Rough idle is back with vengence

[swathdiver]

I was helping a friend diagnose his truck yesterday and it would stumble ever so slightly at idle in a rhythmic fashion. Idle was stock and a little low. No stumble at all if idle raised even 25-50 rpms. The Tech-2 showed misfires on 2 cylinders, neither AFM, and one was much worse than the other. There was no correlation between what we could see on the scan tool and what we could feel or hear. Dmitry, did yours have some misfires?

 

[smokey_mountain]

There are no PIDs for misfires, never were logged PIDs ever for misfires. When I say I see misfires, I am only talking about OBD II $06 monitor misfires. $06 monitor gave me 4 across all 8 cylinders in 30 min drive, and what I was told that AFM causes those that I can see in $06 mode. These GDI engines of 2018+ have an implementation of AFM that causes $06 to show misfires. For PID to be created they should be in 500+ range. One of the 2018 owners that I stay in touch who used to have the rough idle, at some point had plug wire partially off the plug but truck still would not throw the PID. @swathdiver I have an update on RPMs too.

I was helping a friend diagnose his truck yesterday and it would stumble ever so slightly at idle in a rhythmic fashion. Idle was stock and a little low. No stumble at all if idle raised even 25-50 rpms. The Tech-2 showed misfires on 2 cylinders, neither AFM, and one was much worse than the other. There was no correlation between what we could see on the scan tool and what we could feel or hear. Dmitry, did yours have some misfires?

 

[OR VietVet]

Maybe a tune that resets idle at 50-100 rpm's higher, does the trick?

 

[BG1988]

Good folks of this forum,
while my other thread is locked, I have to open this one.

This is a problem for technically-curious, it is far beyond my troubleshooting ability.

Situation:

• Idle was relatively smooth with AC off season(and some other improvements)
• With light feather throttle tip in between 40-50 MPH to maintain the speed uphill engines pings even on 89
• Now AC back on and last night truck was shaking and rocking so that my wife called me asking to come over as she was afraid it would stall
• Mode $06 shows 3 misfires only across all 8 cylinders (much better than with original spark plugs)
• Fuel is Exxon 89 from 3 different stations
• No codes or MILs
• I have noticed that shaking and vibration is more noticeable as tank gets closer to empty, no idea if it has any bearing on fuel pressure, but again, no codes of any kind
• Just a reminder, I did new OEM plugs, new wires, new engine and transmission mounts, new OEM air filter, new oil change.

Appreciate your thoughts/ideas. I am 11-14 months away from my next truck, that is how long it takes to get that Kia, so I need to get this truck to run.

My logs are here https://docs.google.com/spreadsheets/d/14_bFRJg-Ew5v__t3jPefyXJLdsCx6D8YC-SclhXSGo4/edit?usp=sharing

the battery is not charging correctly

14.9 (in drive/park)-15.2(in M mode W/trailer on) volts stable is where it should be at idle cold start


new battery? might be a loose wire, mine runs rough it's the battery cable, it starts getting loose? sometimes to the point where it does not move at all even though the voltage looks good on paper AMPs x OHMS X volts =fail to deliver


my voltage has HIGH ripple (gotta figure that out) yeah that is going to cause issues...

my cable is pretty dirty i did clean it lightly(worked for a little while) but going to do a Deep cleaning tomorrow

with iso alcohol luckily I bough a couple of bottles before the MAD rush for supplies



but DONT reconnect the cables tell it's 100% dry other wise it will catch fire

like 5-10min you may also need to reset the time

 

[BG1988]

so I cleaned the up cables I pulled the screws to find some "dirt" aka impedance the voltage is good and the ripple evens it self stable once the battery is charged up I was at 14.835 volts to 14.865volts the ripple on dipping and rising from 14.2 to 14.7


you need a 10mm socket under the positive cable but i cleaned up both of them the negative cable was ok



that dirt keeps it from making good contact that was probably why when i adjust them it would fix it for a tiny bit of time

It did fix the oscillation.or upto 2000rpm high idle.. but not the rhythmic rough idle



But the rhythmic of the current idle is different then the rhythmic rough idle in the past

(not talking about the Oscillation idle that is a whole another issue that has been taken care of)




there is no reason to suspect misfires for me as the Cat temperature is perfectly stable at 350c when at idle. the TP % is at 20.4% idle or 610rpm in M4 gear he is talking about a 2018 but it's posted in the NNBS?


Watch the CAT temp see if it keeps going up and up? normally when it misfires it gets very hot. like in to the 900-1000C range 450-750C is normal driving temperature..

 

[smokey_mountain]

Folks, my rough idle puzzle is this:
1) Brand new OEM plugs and OEM wires -> no effect
2) RPM bump -> no effect especially with AC ON
3) Drop-in air filters AEM, Wix, OEM -> no effect
4) Brand new engine and transmission mounts -> no effect
5) Checked battery with a load test and all connections -> no effect
6) Fuel Exxon 89 or 93 -> no effect
7) DI Fuel injector and intake cleaners -> no effects
8) Lucas upper cylinder lubricant -> made things worse
9) $06 Mode 1-2 misfires total -> was told on DI 1-2 misfires on $06 is due to AFM
1

With all the above tried, truck rough idles in both D and P. Sometimes it shakes really bad for 1 second and back to mild shaking.

The strongest correlation that I am researching and need your help is this - if tank is full it seems to idle better, once tank <3/4 full, it starts to shake more, and once at 1/4 it is really bad. I have no idea how that can impact anything. On this DI engines there are two fuel pumps for low and high pressure. It seems to me that somehow fuel system is involved, I asked dealer and he said "all pressures are within spec". I have to believe what they are saying given that they are GM dealers, they know these trucks.

 

[Floep]

There is a hose connection between the actual fuel pickup and the pump suction connector, the pump sucks air as the fuel in the tank gets low, rough idle when standing still (no sloshing)
Higher fuel level in tank, pump sucks fuel.

 

[cam3439]

Did you do the oil change yourself, or get it done somewhere like the dealer?

I’d have to try to find the post, but someone else with a newer Yukon or Tahoe/suburban found that the dealer was putting in some lower quality oil that wasn’t the correct weight either. He would take it in for idle/rough engine issues but they never dealt with it. He did the oil change himself with a good quality oil and the correct weight, which solved his issues. Be chalked it up to the valve-train being hypersensitive and needy. I’m just passing along someone else’s experience.

 

[smokey_mountain]

I am trying to understand this point, am I reading it correctly that hose could be disconnected? What is the fault condition and how do I determine it? Certain pressure? Dealers are saying no codes of any kind in my truck.

There is a hose connection between the actual fuel pickup and the pump suction connector, the pump sucks air as the fuel in the tank gets low, rough idle when standing still (no sloshing)
Higher fuel level in tank, pump sucks fuel.

 

[smokey_mountain]

I did the oil change myself with 0w20 Mobil1, made sure that level is perfect just under the "Full" mark. Used Mobil1 premium filter as well (supposedly as good or better than OEM filter).

Did you do the oil change yourself, or get it done somewhere like the dealer?

I’d have to try to find the post, but someone else with a newer Yukon or Tahoe/suburban found that the dealer was putting in some lower quality oil that wasn’t the correct weight either. He would take it in for idle/rough engine issues but they never dealt with it. He did the oil change himself with a good quality oil and the correct weight, which solved his issues. Be chalked it up to the valve-train being hypersensitive and needy. I’m just passing along someone else’s experience.