Throttle position sensor reads 19% at idle. What's going on?

TheFleshRocket

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I recently picked up an Auterra (www.auterraweb.com) and have been using it to monitor my Cobra's vitals. All of the sensor readings seem to be accurate, with the exception of the throttle position sensor, which reads 19% when the car is at idle with my foot off the gas. When I accelerate, the throttle position climbs correspondingly. For example, when I'm cruising at 50 or 60mph, the TPS reads around 24-25%. At that point, I'm most likely at about a 5-6% throttle opening, so it appears accurate. The only thing inaccurate is the idle reading. (I haven't tried going WOT to see if it goes to 100% properly but will do that this afternoon.)

Drivability is fine for the most part, although the engine sometimes feels like it has a misfire at idle. I'm wondering if the inaccurate TPS reading is causing the ECU to inject too much fuel, possibly causing the misfire. And if it's injecting too much fuel at idle or part throttle as as result of the incorrect TPS reading, that's most likely hurting my gas mileage.

I have tested the Auterra on my GF's car and it accurately reads 0% from the TPS at idle, so I have no reason to suspect the Auterra is at fault.

My questions:

The car has a custom tune, but I'm assuming that the tune wouldn't try to fool the ECU by misreporting the throttle position. Is that a safe assumption?

Is the TPS adjustable? IE, is it possible that it is just installed a bit "off" and needs to be corrected?

I'm pretty sure that the previous owner had an Accufab throttle body installed at some point, so is it possible that he screwed up something with the TPS when he put the stocker back on?

Thanks everyone!
 

redsvtcobra

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Measure the voltage at the TPS. It should be about .99 volts. You can adjust by loosening the two screws and twisting it a bit.
 

TheFleshRocket

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Okay, so it IS adjustable. (I need to get a shop manual so I don't have to ask silly small questions like this.) I'll have a look at it tonight when I get home and see if I can't get it adjusted to where it's supposed to be. Thanks guys!
 

TheFleshRocket

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I pulled off the TPS, and sure enough, with the throttle closed, the sensor is pushed several degrees, which is what was resulting in it reading 19%.

I had the two screws off completely and it does not look like there is any real room for adjustment by just turning the whole TPS unit--the screw holes are big enough for the screws with little free play.

Is it possible to adjust the piece of plastic inside the TPS that moves? Or to adjust the little flap of metal on the throttle body that activates the TPS? I tried moving both of those with a little bit of force but they didn't move. I didn't try any harder for fear of breaking something. Anyone have any ideas?
 

TheFleshRocket

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I checked the TPS voltage using my Predator. At idle, it reads 1.4v and at WOT it reads 4.65v. I then took the TPS off of the throttle body. At the lowest setting, it was at 0.4v and at the highest it was still at 4.65v. So the high end is fine but the idle is definitely off.

I have seen no way to adjust the TPS itself. I am wondering if the base idle screw just needs to be set lower. The car idles at around 750-800rpm but could the idle screw be set a bit high and the IAC be compensating for it?

Tomorrow night I will see if I can adjust the idle set screw, but if anyone has any suggestions in the meantime, please let me know.
 

TheFleshRocket

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I found this posted at mercurymarauder.net:

I posted this in the group buy thread, but thought I'd meniton it here as well in case it helps you. On my throttle position sensor (TPS) I could never rotate it enough to get the voltage within spec (.98 to 1.00 volt). I ended up taking it off and drilling out the metal sleeves that are in the screw holes. This gave me the slack to rotate the sensor sufficiently to get it to .995, right on the money.

It sounds like most people haven't had to remove the metal sleeves, though, so definitely try it first before taking this step.

Also, for the TB bolts, the book says 9nm (80 in-lbs). And I did disconnect the battery so that the computer would have to relearn everything. I think this is a good step any time you alter the air intake path.

http://www.mercurymarauder.net/forums/showthread.php?t=26728

I'll try drilling out the metal so I can turn the TPS--that ought to do it!
 

redsvtcobra

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It's possible that you don't have enough wiggle room. I didn't remove the screws, just loosened enough to where I could barely move it and watched the voltage on the predator as I moved it. I couldn't really tell if it was moving or not, but the voltage change says I was.
 

