RPMs surge at idle

Katana200sx · Sep 18th, 2003, 05:03 PM

sometimes when i come to a stop by car will drop down to 800 and surge up to 1500 or so like i am revving it quickly. other times the car will idle normally at 800rpms then sometimes it will idle at 1500 or 2000 rpms. anyone have any ideas why? i thought it was my iac vavle but my friend said it isnt. any ideas are appreciated.

96sentra · Sep 18th, 2003, 06:26 PM

i dunno, but i had this exact problem a couple days ago. but it went away.

Katana200sx · Sep 18th, 2003, 06:46 PM

mine comes and goes. another thing....everytime but once if the car is idling at 2000 and i cut it off and start it again it idles normal.

toolapcfan · Sep 18th, 2003, 07:04 PM

Pay attention and see if it does it only when the engine is cold and quits once it's warmed up. If so then it's probably you IACV Air Regulator or the hoses connected to it are leaking.

Katana200sx · Sep 18th, 2003, 07:20 PM

its all the time. at the first start up it normally idles at around 1200 or 1300 like it always has till warm. then sometimes it acts funny

toolapcfan · Sep 18th, 2003, 08:39 PM

Well, here's what my FSM says about unstable idle for a GA:

1 Check EGR valve for sticking.
2 Perform Power Balance Test; Wtih engine running, disconnect one injector at a time, replacing each one before you pull another one off. Notice if there is any one cylinder that doesn't produce a momentary engine speed drop. If one does then you could have a bad injector or bad wiring for that injector.
3 Check injectors by using a screwdriver to listen for a ticking sound. (This seems pointless since you'd have figured out if an injector was not working while doing step #2)
4 Check ignition spark by pulling one plug wire off at a time, connect a known good plug and ground against something and crank the motor to check for spark.
5 Check plugs by removing them and check for fouling, etc.
6 Check fuel pressure; release fuel pressure by pulling the fuel pump fuse and start car. Once car dies, crank engine a couple times and put fuse back in. Install a fuel pressure gauge just after the fuel filter. At idle it should be 36 psi.
7 Check O2 sensors. This will be different since you have OBDII.
8 Check for intake air leak by pinching the blow-by hose and see if engine speed rises. If so you have a leak.
9 Check Idle Adjustment Screw for clogging; Pull tps off and start car, and see if you can adjust the idle to 600 rpm +/- 50 rpm for manual, 725rpm +/- 50 for auto. If not then your TB or that screw could be clogged.
10 Check compression; 192 psi is standard 164 psi is the minimum. no more than 14 psi variance between cylinders.
11 Check ECU harness connector
12 Try a known good ECU

Katana200sx · Sep 19th, 2003, 09:50 AM

thanks tool i will try some of that today. heres somethin i didnt post. when i unplugged the IAC valve...the car slightly idled up..dunno if that matters.

McLaren F1 2003 · Sep 19th, 2003, 01:46 PM

does it happen when your a/c is on?

AznVirus · Sep 20th, 2003, 04:26 AM

i actually have that problem now too. sometimes it drops from the 2000 RPM range back to normal, but sometimes it sticks there for a long time. is your check engine light on as well?

Katana200sx · Sep 20th, 2003, 07:36 PM

i havent used the a/c in over a year so i dont think its that. yes my check engine light is on, but it has been for about 3 years. everyone i took it to said it was somethin w/the speedometer so i ignored it. my airbag light just came on as well. i'm going to go get some codes pulled monday.

Katana200sx · Sep 27th, 2003, 10:55 AM

found the problem the other day. i was messing with the ecu and pulled some codes. they were speed sensor and egr malfunction. i was going to delete the egr when i put my header on anyway. but the other day i looked around that area to trace some vaccum lines and the one from the bottom of the egr that goes to this thing i dont know what it is yet is completely cut. serious vaccum leak! i guess i wasnt careful enough when i bypassed my throttle body coolant lines and cut this one.