Failure Mode for Throttle Position Sensor

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admiralbrown
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Failure Mode for Throttle Position Sensor

Post by admiralbrown »

How does a TPS fail? Is it electrical and you lose conductivity or is it mechanical and an internal spring fails?
I have the intermittent 2k rpm idle problem. And I've searched the archives for solutions.
I swapped out the TPS with two others and still had the problem.
I have tried a new IAC, but it did not help, when unplugged the idle does drop back to normal.
I have a wealth of parts accumulated over the last 23 years and tried a different BAP.
I tried a different thin film on the distributor.
I swapped out one big VAM for another
I searched for air leaks from the VAM through the intercooler and into the intake manifold and everything looks solid.
I am running an LA2 computer repinned by Ian Potter and was thinking of trying to repin an LA3, but have not gotten that far. Then I started thinking. Could the spring return in the TPS get weak and not fully return to a throttle closed position? Both TPS's I tried were set to 0.94V at Key On Engine Off and the WOT voltage was only 4.04 or 4.05. If the TPS didn't fully return it wouldn't need to be off by much to give 1.1 or 1.2 volts. I did notice a difference in the frequency of the high speed idle occurring between the two different TPS. And with the better acting TPS sometimes the idle will come back to normal. Shutting the car off and restarting resets the idle back to normal.
Any thoughts?
1987 RS500 clone courtesy of the little blonde Merkur fairies of Minnesota
2000 Z3 most dependable, least practical car I could find
2009 Jetta Sportwagen TDI 44 mpg
2012 Ford Focus SEL 5-door, the wife's car and the only automatic
fdt93
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Re: Failure Mode for Throttle Position Sensor

Post by fdt93 »

I guess you could say it's an electrical conductivity failure due to mechanical friction. My understanding is, the TPS has a contact point that slides along a variable resistance contact, thus conducting a different voltage depending on where the throttle is. This constant physical contact as the contact point slides along the throttle body's axis of travel will eventually cause dead spots.
This will usually cause the engine to stumble as you move the accelerator/throttle body across a dead spot. When it gets bad, it can also cause a high idle.

Were the 2 other TPS's brand new, or at least KNOWN to be good? If you still have an idle problem with after installing a new TPS, make sure you've reset base idle and recalibrated the TPS to the new base idle.
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Re: Failure Mode for Throttle Position Sensor

Post by Ed Lijewski »

I bet you have a vacuum leak...somewhere-there are lots of possibilities...

YMMV ;-)
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Re: Failure Mode for Throttle Position Sensor

Post by Merkur »

My checklist would be:
- Clean the throttle body
- Blow out the passageways in the throttle-body that connect with the ISC
- Check the TPS moves smoothly and the voltage doesn't jump using an analog VOM
- Ensure that the bushing/seal is on the TPS shaft to prevent boost air from blowing oil/exhaust gases into the TPS
- Set the base idle and calibrate the TPS
- Work out why the WOT voltage isn't in spec. Is the signal from the EEC 5V without the TPS connected?
- check for vacuum leak using a mighty-vac on the main connection to the front of the upper throttle body
- Replace ignition leads, distributor cap and plugs if you haven't done that in the past two years. Use the stock Motorcraft AGSF32 plugs - not Platinums. (A good $50 investment)

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Re: Failure Mode for Throttle Position Sensor

Post by thebronze »

It's the IAC... had the exact same problem with mine... left it unplugged for a long time... I finally cleaned it and no problems since
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Mike McCreight
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Re: Failure Mode for Throttle Position Sensor

Post by Mike McCreight »

When ALL else fails, try a new distributor.
Seriously.
That tale of replacing everything has been heard in my garage a few times.
And fixed.
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Re: Failure Mode for Throttle Position Sensor

Post by Oldmaddog »

I had the same problem till I found out it was the throttle plate shaft having to much side to side play. The throttle plate would become off center and get hung up on the side of the bore inside the throttle body. When you have high idle, press on the throttle plate shaft linkage and see if you rpm drops. If that the case, you need to fix you shaft side to side play.
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Re: Failure Mode for Throttle Position Sensor

Post by admiralbrown »

