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Old 02-16-2011, 04:24 PM   #1 (permalink)
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Default Hanging/irratic idle - searched, remedy not found

Well for about a year I've had this problem, though I forgot about it until I took the car out to get an alignment done today and it started to occur on my drive there and back. Basically the car will increase the RPM when I press the clutch pedal, and will not settle until I come to a complete stop. Other times a quick blip of the throttle will allow it to return to normal, and it seems to iron itself out after some "spirited" driving. Here is what I've replaced thus far:

Intake manifold gasket
Injector O-rings
TPS sensor
TPS rewire
IAC sensor
Boost leak test (have none)
All vacuum lines are secure

Only thing I have not tried is actually removing the throttle body to inspect it, though when I did have it off for a clutch job about a year ago it appeared fine inside.

TIP line is open to atmospheric pressure...running a built block, 35R turbo and all the fuel goodies + methanol, etc. TDS tune, too. I read a few things regarding the TIP line being a potential culprit but I don't see how when it's been disconnected long before this problem arose its ugly head.

Any help on what else to check would be greatly appreciated. Search didn't yield much but people fixing it with a TPS rewire and I've already done that AND replaced the sensor to no avail. Thanks.
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Old 02-18-2011, 09:00 PM   #2 (permalink)
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Quote: Originally Posted by BoostinBen View Post
Well for about a year I've had this problem, though I forgot about it until I took the car out to get an alignment done today and it started to occur on my drive there and back. Basically the car will increase the RPM when I press the clutch pedal, and will not settle until I come to a complete stop. Other times a quick blip of the throttle will allow it to return to normal, and it seems to iron itself out after some "spirited" driving. Here is what I've replaced thus far:

Intake manifold gasket
Injector O-rings
TPS sensor
TPS rewire
IAC sensor
Boost leak test (have none)
All vacuum lines are secure

Only thing I have not tried is actually removing the throttle body to inspect it, though when I did have it off for a clutch job about a year ago it appeared fine inside.

TIP line is open to atmospheric pressure...running a built block, 35R turbo and all the fuel goodies + methanol, etc. TDS tune, too. I read a few things regarding the TIP line being a potential culprit but I don't see how when it's been disconnected long before this problem arose its ugly head.

Any help on what else to check would be greatly appreciated. Search didn't yield much but people fixing it with a TPS rewire and I've already done that AND replaced the sensor to no avail. Thanks.

Im in same boat.

TIP red line open to atm
IAC replaced
TPS replaced
wire to IAC and TPS replaced
Vacuum leak checked

Car hangs at 2500-3000 when coming to stop, At zero it drops or I can rev a bit and start braking and it will drop.
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Old 02-19-2011, 06:26 AM   #3 (permalink)
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Looks like my new TPS sensor may have been faulty, as I threw the high voltage code. Wires are good, so I swapped it out for a used unit and reset the ECU. Things seem better but not 100% correct, though idle is significantly better, so is idle coming to a stop. I guess I'll see how long this lasts but really I'm unsure of what to check from here on out aside from re-doing the rewire despite them being solid and freshly soldered from like a 10 months ago.
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Old 02-19-2011, 02:57 PM   #4 (permalink)
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been doing some poking around:

I disconnected the TPS, car idles a bit rougher and STILL produces hanging rev when decelerating.

I disconnected TIP sensor, car wont even idle at normal speeds; Revs up to 3k, then slowly lets off to about 2.5k

This queues me to believe that my problem is caused by the TIP because It still happens despite TPS control.

I have a DSP-T and I can view the voltages to each sensor in real time. The TPS voltage looks consistent and does not jump or hang at a certain voltage. As a test, I slowly let in and off of the accelerator and logged it; no discrepancies.

My general question:
WHAT JOB DOES THE TIP HAVE in regards to loop controls???
IE: changes between open loop, closed loop, Idle closed loop, ect.

I have tried capping red line, does not seem to do much.

BoostinBen: Try to disconnect the TPS and take it for a ride; Tell me what happens; I suspect you should have the same results; It should do the same exact hanging thing as if it were connected. Next, If im correct, try reconnecting the TPS and disconnecting the TIP sensor, watch ur idle go CRAZY when u turn on ur car. At least then we can can isolate our specific problems because I understand this hanging rev can be caused by many different things, or many things in correlation...

Through my research as the most common culprits (from most likely to least likely):
1. TPS wire or wire harness is frayed/pulled out.
2. TPS sensor bad.
3. A large vacuum leak.
4. IAC wire bad.
5. IAC motor bad.
6. TIP not set up correctly (still alot of talk as to the best way to have red vacuum line)
7. Solenoid bad.
8. ECU bad.

