Electrical TPS Voltage / High Idle

KD5.0

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I recently installed Long Tube Headers and new O2 Sensors onto my 89 Mustang GT. When I went to pick it up from the mechanic it had a high idle (1500 - 2000RPM) and he could not figure out how to fix so I brought home to do a base idle reset. In the process when I check the TPS voltage after setting the idle screw I am getting a reading of 2 V instead of anywhere in the range of .75 - 1.2 V.

What would be causing such a high TPS reading? Do I need a new TPS?
 
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KRUISR

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Apr 16, 2015
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How far in base idle reset did you get? If you unplug the IAC and spout connector, what is you idle speed?

If your throttle body idle screw is turned in far enough you will have a higher than 1V reading. That is why I ask.

With the IAC and spout disconnected, try backing it off down to 600-700 rpm. Then see what your TPS reading is.
 
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KD5.0

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Yes I pulled IAC and set idle screw to around 650 RPM, after that when I take the TPS reading I am getting 2.1 V so not sure what would be causing this
 

manicmechanic007

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Not really supposed to play with the screw in the throttle plate
The adjustment procedure is there (in the book) in case the coating gets cleaned or washed off the plate
If someone already has adjusted the screw it is best to replace the plate
You can readjust with some trouble
Back it all the way off then try to get .060 gap in the plate and try that
You are looking for .8v at closed throttle
Find another plate or two at the wrecking yard
 

KRUISR

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Yes I pulled IAC and set idle screw to around 650 RPM,
Did you disconnect the spout connector on distributor too? You need to do both or PCM will try to adjust idle through timing adjustments while you are adjusting the throttle screw. I was chasing idle quality for a while after some upgrades until I disconnected both and then set the idle screw. Then following the rest of the base idle reset.
 
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nickyb

WAIT,you now have a pair?
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Headers didn't screw your idle, o2's ? Maybe. If so called mechanic didn't touch anything then you should not have had any issue.
I have installed long tubes with existing o2's and no issues, do you have the o2's that were on there before?
 
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manicmechanic007

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2 volts means the screw is jacked in a mile
Something more than the screw is amiss
Replace the plate with a known good part
That will come with a tp sensor too if you get it from the wrecking yard
You should only have to pull the intake once
Close inspect or better yet replace all the intake bolts
 

Mustang5L5

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Yes I pulled IAC and set idle screw to around 650 RPM, after that when I take the TPS reading I am getting 2.1 V so not sure what would be causing this

Measure your reference voltage. Make sure Vref is 5.0 volts.

Assuming Vref is fine, you are seeing 2.1 volts at idle? If open the throttle body blade fully (key on, engine off), what voltage do you see there?

Is this a Ford OEM TPS? If so, those are usually spot on. If it's aftermarket, i've seen those be WAAAAY off at idle and need quite a bit of slotting to get it to the 0.5-1.25 range you want to be at idle. I really recommend a motorcraft TPS here if you don't have one, but check your Vref and WOT voltages first.
 

90sickfox

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More than likely you don't need a new throttle body. That voltage is not right. TP sensors are better tested with an analog ( needle and gauge ) type multimeter so you can see the needle jump while testing. I choose them over the digital ones for this.

More than likely it's a vacuum leak caused by the guy moving stuff around to drop in the headers. The TP voltage may have already been out of spec. Pull codes and post here.
 
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Dan02gt

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Your TPS probably failed. These new auto parts store sensors are junk. Go with Motorcaft is possible. Pull it off the throttle body and test it with the wires still connected (eliminating the TB screw as being the issue). If it's still at ~2v replace it. Mine failed in the same way. I pulled the car out of my garage to do some cleaning and it wouldn't idle down out of the blue. Quickly tracked it down to the TPS.
 

manicmechanic007

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Damn straight they fail
And on a regular basis
The TPS is the only moving part besides the injectors and EGR valve
They get dead spots and go nuts
A whole rash of codes they can set
Arrrgggghhhh
Thank god they are cheap
All of you should buy Motorcraft parts if you can afford them
 

KD5.0

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Thanks for the response everyone

I ended up just buying a new TPS since they are relatively cheap rather then spending hours trying to diagnose the issue. Car idles and runs great now

I guess it was just bad luck the TPS went as soon I did an upgrade??
 

manicmechanic007

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Great job KD5.0
A Motorcraft or Ford one should last 100k miles at least
You saved 200 bucks at least fixing it yourself
And the job satisfaction is worth more than that
 

Mustang5L5

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I guess it was just bad luck the TPS went as soon I did an upgrade??

The parts being made these days for our cars are just dropping off in quality. The vast majority of 302s that used this TpS are now in junkyards so there isn’t a lot of incentive to make quality parts for a dwindling market with fewer competitors.

Even Motorcraft has fallen off. I’ve found the Motorcraft made parts for 302s made 10-15 years ago are much better quality than the current suppliers of mustang specific parts.
 
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