?

A back pressure test is done thru the 02 sensor bung

 

[Fless]

trims removing 10% fuel from each side is a signs it's NOt happy.

can your scanner read alcohol percentage?


how does one do a fuel trim reset? I've seen it posted but don't know how this commonly done?

Good catch; I forget about the alcohol content issue with some of these. It might think the fuel has more alcohol than it does, and is adding fuel, but then the O2 sensors tell it to take it away.

LT fuel trim reset is done with a scanner. STFT resets with ignition cycle.

 

[j91z28d1]

Good catch; I forget about the alcohol content issue with some of these. It might think the fuel has more alcohol than it does, and is adding fuel, but then the O2 sensors tell it to take it away.

LT fuel trim reset is done with a scanner. STFT resets with ignition cycle.

ahh, I've not noticed ltft reset on mine. so my thoughts is, you check alcohol %, if zero. reset ltft, unplug the front o2 and see how it runs on base fueling. if it idles smooth and doesn't break up at the 1200-1600rpm then at least use know the fueling is correct. Just gotta find out why the trims are chasing lean.

if it still runs badly, I'd say your coils making noise are worth checking out. pop the hood at night and look for arching. that clicking noise might be spark breaking out to a ground instead of firing the spark plug.

 

[Fless]

ahh, I've not noticed ltft reset on mine. so my thoughts is, you check alcohol %, if zero. reset ltft, unplug the front o2 and see how it runs on base fueling. if it idles smooth and doesn't break up at the 1200-1600rpm then at least use know the fueling is correct. Just gotta find out why the trims are chasing lean.

if it still runs badly, I'd say your coils making noise are worth checking out. pop the hood at night and look for arching. that clicking noise might be spark breaking out to a ground instead of firing the spark plug.

The reset for alcohol percentage is just that, and I believe it will also reset the LTFTs. Now I'm not sure if there is a reset just for the LTFTs, but that could vary by year and fuel system. I would think it would need to be separate, as you may not want to do both at the same time.

 

[2009TahoeLT]

Use a scanner on it that can count individual cylinder misfires to see what the counts look like.

The IAC (you know, Idle Air Control) on the throttle body may need to be cleaned or replaced.

The LTFTs are a bit high -- it's learned to take away fuel so you may want to check the operating fuel pressure to see if it's high.

It doesn’t misfire on idle, but misfires on all cylinders when it revs to 1200-1600rpm

 

[2009TahoeLT]

trims removing 10% fuel from each side is a signs it's bit happy.

can your scanner read alcohol percentage?


intake leaks? exhaust leaks? missing any exhaust manifold to head bolts? they are common to be missing.


how does one do a fuel trim reset? I've seen it posted but don't know how this commonly done?

Yeah it’s at 11.480. Intake leaks shouldn’t be any. I definitely have exhaust manifold leaks. No broken bolts though. Just very rusted. I did a fuel trim reset through the scanner I have

 

[j91z28d1]

The reset for alcohol percentage is just that, and I believe it will also reset the LTFTs. Now I'm not sure if there is a reset just for the LTFTs, but that could vary by year and fuel system. I would think it would need to be separate, as you may not want to do both at the same time.

yeah. reset both at the same time would be good, just to make sure. Basically my thought is if you can get it running without any correction going on. everything at zero and then unplug the o2 so it stays there while running. if it runs better then it's correcting incorrectly. like pulling fuel based off leaks. then you'd atleast have a place to start. if it runs even worse, then that could mean a injector is stuck open or something making it rich. followed up by checking the plugs to confirm rich or lean running badly. it's crazy how both can kinda feel the same sometimes.


but as it is with the ecm removing a bunch of fuel, we don't know if that's helping or hurting.

 

[2009TahoeLT]

yeah. reset both at the same time would be good, just to make sure. Basically my thought is if you can get it running without any correction going on. everything at zero and then unplug the o2 so it stays there while running. if it runs better then it's correcting incorrectly. like pulling fuel based off leaks. then you'd atleast have a place to start. if it runs even worse, then that could mean a injector is stuck open or something making it rich. followed up by checking the plugs to confirm rich or lean running badly. it's crazy how both can kinda feel the same sometimes.


but as it is with the ecm removing a bunch of fuel, we don't know if that's helping or hurting.

I did also go through all the injectors in my scanner and they all open and close. Started running rougher when I closed them. I’ll try and see if I can reset both and unplug o2 and see what happens and what readings I get. So from the readings I mentioned it does look like it’s removing fuel? I’m not sure what the normal range is.

 

[2009TahoeLT]

I did also go through all the injectors in my scanner and they all open and close. Started running rougher when I closed them. I’ll try and see if I can reset both and unplug o2 and see what happens and what readings I get. So from the readings I mentioned it does look like it’s removing fuel? I’m not sure what the normal range is.

The other thing I did and forgot to list was I unplugged the maf sensor while running and it ran the same. No change in idle whatsoever.

 

[2009TahoeLT]

yeah. reset both at the same time would be good, just to make sure. Basically my thought is if you can get it running without any correction going on. everything at zero and then unplug the o2 so it stays there while running. if it runs better then it's correcting incorrectly. like pulling fuel based off leaks. then you'd atleast have a place to start. if it runs even worse, then that could mean a injector is stuck open or something making it rich. followed up by checking the plugs to confirm rich or lean running badly. it's crazy how both can kinda feel the same sometimes.


but as it is with the ecm removing a bunch of fuel, we don't know if that's helping or hurting.

So it’s running the exact same with o2 sensors unplugged. This is the long term and short term trim values at idle with them unplugged

 

[Fless]

What's different in the captures? RPM? Can you get these values with it showing if fuel control (closed loop) is on, and the RPM at the time?

What is the MAF reporting in g/sec at hot idle? And what's the corresponding idle speed?

 

[2009TahoeLT]

What's different in the captures? RPM? Can you get these values with it showing if fuel control (closed loop) is on, and the RPM at the time?

What is the MAF reporting in g/sec at hot idle? And what's the corresponding idle speed?

What's different in the captures? RPM? Can you get these values with it showing if fuel control (closed loop) is on, and the RPM at the time?

What is the MAF reporting in g/sec at hot idle? And what's the corresponding idle speed?

Those are both at idle. But that’s what it’s jumping between value wise. I can probably get the values with closed loop and possibly rpm at the same time. I’ll also grab the maf and idle

 

[2009TahoeLT]

What's different in the captures? RPM? Can you get these values with it showing if fuel control (closed loop) is on, and the RPM at the time?

What is the MAF reporting in g/sec at hot idle? And what's the corresponding idle speed?

Looks like I can only show loop status or hot open loop.

 

[j91z28d1]

that's kinda weird it's still removing 8% fuel with the o2s unplugged.

is it possible to clear/reset those? I was expecting trims to be stuck at zero with the o2s unplugged.

edit, and it's in closed loop. yeah that didn't work at all lol.