TheFleshRocket

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Hmm, I didn't try moving the TPS and checking the voltage. It just seemed like it wouldn't move at all. I'll monitor the voltage and try moving it and see what happens. If that doesn't work, then it's on to drilling out the metal inserts.
 

AMB

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Tps

The 99 on up Fords with the EEC-V doesn't NEED/HAVE to be adjusted to .99v. like the Fords with the EEC-IV computers. The EEC-V computers AUTOMATICLY checks/calibrates the TPS idle voltage every time you turn the ignition key on!!!

AMB
 

TheFleshRocket

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Well damn, of course I read that now. Thanks though.

After I pulled the TPS and reinstalled it, I noticed that the Auterra said that the throttle position was at ~22-23% at idle. The Predator said that the voltage was around 1.2-1.3v.

So I drilled out the holes in the TPS a bit until I was able to get the voltage adjusted to .98v. I then checked the throttle position on the Auterra, and it read 19.6% again. So I bet the TPS was properly calibrated initially before I started messing with it!!

Anyway, all is well again. I just wish I'd known this before screwing with it in the first place Not that I wasted a whole lot of time on it, but enough to be annoying.
 

01silverstang

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So this is a very old thread but wanted to provide an answer...

A lot of the cheaper scan tools measure TPS as a percentage of voltage, so if 5v is max then .196 (19.6%) x 5v = 0.98v which is within the range you want. Hope this helps, your throttle is not open 19.6%...
 

13COBRA

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Interesting.

My OBDII reader on my Viper at full WOT shows 76%. I didn't know it would be adjustable.
 

01silverstang

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Mine also reads 19% at idle


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Again, you are reading voltage percentage at idle. If you were at 20% throttle at idle you would know it. You can hook a volt meter to the TPS plug and adjust the idle screw to get the exact measurement you want.
 

srdigital

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Again, you are reading voltage percentage at idle. If you were at 20% throttle at idle you would know it. You can hook a volt meter to the TPS plug and adjust the idle screw to get the exact measurement you want.

wrong. every time you start the car, the ECU resets TPS 'position' based on the voltage at startup. this is not an adjustable part!!! it's not supposed to be! the computer does it for you. idle percentage is correct, and anything above 60% is WOT strategy i believe
 

01silverstang

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wrong. every time you start the car, the ECU resets TPS 'position' based on the voltage at startup. this is not an adjustable part!!! it's not supposed to be! the computer does it for you. idle percentage is correct, and anything above 60% is WOT strategy i believe
We both weren’t clear enough, my response was regarding 94-98 SN95 which this absolutely works on… your response I believe was for 99-04 where like you said the ECU zeros out.
 

TheEel

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I don't have a Cobra but this thread popped up on a search for info about the throttle sensor on a 2002 Mustang GT. My scan tool shows 20% open throttle, also, even when the plate is closed. I replaced the TPS and power reset the ECU and it still showed 20% open throttle. The TPS output at idle is 1.02 VDC. Apparently this is normal. Kind of strange that my scan tool (BlueDriver) shows 20% at idle on the Mustang while showing 3% at idle on the 2014 Xterra (as a comparison).
 

Dusten

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I don't have a Cobra but this thread popped up on a search for info about the throttle sensor on a 2002 Mustang GT. My scan tool shows 20% open throttle, also, even when the plate is closed. I replaced the TPS and power reset the ECU and it still showed 20% open throttle. The TPS output at idle is 1.02 VDC. Apparently this is normal. Kind of strange that my scan tool (BlueDriver) shows 20% at idle on the Mustang while showing 3% at idle on the 2014 Xterra (as a comparison).

Typically about .99v at idle is normal.

This is a count of about 200.

Max counts is 1024. So 204/1024= 19.9%

The sensor is a 5v sensor and 1024 counts. This is 204.8 counts per volt.
 

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