Hey guys, I've been noticing that when I shift out of gear and into neutral in preparation to stop, my RPMs will drop really rapidly until they hit about 400 then they surge back to normal idle of 600-700. I think there's a greater problem though because my gas mileage has suffered immensely. I was getting about 430 to a tank, now I'm looking t about 330 I think. Any ideas or even places to check? Thanks!

--Ben.

 

any codes in the ECU....then there is always clean the ol TB

 

check for codes.
test TPS.

 

yeah, try cleaning the TB and IAC Valve. I had a similar issue and this was brought to my attention. Easy enough to do and it definitely stabilized the rpms. You might have to reset your ECU afterwards though. I noticed after the cleaning, the rpms were way up. Reset the ECU and we're back to normal...

 

Base idle is too low. Clean TB (normally makes idle high for a few days) and if it happens again after a week, test and adjust base idle (ie. Throttle position plug removed, adjust IACV air entry screw or throttle plate screw).

 

This topic is hard to suggest a fix for, because this problem has sooo many causes...and I've tried almost every single one.

Cleaned TB
Cleaned IACV/Replaced just in case
ALL maintenence (plugs, filters, everything)
Battery and Alt seem to put out good voltage.
One of the only things left to do is check the vacuum pressure, which I think is off due to a leak between the LIM and UIM due to a worn gasket or something.

 

And I also don't believe this to be the cause - or else it wouldn't idle at 6-700 rpms consistently AFTER falling past the set idle speed.

 

Yeah I've had the same problem lately, only my gas mileage isn't suffering. I was also told that it was probably a vacuum leak by some of my more car-savvy friends.

My car has been doing this on and off for a few months now and has actually died on me twice due to REALLY low idle, but that was a while ago.

 

ECU will attempt to normalize the idle if it is too low. It is an easy test to see if the base idle set is too low.--> run car to normal operating temp. Turn off car and unplug TPS harness. Start car and rev a couple of times and let car idle. If idle is too low or car stalls out, base idle is too low.

The vacuum leak guess is also a good one. I would deem it less likely if no manifold work was done on the car.

 

NO ONE has ever, ever suggested this. I swear to all that is unholy that if my car just suffered from too low of a base idle...so help me. THE first thing I ever did mechanically was clean my throttle body, and I remember having to turn the IACV idle screw WAAAYYYY down. Man, I hope this fixes it. I will feel like a complete a.ss if the reason I hate driving my car sometimes is because the base idle is set too low...For my sake, I hope your right, and then I will eat my words.

And by adjusting the base idle, you mean to use the IACV screw, correct? Damn I feel noobish.

 

1) check intake, throttle, iacv, and air hose cleanliness.

2) check Throttle position sensor

3) if the above two doesn't work. Check your knock sensor

good luck

 

Vacuum issue with the brake booster. I've noticed it on all 3 of my maximas and that's the conclusion I've come to. something about the RPMs dropping so rapidly when you press in the clutch combined with the vacuum used by the brake booster. I can avoid it if I time my clutch/brake pedal depression correctly.

 

I have the same problem, but it started out of no where. I do only notice it when i depress the clutch and then brake. The car idles so low that it would stall if i didn't heal-toe....

 

The plastic IACV screw can only adjust rpm over a 300 rpm range or so. If the base idle is so low that even maxing out the rpm with the plastic screw doesn't increase the idle any more, than chances are your throttle plate screw needs adjustment--it is what the throttle body's butterfly assembly rests on with no throttle. It opens the TB slightly at idle to increase rpm. By disconnecting the TPS, the ecu no longer adjusts for idle rpm. If base idle is really low, you may have to adjust the throttle plate screw first and then the IACV screw once rpm is in it's range. Make IACV screw adjustments without disconnecting the TPS is pointless.

Definitely also check rubber hose going to the manifold for leaks/cracks of any kind. Although I've had to adjust base idle twice in the last 2 years (the first time, it was super low!), I've also had a crack in that big hose going from the IACV to intake which also funked up the idle.

 

I'll be damned. I unplugged my TPS, and the car would turn on, and then stall out immediately. My base idle was 'slightly' low...I doubt this solved the problem completely since I am sure that I have a vacuum leak around my manifold, but in the initial 5 minute test run after resetting my base idle, the car hasn't gone past 500 rpms (compared to 2-300 rpms...big difference...). Thanks for the good tip - I can't believe no one's ever suggested this before.

 

This didn't fix the problem totally, however, I'm sure it helped a little bit to stabilize the idle. Thanks for the suggestion, now I just have to fix that intake vacuum leak and it should be good. Thanks for the suggestion.

 

Hey guys, I cleaned my throttle body and tightened the intake manifold a bit and it seems to have fixed the problem. Not I have an idle of about 700 rpms, but at least the original problems is gone. Thanks a lot!


--Ben.