The TPS on the OBD tool will never read below 11 or 13 % at the idle stop as that unit of measurement

This is referred to as a PID ( parameter ID ) and what you will see on the tool vs. a voltage reading

Just the way the software was written

 

Typically, I see 12.9% when idling.

Recording the volts DC using a digital multimeter would be the way to go for me. Make sure that you have the same volt reading when replacing the TPS, throttle body assembly, or even just monkeying around with the throttle body, like when cleaning it! With this information, you should be able to fix any abnormal idle issues, not attributed to failing sensors!

 

See here for detail on how to check and adjust the gap:
https://www.jaguarforums.com/forum/x...2/#post2231908

It's a slightly different throttle body as it's from xj40 but the idea is the same.
There is a number of Jag TSBs refering to your issue in that topic too.
Re your Autel, plug it in and read live data, you'll see the TPS value there easily, will be very obvious. I think Autel will show the value in %, as far as I remember, mine sits at 12.37%. This value is crucial for idle and is stored in ECU.

 

OK, so I measured the TPS voltage on the car and then took the throttle body off for another look.

Using a Fluke 87V digital meter measuring from a nick in the center green/yellow wire to the car frame/ground and the stop screw restored to its original position I get 0.708 volts.

Measuring the resistance of the TPS, and naming the pins Left-Center-Right, Left-Center is 0.667 ohms, Center-Right is 3.545 ohms, Left-Right is 3.991 ohms.

The throttle body throat is still clean, there looks to be a .002 clearance almost entirely around the butterfly and it moves freely no matter how I push the shaft towards or away from the manifold or longitudinally to mimic vacuum on the butterfly, there’s no sign of wear or ridges in the throat around the butterfly.

The springs on the shaft all look undamaged and move freely, and as previously noted the engine starts and idles just fine.

I can see nothing that would cause the butterfly shaft to hang in the idle position with the engine running, but testing by hand there’s no doubt that’s where it is. Nothing to do with the cables or cruise control links. Hopefully someone knows what this is, otherwise I guess I’ll try an exchange unit.

 

How about that set screw stop? Any indication that its started to dig into the metal where it makes contact?

It's gotta be something.....

(BTW, 0.708 is higher than I would have guessed possible for idle, so its great that you were able to record that)

 

The 0.7 volts is high and I would personally start there with your original parts

Your ECU would have never been oriented or matched to this value as the point of idle or null

One of the other 3 wires on the TPS is a ground and the 3rd should be a very controlled reference 5.0 volts DC

The ECU has some ground wires through the ECU connector and a dedicated ECU case ground strap on a ECU mounting bolt

There is very little wiggle in the TPS mounting to change the original 0.60 volts but you can drill up a size on the plastic body TPS to be able to twist to get to the 0.60 tolerance

But I would look at it electrically before adjusting the TPS mounting , the ground being the most suspect

 

I might start and see if you can find a used TPS online for cheap. Maybe get a bargain and find a cheap throttle body with the TPS still on it?

something does not look right with a TPS with that much voltage at idle.

The problem here is that adjusting it down, will likely require a TPS reset, requiring a PDU, WDS, etc

Not sure what the right move is here. As long as you know where you started from, Parkers advice shouldn’t take you to some point of no return.

 

I believe the Allen key is 5.5 mm. It's unlikely to be an imperial thread. 5.5 is not uncommon in the automotive industry, as I recall it was used on the brakes of my old cavalier.

 

Yet again, thanks for the facts and figures. To be sure that I was measuring to a true signal ground I nicked all 3 leads, plugged the throttle body back in, key on but engine off, and measured voltages to the black, obviously ground, lead. Voltages on the other two leads were 5.054 and 0.708. So measuring to the ground from the ECU is the same as measuring to the car frame, so presumably the ECU ground is OK.
To answer Vee, no sign that the set screw is digging in, and as noted the butterfly seems to have clearance all round, no signs of any digging in or mechanical fouling.
I've had the car for 15 years and during that time no-one has touched the throttle body or done anything that might have changed the switch. What does it mean to have an idle voltage of 0.7 instead of 0.6 - would it be running too rich? It's always been quite heavy on gas but I've never worried about it and it's shown no problems.
To recap with the benefit of hindsight, all was well until a few weeks ago when the throttle body suddenly started to stick in the idle position. I took the body off, found it quite dirty, cleaned it and put it back, it worked Ok for a few days and then reverted to sticking again. The only thing capable of exerting that much force on the TB shaft is the butterfly, and since it's not fouling mechanically it can only be the manifold vacuum. What does that other electrical device on the throttle body do - it seems to be some sort of air bypass, is it supposed to limit the vacuum, and if it was blocked or stuck would it cause my problem?
I have a spare throttle body but its switch is bad. And if changing it requires rematching to the ECU, I don't know where I could take it to do that. My favorite mechanic here got Covid and didn't survive, a great loss. I'd rather sort my mechanical problem first before starting down a new road.

