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Another sentra question


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Yes I know this is a pathfinder forum, I have this posted in a sentra forum that I frequent too but it's pretty dead over there recently so want to ask here as well.


90 sentra 5 speed, when the electric fan comes on the rpm's drop to around 300-400 sometimes when parked the engine will stall, normally idles at 850 rpm with nothing on (fans).


When I turn the AC on to activate both electric fans ( I have no AC right now and I removed the compressor belt so turning the AC on just turns the fans on), the rpm's will rise to about 1100 rpm which is what it is supposed to do,


question is why do the rpm's not raise when the cooling fan alone comes on?? Isn't there something that is supposed to raise the rpm's when the engine is under a load??


No power steering sensor since my car has manual steering.



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I would look at all of the following given the vehicle's age:


IACV (Idle Air Control Valve)

An IAC motor is designed to adjust the engine idle RPM speed by opening and closing an air bypass passage. An IAC motor can fail one of two ways; either the motor short circuits and stops working or the motor will develop high resistance and cause the IAC control motor to react slowly. It can also become clogged with carbon build up and not allow the valve to open or close all the way as needed. Either failure can cause the engine to operate at a low idle.


Vaccum hoses for a vaccum leak A vacuum leak in a vacuum hose can cause the engine to idle low.


Air intake duct for cracks causing an external air leak into the duct. If this duct develops a tear or crack post MAF it will cause the system to not monitor the air intake volume correctly causing a low idle condition


Dirty or 'coked' throttle body. It can cause low idle or stalling if the plate and bore are too dirty.


Improper vehicle timing.

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thanks, I pulled the whole IACV assembly not too long ago and cleaned the whole thing, vacuum lines have all been replaced,


It's funny the engine idles perfect with nothing on, 850 rock steady, the idle increases when the AC is turned on (both fans), but only when the cooling fan comes on by itself does the idle drop??


Only other time I notice the rpm's drop is sometimes when I slow for a stop sign or red light and press the clutch in sometimes the rpm's will drop and flutter for a moment, If I rev the engine one time quick it will idle as it's supposed too.


I do not have the coolant lines connected to run through the TB any longer as one of the elbows going into the TB broke off, I do have the coolant line still connected to the thermo element so that functions correctly.


Car has 242K miles on it, timing chain and water pump have never been replaced, think it's about time possibly??


Engine runs pretty good I will say, I removed the cat as it was clogged and my MPG went up 4 MPG, I can run 80-85 mph to and from work 4 days a week and last fill up I still got just shy of 30 MPG, so it's doing pretty good.

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That is great that it is getting good milege still!


thought: check the vacuum lines and the check valve for the power break booster, it also pulls vacuum from the engine.


Clogged PCV valve woudl also cause idle problems from a lack or low vacuum.

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Hi, I'm not sure about the correct name but I will try to explain as well.


There is a Idling motor to control rev when AC ON or cold start. In previous age we need an extra unit for this purpose but as I know Sentra just gives more gas+air automatically when AC ON or cold start. You won't see rev raise bcos this function is nothing with cooling fan, nor the engine temp.

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