please help diagnose -low cold idle

[vlado11]

OK here si my problem... At temeratures below 45F when first started, my car idles at only 1100rpm. Doesn't hessitate, doesn't shake or nothing like that, just idles low as if the engine was warm. Every once in a great while it would act normal: idle at around 1500rpm on start-up than go down as engine warms up. I know this is abnormal since it didn't use to do this the first 80k of its life. This is a '97 Altima with 145k mi. What could cause my car's low start-up idle in cold weather??

 

[wiswind]

One thing that could do this is a defective coolant temperature sensor reading to the engine computer. This could be the coolant temperature sensor, or a bad connection. Note that some vehicles use different sensors for the computer and dashboard gauge. I am not familiar with the altima, but the idle speed will be controlled by the computer, based upon various conditions.

 

[jamesn]

you probably have a bad idle air control valve. it is what actually controls the idle. on some nissan models, it attaches to the throttle body on the bottom and has 2 coolant lines running through it. as the temperature of the coolant rises, the IACV slowly closes a port and lowers the idle. this is why the idle does not drop suddenly like a carburated engine does. your iacv is most likely stuck in the closed position. keeping the idle low. here is a link to one on ebay: http://tinyurl.com/yw43pa

 

[Gary Allan]

It could be either of the aforementioned conditions. To test, engage the AC compressor (you probably do with defrost) and see if the idle reacts normally. Run the PS lock to lock (don't make a practice of it - but for a test) and see if the idle compensates without "overshooting" and hanging up (jerking around to settle). If it appears "normal" then I'd go with the temp sensor reading warm. Gauges or idiot lights? Most OBDII use the PCM to integrate and relay the sensor data.

 

[jamesn]

a dead temperature sensor should throw a check engine light before too long. it will on my OBD2 acura. the engine is missing a necessary input. a bad IACV will probable not throw one, it has no input to the computer other than it reading warm or not warm.

 

[mechtech2]

The idle air motor is dirty or otherwise faulty. That is #1 on the list.

 

[vlado11]

Coolant temp gauge functions proprerly. No SES or other idiot lights. If the temp sensor or the IACV is bad, is it possible for the car to act normal at cold start-up every once in a while? The problem I have happens most of the time, but not always. Sometimes even at very cold conditions acts normal. Is it possible that the IACV valve is stuck in the open position most of the time, and sometiomes it opens? Could it be cleaned?

 

[Gary Allan]

Cleaning it is fairly easy on most engines. Depends on how much plumbing they put in the way. Again, if it's a stuck or sticking AIS ..it WON'T compensate for changes in load at idle. Your idle will be jerked up and down with every surge of load ...and will (should) overshoot and hang if it's "sticking". You're not reporting any stumbling with stuff like turning the steering wheel at cold idle ..nor great impacts to idle or great fluctuations with no pedal input ..etc..etc. That's not to say that it's not the cause, just that the symptoms that should also be there ..aren't.

 

[vlado11]

Gary.... I did what you suggested today. Engaged the air compressor and turned the wheel to the left, than almost all the way to the right, and the idle did fluctuate for around 300rpm, I would say. Is this excessive and would it suggest a faulty IACV?? Thank to everyone for all your help!

 

[vlado11]

OK.. from what I read in the Haynes manual it's definitely the IACV that's the problem. Around $130 for the valve, no big deal, but removal/instalation on my car requires removal of the intake manifold! One more question please: Can I just say screw it, and press the throtle lightly on cold starts and hold the rpms at 1400-1500 for a minute or so, in an attempt to mimic what the IACV does???? Like I said before, the vehicle has no problems starting even at temps well below zero, but idles (smooth) at only 1100rpm.

 

[Dave H]

I just replaced the IACV on my '97 Maxima. My idle was too low. It eventually lit my CEL, so I pulled the codes and determined the cause. Replaced it, and everything is fine. On my old one, the "pintle" (cone shaped protrusion that seals the air opening) was visibly worn. It wasn't a smooth cone shape, but had a ridge worn where it sealed the opening. Mine had ~125,000 miles on it. As for your question of manually increasing the rpm's when cold, I'd say it that is a very temporary fix. The IACV controls the idle at all times, not just when cold. I think your situation will only get worse. Meaning it will soon progress to not only happening when cold, but more frequently. So I would replace as soon as practical (luckily for me, mine is easy to access. I even remove it with my throttle body every 10,000 miles or so when I clean my TB). Dave