 

The device attached to the TB is the idle air control valve that schedules down in steps as you release the gas pedal to a not fully closed IAC valve position

A too open final postion will have a higher idle speed

The mounting bolts have a habit of snapping off

The EGR and EVAP valve will be at fully closed position at idle

Measure the resistance of the ground wire from the TPS connector to the ECU connector at socket Red 7

Corrosion can accure on the wire side of the socket in the ECU connector

The 0.7 TPS signal should be a rich mixture different then the target of 0.0 for the short term fuel trim in all areas of throttle region

This if the IAC valve is in the correct position as a variable of consideration

 

If the ECU has learned, over time, as it is intended to do, that 0.7 volts is idle, then you are fine. The engine will not over fuel, and should respond normally.

I wonder what your top end voltage is at wide open throttle? I think after 4.5V, you are as far as the ECU can read.

Could the TPS itself be causing the stickiness? That butterfly is directly tied into the spring loaded TPS at the bottom.

 

If I am still following this correctly, we are still trying to diagnose a sticking throttle, rather than a high idle, and the fault preceded the initial cleaning of the throttle body?
You have stated that the throttle moves smoothly when you open it by hand with the engine off, but sticks with the engine on. I take it you are opening it by hand when you have the engine running, and are comparing "like with like" ?
I ask, as I can imagine how the throttle might open ok by hand, but stick when using the pedal.
Assuming you are comparing like with like, I would be interested to know whether there is any difference with the traction control / throttle relaxer disconnected. One of the small number of differences with the engine running is that it has power, and it is directly connected to the throttle cables.
If that makes no difference, you are back to the throttle body, and a close examination for wear/ bend in the spindle. Your mileage would make me doubt that wear is an issue, but stranger things......

 

I would check OBD TPS value first and confirm its the same as your fluke meter reading, 0.7V seems awfully high and at the end of the day what ECU sees is what's important, not what you see at the sensor itself.
Also, when you say sticky and you have to tap on the pedal, I assume you mean the car still runs at about 1500rpm when going fairly slowly and foot off the pedal?
Maybe your issue isn't with the sticky plate but with TPS reading going above base voltage. Basically imagine going down the road, foot off the pedal, you're going 15mpg to a stop but engine RPM sticks at 1500 and you have to tap the pedal to 'unstick' it and for it to go to around 1000RPMs. Problem here is, you won't know if it's actually butterfly plate that is sticking or if it is TPS voltage going 0.02v over base TPS voltage. If that happens your IACV switches off in partially open position, allowing extra air going into intake and ECU supplies extra fuel to engine and it gives you exactly same results as sticky throttle body, in which case actually backing out that little stopper nut might help, not turning it in.
Only way to find out would be with checking live OBD data while you're driving.
Re swapping throttle body, this is easy and you can do it, two things to remember though. First - you need to make sure that transferred TPS shows exactly the same base voltage(or slightly lower), and second: when transferring IACV, make sure to heat up it's little bolts, before trying to undo them, they have loctite on them, if you try to remove them without heating them up first you'll snap them straight away.

