Advanced high idle issue. TPS mode sticks in PT (Part Throttle) sometimes

[texas_snake]

So I have been chasing a random high idle issue on my 03 Mach 1 for some time. I have done the standard updates:

MAF (had a broken screw)
Throttle body (was very nasty inside stock tb and idle screw had been messed with), have accufab now
IAC
TPS

Nothing fixed it. I finally got a good scanner (using forscan on a laptop). I can now see that the tps mode is sticking in PT (Part Throttle) when I rev it from CT (Closed Throttle). This opens the IAC and advances timing which causes the high idle (from what I can see).

So I think that is the issue - Part Throttle stuck when not touching the gas. I have checked wiring and it seems OK to the tps (5.05 volts ref).

It looks like tps voltage minimum drops (might be .92 at start, but I see it drop to like .86 sometimes when running). I believe this is what can cause a PT condition (the PCM sees tps voltage is at say .92, but minimum was down to .86 so it decides I must be pressing the gas). I think Ford uses +/- 3 percent to make that determination. So if this is true, it is a voltage drop from startup that is causing the issue. So then what causes it. I can only think ground issue or pcm.

Help! Thanks.

 

[gt347mustang]

First of all is the car tuned? 99% of the time a poor tune causes high idle.

Second you need to see what tp ratch is during these logs.

 

[NickC03]

Don’t hit me but have you checked for vacuum leaks with a smoker? What’s the purge solenoid duty cycle when this event is happening ?

 

[texas_snake]

Very good point on the tune. I bought the car 3 month back, so I have no history on that. But I have felt it might be a likely suspect (perhaps it was tuned and had some mods removed before selling it, not sure). Had headers and a catback when bought. So I have been considering a Ford reflash. But... I am trying to also decide if there is a PCM issue physically (not really thinking so at the moment). I inspected the pcm physically and it looks fantastic (along with checking it was the correct pcm part #). I have been taking the "shade tree mechanic" out of the car since I bought it (list of issues, but got it at a very fair price) and I am nearly done.

Trying to avoid being wrong and wasting money on a reflash only to need a pcm rebuild which includes the reflash.

Regarding TP Ratch, I believe that is the forscan TP CT which is lowest closed throttle voltage. when TP mode stuck in PT and idle was up:

TPCT was .86
TP_V was .91

Voltage at new Ford TPS with new Accufab TB:
Ref: 5.05/04 at battery negative
TPS: .91 (with new motorcraft tps). Would need to drill to get closer to .99, but from info I read, this is a myth.

When in PT:

IAC goes from 30% ish to 40% ish
timing advance goes from 6% to 40%

This looks to be enough of a differences in TP voltage to go PT from CT I believe. So just trying to understand the cause. It had the same issue with the stock TP and original TPS (set at .98).

I have reset KAM a few times. But oddly forscan shows KAM power in fault, but I checked against my 03 cobra with no issues and it also shows fault. So must be a bug with forscan and our mustangs.


On the second question, I have also tried chasing the idea of a vacuum leak, but have not gone all in with it (like a smoker). Nothing found so far, but can't rule it out. I will check into the purge solenoid duty cycle with forscan and see (need to look that up).


Thanks!

 

[texas_snake]

I am switching to the old stock (but cleaned) throttle body and tps. Will watch with forscan. Have TPS set to .97. Also will reset the KAM.

Will see how it acts. But I am suspecting there is a tune on it.

Any way to identify a tune with forscan? Parms to watch, etc. Thanks.

 

[texas_snake]

Fixed the nightmare high idle issue!!!! Here are the notes for others should their idle issue be as hard as mine. I could see with Forscan the ratch drop while idling and when the A/C was on the min tps voltage was not stable. So it would pop into PT mode from CT (and that was the idle issue, timing hopped up as well as IAC % open):

1 - If your A/C has too much Freon and you reset your KAM memory, it may not have enough time to learn the idle with the A/C on when first starting and going through initial learning with A/C turned on. So idle goes crazy when you turn on the A/C. The fix was to reduce the Freon so the A/C clutch will be on long enough for the PCM to determine a quick idle with A/C. This looks to be about 10 seconds

2 - Put washers on my TPS

3 - Cleaned every freakin ground on the car (every one!). Found the engine to frame was VERY DIRTY (my guess is this was the issue)

4 - Added an extra 14 gauge ground from block to frame (for insurance)

5 - Fixed a wire going to the radio suppressor behind the passenger head (this likely didn't do anything, but it was on my list since the wire was bad on it)

Hopefully this helps the next person. This was far from an average idle issue.


Next, I suspect there is a tune on the car. The timing drops slowly when back to idle during driving. This is not a big deal. But heading to Ford for a factory pcm reset (again for insurance of no other issues).