Low Power at Low RPM
#1
Low Power at Low RPM
2008 Yukon XL Denali 6.2l No Mods
Drove through a huge rainstorm on the highway, check engine light comes on, read codes for knock sensors on both banks, symptoms are low power until 2k rpm is met, then runs like a dream, replace knock sensors on both banks, opened ECU connectors checked for water corrosion, was a small amount of corrosion, cleaned it and used di-electric grease, reconnect connectors, take the truck for a test drive, first 4 minutes it runs great through all RPM ranges, suddenly the same symptom returns, low power at low RPM, no check engine light. Any ideas?
On a side note, I was attempting to climb my hill to the house, would barely move up the incline, switched to manual mode, it was in 3rd gear which i thought was odd, shouldn't the tranny drop a gear if under load and getting a lot of peddle? So I manually placed the tranny into 1st gear, it jumped with plenty of power, then put it in 2nd gear, climbed the hill no problem......
I appreciate any and all help. Thank you. Jon
Drove through a huge rainstorm on the highway, check engine light comes on, read codes for knock sensors on both banks, symptoms are low power until 2k rpm is met, then runs like a dream, replace knock sensors on both banks, opened ECU connectors checked for water corrosion, was a small amount of corrosion, cleaned it and used di-electric grease, reconnect connectors, take the truck for a test drive, first 4 minutes it runs great through all RPM ranges, suddenly the same symptom returns, low power at low RPM, no check engine light. Any ideas?
On a side note, I was attempting to climb my hill to the house, would barely move up the incline, switched to manual mode, it was in 3rd gear which i thought was odd, shouldn't the tranny drop a gear if under load and getting a lot of peddle? So I manually placed the tranny into 1st gear, it jumped with plenty of power, then put it in 2nd gear, climbed the hill no problem......
I appreciate any and all help. Thank you. Jon
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CathedralCub (06-24-2023)
#2
Senior Member
True Car Nut
Is it starting in second or third gear when it is having this low power symptom?
#3
Not sure. when I moved the shifter to manual, it was in 3rd gear at around 3mph. I can check in the morning and report back. Is it common for the yukon to start out in 2nd or 3rd gear? is that a problem?
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CathedralCub (06-24-2023)
#4
Senior Member
True Car Nut
No.
3rd gear at 3MPH is a problem if it is actually engaged in 3rd gear at 3MPH.
Last edited by CathedralCub; 06-24-2023 at 12:52 AM. Reason: Added a missing "f"
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CathedralCub (06-25-2023)
#6
Used my scan tool to do some tests.
Fuel trims are normal, O2 sensor Voltage acts normally.
When the computer commands a certain gear, the transmission immediately shifts to that gear.
truck starts out in 1st gear each time. shift solenoids are ON. 0.25 seconds to shift.
spark is 18 degrees. no retard from knock sensor, never.
vehicle speed sensors alive and working.
No misfires. MAF sensor numbers are normal.
Today I got another knock sensor low input code (is has two new sensors) so maybe there is a wiring short or corrosion in a connector?
Also the electric cooling fan sometimes comes on before the truck is up to normal operating temperature.
Fuel trims are normal, O2 sensor Voltage acts normally.
When the computer commands a certain gear, the transmission immediately shifts to that gear.
truck starts out in 1st gear each time. shift solenoids are ON. 0.25 seconds to shift.
spark is 18 degrees. no retard from knock sensor, never.
vehicle speed sensors alive and working.
No misfires. MAF sensor numbers are normal.
Today I got another knock sensor low input code (is has two new sensors) so maybe there is a wiring short or corrosion in a connector?
Also the electric cooling fan sometimes comes on before the truck is up to normal operating temperature.
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CathedralCub (06-25-2023)
#7
Senior Member
True Car Nut
Used my scan tool to do some tests.
Fuel trims are normal, O2 sensor Voltage acts normally.
When the computer commands a certain gear, the transmission immediately shifts to that gear.
truck starts out in 1st gear each time. shift solenoids are ON. 0.25 seconds to shift.
spark is 18 degrees. no retard from knock sensor, never.
vehicle speed sensors alive and working.
No misfires. MAF sensor numbers are normal.
Fuel trims are normal, O2 sensor Voltage acts normally.
When the computer commands a certain gear, the transmission immediately shifts to that gear.
truck starts out in 1st gear each time. shift solenoids are ON. 0.25 seconds to shift.
spark is 18 degrees. no retard from knock sensor, never.
vehicle speed sensors alive and working.
No misfires. MAF sensor numbers are normal.
This might be because the air conditioning or defroster is turned on.
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CathedralCub (06-26-2023)
#9
I bought some contact cleaner I am going to unplug every connector in the engine compartment and clean out, dry it out, remove corrosion, apply dielectric grease, see if that might remedy the issue...could be water hanging around somewhere?
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CathedralCub (06-26-2023)
#10
Senior Member
True Car Nut
Sounds like a good idea.
Some people go crazy with the dielectric grease. Personally, I use it around the connector seal and sometimes in the back end of the connectors where the wire comes in.
Some people go crazy with the dielectric grease. Personally, I use it around the connector seal and sometimes in the back end of the connectors where the wire comes in.