Qx4 Low RPM at Idle

[tcschweitzer]

I have a 2001 Infiniti Qx4 4x4 with 147,000. Engine typcially runs smooth as silk, however, lately when I'm in drive with my foot on the brake, the engine's idle speed drops to around 550rpm and creates a nasty vibration. It's not a misfire, more of a noise vibration. When I put it back in park, rpms return to 750 and vibration goes away. I have since changed an o2 sensor (after the cat), cleaned the MAF, new airfilter and recharged the a/c. This did not help. Any suggestions?

[ARKQX33V6]

Well as you know idle remains at 750 RPM loaded or unloaded. A drop can be caused by:
vacuum leak
idle set screw setting
Idle volume control
Throttle plate position out of range
low pressure in the fuel rail (gas filter plugged, dirty fuel line, dirty tank)
problems with transmission

[Densetsu]

I had the same issue a few months ago with a Pathfinder I bought last year. Idle was around 550, and at times it'd "surge"... RPMs would drop even lower, then recover, and repeat. After a few generic idle issue solutions that didn't do much (idle relearning, cleaning MAF, changing air filter, changing fuel filter), I realized the vibrations and RPM drop was only in gear -- not in park or neutral.

I checked the service records I got from the last owner, and there was no sign of a transmission fluid change. I did a drain and refill (around 4 quarts of fluid, buy 5 just in case) and it helped tremendously. RPMs in gear are now at 700, just under the idle speed I have in park/neutral. There's still a tiny vibration there when it's in gear, but nothing like it used to be.

[ARKQX33V6]

As Densetsu has indicated the transmission with used oil although it may look clean has many contaminants within it and these particles get into the small passages of solenoids and clog and stick the action of the valves.

Changing the fluid every other year is a very good preventative. Remember the transmission loads up the engine and a 1-2-3-4 gear change loads the engine further but when within the speed range the gear range will equal what the ECM is expecting, but if a valve sticks no matter what the ECM says for the proper gear and range the electric signals go unheard.

It's like trying to start off in third instead of first with a manual, you and your brain soon realize there is a problem and you re-shift to 1st. Your built in wiring corrects the situation, but what if the shifter is broken and physically stuck in 3rd, your brain knows what to do but can't tell your body to do it because of a mechanical issue beyond the brain's control.

You can save a lot of grief, money and time with simple PM, this is a PM that should be done, it will save diagnosing a complex issue with a complex transmission. An oil change out is not complex, a bit dirty and will offer you insight into that box behind the motor via the feel, smell and look of the oil and the particles that come out with the oil.

Auto transmissions have clutches, bands, solenoids, small oil passages that rely on proper pressures and action of a liquid to hydraulically do a job. That oil does a lot but it does it when the oil is dirty or clean, but the dirty oil changes and these changes affect the way thing operate. With very close tolerances dirt is not wanted or needed. Viscosity changes and cleanliness is critical inside any hydraulic component be it:

transmissions
Brakes
Clutches
ABS

everything hydraulic

[Rust-O-Matic]

I had the same issue a few months ago with a Pathfinder I bought last year. Idle was around 550, and at times it'd "surge"... RPMs would drop even lower, then recover, and repeat. After a few generic idle issue solutions that didn't do much (idle relearning, cleaning MAF, changing air filter, changing fuel filter), I realized the vibrations and RPM drop was only in gear -- not in park or neutral.

I checked the service records I got from the last owner, and there was no sign of a transmission fluid change. I did a drain and refill (around 4 quarts of fluid, buy 5 just in case) and it helped tremendously. RPMs in gear are now at 700, just under the idle speed I have in park/neutral. There's still a tiny vibration there when it's in gear, but nothing like it used to be.

Sorry to resurrect such an ancient thread, but I changed the trans fluid in my girlfriend's '01 today based on Densetsu's quote above, and HOLY MOLEY the difference in felt vibration is incredible. It was super bad at every stop sign previously. Felt like the exhaust was going to shake itself right off of the vehicle. I drained and refilled twice with regular ol Dexron III ATF (about 7 quarts total) because thats what I had, and it is like an entirely different vehicle now. I am shocked. I'll do it again in a few months, several times, with a good synthetic fluid.

Just thought I'd share; maybe it will help somebody down the road.