Electrical TPS Voltage / High Idle

KD5.0 · Sep 10, 2021

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?

KRUISR · Sep 11, 2021

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.

KD5.0 · Sep 12, 2021

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 · Sep 12, 2021

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 · Sep 12, 2021

KD5.0 said:
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.

nickyb · Sep 12, 2021

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?

manicmechanic007 · Sep 13, 2021

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 · Sep 13, 2021

KD5.0 said:
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 · Sep 13, 2021

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.

Dan02gt · Sep 14, 2021

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 · Sep 14, 2021

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 · Sep 18, 2021

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 · Sep 18, 2021

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 · Sep 18, 2021

KD5.0 said:
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.

CAMTWO1070 · Nov 22, 2022

KD5.0 said:
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?

When you add long tube headders you in essence push the 02 sensor further away from the cylinders plus you change the way the engine scavenges plus you give the ECU an improperly timed sample to go by resulting in a leaner and cooler exhaust reading so the engine is going to run with a higher idle because it cant add enough fuel to run right and goes into LIMP MODE and thats why your tps is reading whacky because the idle sets at an rpm that will get the car home or off the road......Sometimes it works in reverse and causes plug fouling too when the IAC isnt functioning correctly...

To fix the problem you need to add an adjustable fuel regulator and bump up the fuel pressure a few pounds and also wrap your hedders to keep the exhaust as hot as possible plus to add there are 02 sensors you can buy that are made for long tube hedder use too which is usually done by simply upgrading to 4 wire 02 sensors and wiring in the extra heater circuit wire ,,...

The right way is getting Ford Racing 24 lb injectors and running the X3Z computer or getting a chip..The MAF's are the same..

Another problem will be running issues when driving in rain....I got sick of the issues and just installed shorty hedders and I also wrapped those too...

Good Luck

General karthief · Nov 22, 2022

CAMTWO1070 said:
When you add long tube headders you in essence push the 02 sensor further away from the cylinders plus you change the way the engine scavenges plus you give the ECU an improperly timed sample to go by resulting in a leaner and cooler exhaust reading so the engine is going to run with a higher idle because it cant add enough fuel to run right and goes into LIMP MODE and thats why your tps is reading whacky because the idle sets at an rpm that will get the car home or off the road......Sometimes it works in reverse and causes plug fouling too when the IAC isnt functioning correctly...

To fix the problem you need to add an adjustable fuel regulator and bump up the fuel pressure a few pounds and also wrap your hedders to keep the exhaust as hot as possible plus to add there are 02 sensors you can buy that are made for long tube hedder use too which is usually done by simply upgrading to 4 wire 02 sensors and wiring in the extra heater circuit wire ,,...

The right way is getting Ford Racing 24 lb injectors and running the X3Z computer or getting a chip..The MAF's are the same..

Another problem will be running issues when driving in rain....I got sick of the issues and just installed shorty hedders and I also wrapped those too...

Good Luck

I'm gonna say that I've had long tube headers on my car for a long time and many miles and noticed none of these problems so there may be other factors in play here, you seem to be highly versed in the eec-iv working so what is your thoughts on why I didn't have those problems?

General karthief · Nov 22, 2022

KD5.0 said:
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??

'it' happens,

Electrical TPS Voltage / High Idle

KD5.0

New Member

KRUISR

KD5.0

New Member

manicmechanic007

KRUISR

nickyb

I gotta say i never painted my nuts, Never Ever

manicmechanic007

Mustang5L5

That is…until I whipped out my Bissell

90sickfox

Wasn't a pretty sight...and I've got big hands

Dan02gt

mazing how much gas smell came from that tiny hole

manicmechanic007

KD5.0

New Member

manicmechanic007

Mustang5L5

That is…until I whipped out my Bissell

CAMTWO1070

Active Member

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General karthief

wonder how much it would cost to ship you a pair

General karthief

wonder how much it would cost to ship you a pair

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Electrical TPS Voltage / High Idle

New Member

New Member

I gotta say i never painted my nuts, Never Ever

That is…until I whipped out my Bissell

Wasn't a pretty sight...and I've got big hands

mazing how much gas smell came from that tiny hole

New Member

That is…until I whipped out my Bissell

Active Member

Attachments

wonder how much it would cost to ship you a pair

wonder how much it would cost to ship you a pair

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