 

Thanks folks for the most recent comments. Let me try to give a clearer description, for your questions. With the engine off, the throttle body shaft moves freely both by hand and with the gas pedal. With the engine running, the shaft sticks in the idle position. When I move it by hand it is clear that what is sticking is the throttle body shaft, not due to either the traction control or the cruise control. Once the shaft is forced open from the idle position by a couple of degrees, either by hand or by booting the gas pedal, the sticking goes away and it moves freely for the rest of its travel.
The engine starts and idles normally and also drives normally after I boot the shaft off the idle position. With the engine running, if I force the shaft open by hand and then release it, it snaps back to the idle position freely with no delay, but once in the idle position it sticks again.
With the throttle body off the car the throat is shiny clean and there seems to be an even .002 inch gap around the butterfly. There's no sign of any shaft distortion and it moves freely through its full range of travel with no binding anywhere. The throttle body switch seems to be behaving normally and an ohmmeter shows smooth transition through the shaft range of travel. And the switch is too small to exert the sticking force on the shaft.
While the TBS idle voltage is higher than normal at 0.7 volts, that seems to be a separate issue as there seems to be no way for that to cause the mechanical problem of the shaft sticking in the idle position.
What I am wondering is whether this is due to a problem with the idle air control valve. If it was sticking fully closed at idle, could that be causing a higher than normal vacuum on the butterfly to hold it closed? All the other symptoms are compatible with this - once the butterfly is forced open the force from the vacuum would disappear, but would return as soon as the butterfly closed again. And the original problem began when the throttle body hadn't been cleaned for a long time, so maybe that dirt has jammed the idle air control valve? That would also be compatible with the original symptoms, recalling that the sticking was intermittent.
The idle air control valve has 4 pins. What do these do, is there a way to test it?

 

The IAC is a 4 wire stepper motor that does not have a position sensor to see if it is in agreement to what the ECU commands

Usually will not see a open leg on the ECU black connector with your meter until warmed up and tapping on the stepper motor

You can test the legs for resistance between the 4 squared number of combinations minus the electrical equivalents

Red to Green is electrically equivalent to Green to Red so you have 9

 

Is it possible that the cleaning process has left the spindle bearing surface dry? I would try some ptfe spray on the bearing surfaces.

 

Thanks for clarifying all of that, as it helps to rule out much of the endemic high idle troubleshooting advice. I understand what you are saying about the possibility of the butterfly being held shut by manifold vacuum, and you have certainly got me thinking, but the fact that the car is idling correctly would suggest that the manifold vacuum must be correct, ie the correct amount of air is getting in to sustain idle. It could be the case though that the proportion of the air coming through the butterfly is too low, and the IACV is making up the difference to the correct level (suggesting that your IACV is working fine) Could that be enough to cause the throttle to stick; possibly, but it would seem marginal. It is beginning to sound as if the throttle body is going to have to come off again to be put "under the microscope", probably starting with the grub screw adjustment to make sure you have the specified amount of butterfly clearance.

 

My thinking in respect to butterfly pressure is you would have equal and opposing forces on the surface of the butterfly on both sides of the shaft in a rotating sense

This balances out regardless of the manifold pressure if it is out

But you would have a higher force inwards on the shaft friction slowing down its rotation

Whether or not this friction accounts for your observations .................

The sticking force away from the idle stop .............

The majority of the butterfly circumferential gummy surface contact will be highest at the idle stop

I was surprised how much gunk was on the throat area and had gone forward past that into the inlet air temp sensor

You may have higher then average blowby with your compression and I found Rislone oil treatment product that worked for me on a collage car

There is a TSB on the brreather gas seporator upgrade and a different 3rd party breather gas seporator

03.1-15 (jagrepair.com)

 

Well dang, Parker 7 is right that the vacuum forces on the butterfly would be balanced. And I've tried pushing the shaft to mimic the vacuum pressure on the butterfly, but there's no unexpected shaft friction.
There is gunk looking into the manifold, but no way for that to affect the throttle body shaft.
I have the car off the road and the throttle body on my bench looking for inspiration, but Parker has destroyed my only idea what could be making it stick. Perhaps it's evil spirits.
What I'll try now:
- test the IACV per Parker's very helpful illustration
- re-check the butterfly clearances (but with a lamp behind the butterfly these look right)
- run the car with an exchange IACV
- run the car with an exchange throttle body, but with my original TPS so I get the original idle voltage to the ECU

 

It's not the TPS that saves the original idle voltage, it's the way its installed. You can take your TPS and install it on to 5 different throttle bodies, and you will likely get 5 different voltages. You can take your TPS off and put it right back on, and likely have a different voltage. Even the slightest adjustment to that thing will adjust the voltage by a tenth of a volt or two.

Once you remove, or rather loosen the screws holding it in place, it's game over.