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Old 06-12-2011, 07:52 AM   #1
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Default Weird, sudden high idle, bad TPS sensor?

99 Ls1, no CEL's or codes.

Drove the car fine for awhile last night, parked for a few hours, then went to go home and the car was idling at about 3k rpms. Gauge said 0% throttle also. The car drove fine, just idled really really high. Would this be a bad TPS sensor? Or anyone have an idea what would cause this? I've been driving this fine for months with no issues.

Last edited by TurboGoKart; 06-12-2011 at 08:56 AM..
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Old 06-12-2011, 10:10 AM   #2
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Typically if your idle shoots up like that but the TPS shows no issue, you've sprung a vacuum leak somewhere.
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Old 06-12-2011, 10:18 AM   #3
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Dammit Jim.


Sitting in some traffic last night the temps got up to 228. I parked it, left it for a few hours, then started it back up and weird idle. I hope it didn't blow a headgasket. Time to pull plugs/check oil & coolant. Maybe compression test too. Argh. Lol.
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Old 06-12-2011, 11:04 AM   #4
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you can try to pull your iac see if its gunked up. or if you have access to a scan tool watch the position of the iac to see if it's getting stuck to far open.
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Old 06-12-2011, 11:15 AM   #5
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I do have a scan gauge. Any idea what numbers I'm looking for on a normal IAC?
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Old 06-12-2011, 12:39 PM   #6
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i think at hot idle closed loop it should read between 40 for stock to 60 cammed. i would watch it from a cold start and see if it's moving at all. there also may be an obstruction in its path and keeping it from closing. i would pull it out and clean up the port on the tb and the plunger on the iac and see if this helps.
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Old 06-12-2011, 12:45 PM   #7
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ok just looked it up and it looks like it should be around 60, maybe even up to 80 if cammed. what did you find when checking coolant/oil?
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Old 06-12-2011, 02:00 PM   #8
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Nothing yet. Still at work. Going to check all of this when I get home and report back. Thanks for the info!!
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Old 06-13-2011, 11:48 AM   #9
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Well coolant looked clear so that's good. Cleaned off the IAC before I tried to start it again. Same high idle deal.

Apparently my scanner doesn't pick up the IAC, unless it's called something different. I didn't see an IAC on the data portion of the scan tool. Had to take the kids to the zoo but I'm going to check wiring later today.
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Old 06-13-2011, 07:41 PM   #10
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None of the wires were melted together/solder broken. Motor is in a Nissan 240sx. I know the wiring is right because I've put like 3000-3500 miles on the motor since the swap, it's cammed, and been tuned, so I know it's all been in good repor.

Right now what it's doing is it'll start/idle normal, then if I drive it for a couple mins, the idle will want to hang at like 2-3k rpms, UNTIL you come to a stop. Then after about a second or two of that high idle, it'll come back to normal. It used to do this a little from when I put the motor into the car, until I got it tuned. Tuner made some adjustments to the IAC settings to prevent that, and it's worked, until now. I wonder if my IAC is just going bad? Does this happen often on these motors?
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Old 06-14-2011, 01:50 PM   #11
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**UPDATE**


Grabbed a new IAC, and it's not hanging anymore. However, when I started it up, it would hardly idle, until the car was warmed up. Do you guys think that could be because of the settings the tuner changed getting the old IAC to work right? Or would this be more symptomatic of a MAF problem? (not idling great until warmed up)

Or do these sensors "learn" a little as time goes on?
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Old 06-14-2011, 11:20 PM   #12
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it could be possible that it was tuned with a ( iac going bad ). try cleaning your maf with maf cleaner and see if that helps first. also did you clean the port on the throttle body for the iac?
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Speed Inc. tuned
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Old 06-15-2011, 12:41 PM   #13
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Yup. I cleaned the throttle body out really well, especially the IAC port. After I made that post about it not idling, I drove the car for about 40mins, then shut it off for about 10, drove it again for 20, beating the poo out of it lol, then let it sit for about 20-30mins. So far it's been idling fine again. I will still clean the MAF off.

That's exactly what my first thought was (IAC settings adjusted on a 'going bad' IAC). Going to hook up with my tuner this weekend and have him take a look at the IAC settings with the new one in there. Thanks a TON for your help poltergeist02!!!!!!!
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Old 06-15-2011, 12:41 PM
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240sx, bad, cam, cleaned, high, iac, idle, idling, ls1tech, nissan, put, sensor, swap, tb, tps, traffic, weird


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