I have been having some idle issues for a while now. Rough rich idle, and when I come to a stop, the car will hunt for a while (maybe 10-20 seconds), then finally find idle again. I also have started to develop the dreaded 4k rpm hesitation and I believe I narrowed it down to a TPS issue based on all the info on here. So I decided to check it out and I found something weird. I am unable to get my VTA1 open valve above 3.5V when adjusting it while it’s installed. If I take the TPS off completely and rotate it by hand, I can get my VTA1 value to reach 4.99, but when I set the TPS to it’s max adjustment while installed on the throttle body, the highest I can get it to be at is around 3.5V. All the other 3 values I am able to get within range when adjusting easily, it’s just the VTA1 open that will not cooperate. Based on some other threads, I am definitely installing it correctly. The ears on the TPS are mating with the throttle body correctly.

94 single turbo on a power FC, the idle wax thing has been deleted. I had this motor built and installed by IRP about 10 years ago and have never touched or adjusted it before just as some background.

My troubleshooting so far:

The sweep for both VTA1 and VTA2 looks normal when I open and close the throttle. There are no dead spots and they both look fairly linear as I open the throttle, the VTA1 just stops at 3.5V when adjustment is maxed out. I have attached some photos of my Commander for reference. (I recently changed my dead screen and it displays backwards if it looks confusing. That’s next on my to do list…)

TPS installed with max adjustment (TPS max rotated counter clockwise when looking at back of motor) open throttle:
VTA1: 3.55V
VTA2: 4.98V









TPS fully opened, uninstalled:
VTA1: 4.99
VTA2: 4.98


TPS closed, uninstalled:
VTA1: 0.01V
VTA2: 0.00V






With the TPS connector now unplugged, I am able to check continuity between the ECU and the Connector at the 3I - TPS constant, 3F - Narrow TPS and 3G - Full range. No issues there. (ECU Pinout Diagram below for quick reference) I also get continuity with the body and the ground wire (Brown) on the connector side so all wiring checks out. (I think)

With The TPS connector still unplugged and now the ignition on, I am getting the following values for voltages on each pin:
(Top) Brown -0.00V - (continuity with the body check outs)
Green / Red Stripe - 4.10V
Brown / White Stripe - 4.99V
(Bottom) Black / Green Strip - 4.00V





Oddly, which I don’t understand if anybody has an explanation the Narrow and Full wires on the connector are receiving approx 4V. From how I thought I understood how the TPS works, 5V should go in, it should go through the “potentiometer” of the TPS, and a different voltage should come out the other side depending on where it is in it’s “sweep”. this is strange to me as to how voltage could come from both sides of the TPS, but if this is normal and anyone can provide a reasoning please let me know. The more I understand how all this stuff works the better I can rule out variables while troubleshooting. They BOTH read the approx 4V so I’m not jumping to the conclusion of a short in the wiring harness just yet… Part of me thinks I just don’t understand how the sensor works.

So… Is the TPS just on its way out? Or does anyone believe there is another culprit that I was not able to determine yet. Any insight is appreciated.

 

Hard to think you have full sweep when you have it in your hand but NOT when it's installed. This fact will indicate that you have it installed incorrectly. Being aware of the ears is half the battle. Which direction do you have the tps facing? Towards the iac or away from it?

 

When installed, the plug is facing away from the IAC. When looking at the back of the motor, I place the TPS onto the throttle body with the TPS connector at the 10-11 o’clock position and rotate counter clockwise until the through holes in the sensor line up with the threaded holes in the throttle body.

Before I touched anything i checked the VTA values on the commander and the VTA1 open was in the low 3s. ( I should have written the values down before I took it off). At that point I started diving in trying to diagnose it.

 

dumb question, but does the throttle body actually open all the way?

 

I was actually thinking that on my way into work this morning. I’m going to check as soon as I get home. Are there any common “blockages” or issues that would cause the throttle body to not open all the way that anybody knows of?

I’m also aware that since the issue is on the open throttle side, this is probably not the reason for any of my original idle symptoms as the closed voltages are within spec, but it still needs to be fixed regardless.

 

i'm not aware of anything in particular, mine is an engine swap, so its something that i had to check. and if you put an FD engine in an FC, it doesn't open the throttle all the way

I’m also aware that since the issue is on the open throttle side, this is probably not the reason for any of my original idle symptoms as the closed voltages are within spec, but it still needs to be fixed regardless.[/QUOTE]

for sure having it in spec at idle is the important part

 

How far the throttle body butterflies open (and how far the TPS moves) is a function of the accelerator cable and whether it is adjusted in such a way that it is fully open when the pedal is fully pressed. Worth checking out.

Beyond that, I would expect that the TPS sensor is designed to be able to move further than the full sweep of the throttle body so that it doesn't break if the throttle plates move more than the TPS sensor can move. So while the full range of the TPS sensor might be 0-5, the practical range that the TB moves might be 1-4, for example.

Hope that helps.

 

I took the UIM /TB assembly off to get a better look to see if maybe something was causing the throttle body to jam up/ block something. It seems that when the wax mechanism thing was deleted (previous owner) a peice of it that remains is hitting on a part of the lower butterfly shaft assembly.

After forcibly turning it with a pair of pliers as it was pretty tight, I am able to get a good 20 degrees more throttle movement… i have not checked the vta1 open value again yet but I’m positive it will be fine now. After loosly throwing the UIM back on and feeling the difference in the gas pedal I needed to walk away before wrenches started flying.

I am really hoping that my 10 years of ownership of this car has not been like this. I always thought the throttle seemed like a short window, and I truthfully can’t ever remember feeling my gas pedal hit the floor until now. Im pretty confident it always stopped before it hit the floor and I’m so curious to drive it again to see if I feel a noticeable difference. Over 10k miles, countless track days and I only ever felt what ~~75% throttle on this car feels like. I am both extremely frustrated while also filled with anticipation to drive her again once all the salt is gone to see if there is a noticeable difference.

Here are some photos of the UIM/TB off the car.


Closed Throttle body. Lets call this “zero degrees”:





open throttle body before discovering the issue, approximately 45 degrees give or take?:



open throttle body after rotating the wax mechanism out of the way, id say that’s another 20 degrees atleast:





Spring on the wax mechanism hitting the lower butterfly assembly:




lower butterfly shaft assembly clearing the wax mechanism after rotating it up:




thank you all for the input. Hopefully this helps someone else as well one day.

 

20 years on this forum and I have never seen that before!