I have personally gone through the first 5, and am trying to understand the 6th before I go point my finger at a specific culprit to my problem.
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Old 02-19-2011, 03:06 PM   #5 (permalink)
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My TIP sensor has been open to atmospheric pressure for years...ever since I went big-turbo to the .50 trim 3 years ago and removed the stock solenoids. Not sure if I should cap the TIP sensor, though.

I did remove the IAC sensor and the car slowly stalled, but that's also a brand new sensor and the wiring looks to be in-tact with that, too.

I have not driven the car today, but after swapping out the sensor things seem to have improved, but that also may have been coincidence since I also reset the ECU. Needless to say it's a very annoying issue, and it's literally like I have cruise control.
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Old 02-19-2011, 03:13 PM   #6 (permalink)
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Quote: Originally Posted by Amani77 View Post
BoostinBen: Try to disconnect the TPS and take it for a ride; Tell me what happens; I suspect you should have the same results; It should do the same exact hanging thing as if it were connected. Next, If im correct, try reconnecting the TPS and disconnecting the TIP sensor, watch ur idle go CRAZY when u turn on ur car. At least then we can can isolate our specific problems because I understand this hanging rev can be caused by many different things, or many things in correlation....

Not the IAC, The TPS.

Then the TIP.

I want to know if ur idle goes crazy with tip unplugged, and if u experience the same problem with the TPS unplugged during normal driving.

Just because u've been running it open to ATM for years doesnt mean that the actual sensor or solenoid didnt go bad...
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Old 02-19-2011, 03:17 PM   #7 (permalink)
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My car does the same thing but only when I try to hook the TIP sensor back up to the solenoids. I had this problem forever and when I deleted the solenoids it went away. As soon as I hook the tip back up it does it again. Dunno if this helps at all just figured I would put in my input
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Old 02-19-2011, 03:21 PM   #8 (permalink)
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Quote: Originally Posted by bigblockcoupe View Post
My car does the same thing but only when I try to hook the TIP sensor back up to the solenoids. I had this problem forever and when I deleted the solenoids it went away. As soon as I hook the tip back up it does it again. Dunno if this helps at all just figured I would put in my input

You just disconnected the vacuum line from the tip sensor to the solenoid?

Edit: and capped the other ends at mani of course
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Old 02-19-2011, 03:30 PM   #9 (permalink)
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I'll try unplugging the TPS to see what it does. What exactly would that prove, though? The ECU uses the voltage of the TPS to adjust the throttle? It would make sense for it to get erratic with it unplugged, no?
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Old 02-19-2011, 03:45 PM   #10 (permalink)
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Quote: Originally Posted by BoostinBen View Post
I'll try unplugging the TPS to see what it does. What exactly would that prove, though? The ECU uses the voltage of the TPS to adjust the throttle? It would make sense for it to get erratic with it unplugged, no?

The computer has MANY things the calculate idle, If one fails, it relys on the other sensors.
I want to prove that the TPS is NOT part of our problem.

If the problem is the same before and after TPS instillation, then probably its not a bad TPS; this is assuming that the tps works at all; Mine does because I tested it wit DST; im just ruling out a functional but bad tps.
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Old 02-19-2011, 04:19 PM   #11 (permalink)
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Well if it's not the TPS I'm not sure what else it could be. I've literally traced everything and boost leak tested the car and I have zero leaks. No SES lights since switching the TPS sensor, either. Rewire is solid, IAC wires are solid, etc.

I am kind of wondering if it's the LFS 60mm throttle body itself, but I've had that for years and never had a problem with it until this past year when I've been trying to track this problem down. There are weeks where it will run fine, then it will go batshit insane and start idling erratically.

I'll try your TPS unplug thing, though, and see if the problem occurs.
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Old 02-19-2011, 04:24 PM   #12 (permalink)
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Quote: Originally Posted by Amani77 View Post
You just disconnected the vacuum line from the tip sensor to the solenoid?

Edit: and capped the other ends at mani of course

Yes but u have to leave the tip sensor plugged in electronically
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Old 02-19-2011, 04:46 PM   #13 (permalink)
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Quote: Originally Posted by bigblockcoupe View Post
Yes but u have to leave the tip sensor plugged in electronically

I did and it made my hanging problem much worse. Before unplugging line I could hit accel and break and the revs would drop, after they wouldn't drop at all until I came to complete rest
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Old 02-19-2011, 06:28 PM   #14 (permalink)
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I actually had one of those wires break on the plug to the tip but that threw a code... R u throwing any CEL's?
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Old 02-19-2011, 06:35 PM   #15 (permalink)
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I was throwing a P0123 code for high TPS output signal, and changed the sensor, reset ECU and it went away, and the car started to idle normally. It still hangs coming to a stop a bit but nothing like before.
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