Thanks guys,
I forgot to add that the coolant temperature sensor resistance on the upper intake is roughly 31K overnight cold and about 310K at operating temperature. I don't know the spec but it is not sending a cold engine signal to the computer.
I have manually worked the throttle from under the hood and it is moving fine and always returns fully to the idle adjust screw. It is on my to do list to open the throttle body and clean it with carb cleaner.
The distributor cap and rotor are Motorcraft and have less than 5,000 mile on them. The plug wires are the same age and are MSD, I do not have any doubts they are working OK. Brand new spark plugs bought with the IAC and they still show the fast idle problem that was there with the old plugs. The engine has 113,000 miles on it, I would be willing to replace the ignition pick-up but doubt the distributor is worn out.
I do not believe there is a vacuum leak. I have vigorously shaken all the hoses and connection and could not get the idle to budge.
I have a spare IAC and bought a new one. The intermittent high idle does not seem dependent on any of them and I can't believe all three would be bad. But something is sending a signal to the IAC to open. Is the IAC digital or analog? Does it only open and close or does it have a range of voltages and the computer controls how much are gets through it?
The TPS gasket is in place, no sign that there is air leaking in around it.
The two TPS I had lying around are of undeterminant age but could have been sitting for 15 years. I'll check the voltage without the TPS in the circuit. I'll also check and reset base idle.
I drove to work today and had no idle or throttle problems. I am willing to bet the TPS can wear out and the spring return gets weak over the years. Next time I get a fast idle I will measure the voltage at the EEC and am willing to bet over 1.1V.
I've been spending too much on the Z3 this year, new tires, new shocks and struts, new water pump, belts and tensioners and I just ordered a new soft top. I suppose a new TPS will not break the bank, the new IAC was a waste of money.
1987 RS500 clone courtesy of the little blonde Merkur fairies of Minnesota
2000 Z3 most dependable, least practical car I could find
2009 Jetta Sportwagen TDI 44 mpg
2012 Ford Focus SEL 5-door, the wife's car and the only automatic
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Re: Failure Mode for Throttle Position Sensor

Post by Merkur »

Have you checked the codes? That would tell you if one of the sensors is out of spec.

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Re: Failure Mode for Throttle Position Sensor

Post by admiralbrown »

I read the slow codes and I get an error of coolant temperature sensor voltage out of spec. I am assuming the voltage from the sensor gets out of spec and the car defaults to a coolant temperature of -40 degrees. That causes the the car to think it is cold and runs to high idle to warm back up and may explain why disconnecting the idle air valve lets the idle return to normal.
Since jiggling the wire the sensor voltage has been fine and no more high idles experienced. I am guessing there is a short in the under hood wires. Still not sure why shutting the car off and restarting it clears the error.
Using my scan tool did raise a question about my code reader and my computer.
The car has an LA2 that has been repinned. What car should I tell my code reader it is reading, an 87 Merkur or an 87 Thunderbird?
1987 RS500 clone courtesy of the little blonde Merkur fairies of Minnesota
2000 Z3 most dependable, least practical car I could find
2009 Jetta Sportwagen TDI 44 mpg
2012 Ford Focus SEL 5-door, the wife's car and the only automatic
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Re: Failure Mode for Throttle Position Sensor

Post by DPDISXR4Ti »

admiralbrown wrote:has been repinned. What car should I tell my code reader it is reading, an 87 Merkur or an 87 Thunderbird?
'87 T-Bird. But the reality is that it doesn't matter for the purposes of pulling codes. Either PCM is an early EEC-IV that uses 2-digit error codes.

As for the high idle, this era of EEC-IV PCMs is notorious for displaying this symptom, especially once warm. All the things discussed here can be the culprit, but countless TPS and IAC sensors have been replaced over the years that are perfectly fine. I believe the most common cause is something that can't be fixed - an overly reactive IAC strategy that commands a higher idle when it shouldn't. i.e. The IAC is being told to raise the idle and it's doing exactly what it's told. I sorta proved this years ago when I switched to a later EEC-IV PCM from a '92 Mustang, and the high idle problem went away with no change to the plumbing, IAC, or TPS.
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Re: Failure Mode for Throttle Position Sensor

Post by Ed Lijewski »

See this thread, last paragraph, re changing TPS resolving high idle problem:

http://forum.merkurclub.com/forum/viewt ... 9&start=15

YMMV
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