Runs Fine But Idles Low When In Drive

96nickSn95

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Apr 4, 2016
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So just adjusted the TPS in my 1996 mustang 4.6L V8. It's pretty much stock, just replaced some minor parts with OEM equivalent parts (like maf sensor and IAC and such). The biggest after market part I have bolted on is a Dorman intake manifold for NPI heads. Now the other day my stang died on the way home from work, pulled up to a stop light and died, and I mean the hazards didn't even work. I was able to get it off the road and got a jump from a friend. After the jump, and pulling the ground on my battery on and off a few times, I was able to drive home another 6 miles just fine. The next morning however, the engine would turn over and then dump the RPM's to below 500 and die. I then noticed revving the car before it died kept it alive and sometimes after a few low revs I was able to keep it alive at 250RPM's. Now I just replaced the altinator and battery because I thought they were the cause. But no luck. So then I drew a code for a TPS and adjusted the it and it runs like a dream now. The only thing that bugs me is that in drive or reverse, the car idles steady at 500 even. But In park or nuetral, idles around 850 or so. I'm lost as to why it idles so low in drive. Its not a rough idle however, everything's smooth. I just don't like to see that kind of little stuff wrong with my car and I'm at a loss of how to fix it. On a side note my Bluetooth obdii plug in (dash command) shows that the catalyst, o2 reading and o2 heating Ran incomplete monitoring (not to be confused with unsupported) just curious as to what the f*** is goin on. Please help me
 
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96nickSn95

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Apr 4, 2016
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What have you done to rule out a bad battery, battery terminal, Ground, or bad alternator?
I've replaced the battery and the alternator and tested the grounds with a multimeter. But it idles and drives fine, unless I'm in drive (or reverse) at a stop. Then it idles at 500 steadily
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
For an automatic transmission with the motor fully hot, 500 RPM in drive sounds perfect to me. Both my 1996 GT and 2003 GT did it.

OBTW, on the 1996 and above the TPS isn't really adjustable. The PCM re-learns the new TPS setting on each start. So the most vital thing is for the TPS to produce the SAME reading each time the throttle is closed.
 

96nickSn95

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Apr 4, 2016
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For an automatic transmission with the motor fully hot, 500 RPM in drive sounds perfect to me. Both my 1996 GT and 2003 GT did it.

OBTW, on the 1996 and above the TPS isn't really adjustable. The PCM re-learns the new TPS setting on each start. So the most vital thing is for the TPS to produce the SAME reading each time the throttle is closed.
What do you mean? A little confused sorry lol but I was able to watch the voltage to the tps change with a multimeter while adjusting it. But you said something about the pcm relearning the tps every start up?? Oh and after adjusting the tps the idle dropping below 500rpms and dying went away. and thank you for telling me that yours idled around 500rpms, made me a little more comfortable haha
 

wmburns

SN Certified Technician
Aug 14, 2009
5,611
464
164
Houston Texas
"adjusting" the TPS is a procedure that applies to the 1995 and older Model year Mustangs. Adjusting the TPS implies moving or indexing the TPS to give a set voltage value at closed throttle.

The 1996+ Models don't really have a factory method to "index" the TPS in order to "adjust" it.The adjustment is no longer necessary because the PCM "learns" the TPS initial value at each start up. Once learned, when the PCM sees the same value it assumes that the throttle is closed.

So you see that adjusting the TPS for a specific voltage value has no effect.

Of course the voltage will change when the throttle is moved. But that is how the TPS works (not an adjustment).

If you run across any write-up that explain how to wallow out the TPS mounting holes in order to add the ability to adjust the TPS, it's pure snake oil for the 96+ model years

The TPS is responsible to signal (3) states to the PCM. Closed throttle, part throttle, and WOT. As stated what is VITAL for correct TPS closed throttle determination is for the TPS to produce the SAME value each time it is closed. I really thing that the improvement seen is because some looseness or slop was removed from the TPS.
 

96nickSn95

New Member
Apr 4, 2016
6
0
1
23
"adjusting" the TPS is a procedure that applies to the 1995 and older Model year Mustangs. Adjusting the TPS implies moving or indexing the TPS to give a set voltage value at closed throttle.

The 1996+ Models don't really have a factory method to "index" the TPS in order to "adjust" it.The adjustment is no longer necessary because the PCM "learns" the TPS initial value at each start up. Once learned, when the PCM sees the same value it assumes that the throttle is closed.

So you see that adjusting the TPS for a specific voltage value has no effect.

Of course the voltage will change when the throttle is moved. But that is how the TPS works (not an adjustment).

If you run across any write-up that explain how to wallow out the TPS mounting holes in order to add the ability to adjust the TPS, it's pure snake oil for the 96+ model years

The TPS is responsible to signal (3) states to the PCM. Closed throttle, part throttle, and WOT. As stated what is VITAL for correct TPS closed throttle determination is for the TPS to produce the SAME value each time it is closed. I really thing that the improvement seen is because some looseness or slop was removed from the TPS.
the tps was pretty tight on there, it might have just been me taking it off, unplugging it, and cleaning the plug that helped things along. Well thank you for the info! Always like to learn further how things work