Engine Idle Fluctuates At Idle

[ttvr4]

I have a 1998 Nissan Quest with 146,000 miles. The car is experiencing RPM fluctuations when it is idle. This typically happens when I am sitting at a red light and the weather is cold. I've replaced the PCV valve and cleaned the MAS but the problem still exist. Any ideas or suggestions would be greatly appreciated.

-ttvr4

 

[TallPaul]

How much fluctuation?

Likely many things can cause this, one of which would be worn timing chain, but by no means am I suggesting that is your problem. My Aerostar with 140,000 miles has a fluctuating idle, probably within a range of about 50 rpm. I don't know why.

 

[addyguy]

My Cavalier does, too.

 

[StormRider]

So the RPM fluxuations go away when it is warmed up? If you put it in park at the red light does it smooth out? Might be a vacuum leak...

 

[ttvr4]

Originally Posted By: StormRider
So the RPM fluxuations go away when it is warmed up? If you put it in park at the red light does it smooth out? Might be a vacuum leak...


When I kick it in neutral or park, the RPM shoots up to 1500-2000 and then calms down after.

-ttvr4

 

[StevieC]

You have a vacuum leak and the computer is trying to compensate.

 

[StormRider]

Check your vacuum hoses for leaks. If its idling high when its warmed up the Idle air control valve could be sticking as well, assuming no vacuum leaks..

 

[artificialist]

Compare the cold resistance of the coolant temp sensor to the factory manual. Sometimes when the coolant temp sensor fails, the ECU uses the wrong air/fuel mixture. You can't expect the check engine light to come on, because the ECU sees it as a valid temperature, even if it doesn't match the coolant temp.

 

[GMBoy]

Originally Posted By: StevieC
You have a vacuum leak and the computer is trying to compensate.



Vac leaks are possible - but poster says idle shoots to 1500-2000rpms when shifted into N or Park. Sounds like a IAC to me. Idle Air Control motors will cause crazy idles - as will a coolant temp sensors as mentioned above. I've even seen a TPS do this.

 

[Nick R]

I know my dad's jeep has a pretty unsteady Idle. It bounces up and down a good 50RPM just sitting there. Seems to be something fairly common with old school 4 bangers. (Like the 2.5 AMC and the 2.2 GM OHV. Though the 2.2 is an extremely smooth engine most of the time)

 

[ttvr4]

So how do I check for vacuum leaks? Where do I start and where do I end?

-ttvr4

 

[Familyguy]

I generally use some carb/choke cleaner. Spray it near the suspected leak (or along the vacuum line path) and when the idle goes up, you found your leak.

 

[GMBoy]

Originally Posted By: Familyguy
I generally use some carb/choke cleaner. Spray it near the suspected leak (or along the vacuum line path) and when the idle goes up, you found your leak.



X2

 

[mechtech2]

Clean your throttle body bore and plate really clean.
Clean the idle air motor, but be careful here with strong solvents.
Check for vacuum leaks ate very hose and it's connections Also the intake manifold may leak. The PCV system is of special imprtance - ALL the lines , fittings, and the valve.

 

[Vizzy]

Actually at times especially on OBD II or later vehicles a fluctuating idle (of generally less than 100 rpms) is normal. It is the ECU trying to optimize the cars emissions and the ECU can slightly adjust the idle speed to help accomplish this.

 

[PW01]

Idle air control valve?

 

[Vizzy]

Originally Posted By: PW01
Idle air control valve?


This is a possibility also known as the "idle stabilization valve" on some models. You can clean this part fairly easily with carb cleaner.

However most 96 MY (OBD II) and newer cars have this function directly controlled by the throttlebody and ecu.

 

[ttvr4]

Thanks for all the inputs. Besides the rough idle, the car runs great. I would leave it alone, however, it is annoying when you are sitting at a stop light and the RPM revs up and down. I will attempt to take the entire intake system apart and clean out the throttle body as well as the idle air motor.

-ttvr4

 

[aaxb970]

Possible weak battery...