Surging Idle

Does Your 5.0 suddenly increase to around 2500 RPM at idle? Tired of having to quickly turn the key off and on to return the idle to normal? Here are some suggestions:


Caution: Long and Boring, probably confusing...

Look at TPS voltage update on bottom of page when finished reading.

How I solved my Surging Idle

This is what cured my stock 87 GT with 140,000 miles. I removed the intake and cleaned all the carbon out. After Scanning its computer for codes the EGR sensor was replaced, which the surging seemed to occur less but did not fix it. The Problem was with the TPS (Throttle Position Sensor). It has now been 2+ years with a constant perfect idle around 650 RPM. This may not be the solution to your Mustangs problem but this info might help you in detecting yours.

There is a way to test if your TPS is working properly. There are 3 wires on it. With a digital meter and the engine off but key on, place the positive on the green wire and negative on a ground (intake is fine).

A CLOSED throttle should give between 0.97 and 0.99 volts. You can adjust the voltage by loosening the 2 screws (so TPS is still on the throttle body,but so you can slightly rotate the whole unit ) and turn the TPS until you get the voltage (0.98). Update: If you can't get to .98 just get as close to it...see update.

NOTE: The TPS itself can not be adjusted, what can be adjusted is its position on the throttle body. The 2 screw holes allow for minor rotation.

It should never be over 0.99. I'm guessing that anything under a 1.0 tells the computer that the throttle is closed. On full throttle it should read about 4 to 4.5 volts. As you move the throttle you should see the voltage change to the with the exact position of the throttle plate.

Could This be your solution too?

The Odd part and probably what is confusing other mustang owners was that the motor was still acting up after adjusting my TPS perfectly. I gambled and bought a new TPS ($60 from Ford) and have never had a problem since.

buildup will clog sensors

The Intake and its components get clogged with carbon after a few years. I would recommend (if you have a day or so to waste, and it's a pain in the ass) to pull out the upper intake and clean it out, you will need to buy new gaskets (8 holed Plenum, 2 different throttle body, Air bypass) from the dealer.

The EGR and Air Bypass valve will definitely need to be cleaned, use an intake cleaner that is safe for sensors. While you have the intake off, you might as well change the valve cover gaskets if there old or leaky. See Parts section on metal valve cover gaskets.

Quick Fix?
Idle fix kit...


There is an Idle fix kit sold by Ford that some recommend. Before I did anything I tried it, it costs around $10. All it is, is a small spacer that goes between the Air bypass valve and the intake. It has 2 screws to adjust air entering and exiting the valve. It is listed as a repair for F-Series trucks. It is just a temporary fix to try to have the bypass valve function normally on a clogged intake. In general the Bypass valve only affects the motor when its warming up. I tried this spacer on my (stock) '87 5.0 and it made things worse. At least I was able to use the new gasket. ...Hear it works on modified motors (bigger TB, heads, intake to better adjust air flow. ?
Clear memory shut everything off, run car turn everything on
There is another 'solution' which also didn't work for me. Sounds odd, but some claim it works. It involves disconnecting the battery for 10 minutes to clear the computers memory, then starting the car with all accessory's off, no radio, A/C etc. running for 2 minutes. Then shutting off and restarting with everything on, lights, A/C, Radio, .... You can find this in detail in the tech section at (Go to my links page)

Who Knows?

The trouble of finding the cure to this problem is if it's sensor related, is it the sensor that the computer code is telling you it is? Is it a failing sensor in the loop, causing the other sensor to attempt correction of the problem with inputs from the failing sensor, measuring and adjusting a few times every second, resulting in the famous idle hunt. I have been long and boring, and probably confusing, but this is not an easy fix. I would recommend cleaning all the intake parts, and if you change a sensor that might not be broken, it's probably getting old and should be replaced soon anyway. Good Luck.

E-Mail your solution to add it here, and to help others fix this problem.


TPS Adjustment in Living Color
Throttle Position Sensor: The only way the computer knows where your foot is...

The TPS is screwed into the Throttle body and sits directly above the throttle plate. The plate has a tab that extends upward into the TPS. The TPS has a small plastic rotating insertion that directly connects it to the plates (tab) movement.

A simple way to realize what the TPS is, is to think of it as a volume knob, as you turn the knob the volume changes, where the throttle plate turns the TPS the voltage changes.

Check to see that The Throttle Plate is in the closed position. If it's not closed completely turn the idle screw (Flathead screw under Throttle Body) until it's closed. If it's not completely closed your only wasting your time by adjusting the TPS.

Connect your digital volt meter (Engine OFF!) to the Green wire(poke it, or stick a pin and touch pin with meter positive) it's the positive & any good ground (throttle body) is negative.

1) Loosen the (only) 2 screws on the TPS.
2) Key On, Engine OFF
3) Rotate or twist the TPS till desired voltage, around .98 volts.
(see update below)
- clockwise increases voltage

4) Hold TPS in exact position and tighten screws to keep it from rotating

NOTE: The TPS itself can not be adjusted, what can be adjusted is its position on the throttle body. The 2 screw holes allow for minor rotation.

Old age & Heat either moves the TPS out of position or slowly deteriorates it causing its measuring to suffer, after all it works every time you move the throttle.
I don't know if they were calibrated on to the Throttle Body at the factory or just slapped on...


TPS Voltage Update ! ! !

Does the TPS voltage under 1 volt really matter ???

When I bought my TPS 2 years ago I slapped it in on the dealers parking lot, I couldn't wait to see if it would help my annoying idle. Success! Perfect Constant Idle.
So 2 years later I decide to check what the TPS is set at. At closed throttle it was reading .73 volts! Everything I've read states that the TPS should be near .98 but never over .99.
I set it as far as I could to .90 volts with no noticeable change in performance.

So what does that mean? Lower than .98 volts may not have a bad affect with idle but does it change fuel consumption or (supposedly) response ???



Back to FIX IT