Continuing problems with my throttle body. My car is a 1996 VdP XJ6 with about 130k miles. A couple of months ago I wrote in this forum that the throttle body developed a problem. Working the throttle body shaft by hand, it turned freely with the engine off but with the engine running it wanted to stick in the idle position, and to drive it I had to punch the accelerator to get it free. So I took the body off the car and cleaned it very thoroughly with spray and a soft cloth, it was pretty dirty. After refitting it seemed normal for 6-8 trips but has now gone back to hanging up again. Once it starts hanging up, stopping the engine and restarting doesn’t help. Very occasionally it behaves properly for a trip for no obvious reason other than maybe it’s colder.

It seems unlikely to be dirty again after just a few runs. What’s making the butterfly shaft hang up - vacuum perhaps, doesn't feel gritty like dirt? Do these things wear out and have to be replaced?

 

You’re right, it’s highly unlikely that the throttle body is dirty again.

The throttle body is not a commonly replaced part, so while it can solve the problem, I’m curious to see what is causing this. Have you actuated the throttle by hand to see what it’s getting stuck on?

Some people add a return spring to help that butterfly snap back stronger. There is a part number for this spring. If you do a search in this forum, you will find it. Perhaps that’s all you need?

Id still take a closer look to see what’s binding that thing up.

 

If I am interpreting this correctly, the Throttle is sticking in the closed or idle position. If so, the "helper" spring modification would not make a useful contribution, and would make the throttle marginally more difficult to open.
I would suggest examining the sliding joint where the cruise control actuator is attached to the throttle to make sure it isnt binding, then try disconnecting the throttle cable to see whether the throttle moves freely.
In the event that neither of these are affecting the operation of the throttle, I would begin to wonder whether the throttle spindle could be bent or worn. The mileage of your car would make me think that wear is less likely a culprit.

 

Then, of course, a look at the spindle might reveal whether or not it was cleaned all the way...however if you took the throttle body off the car, I'm assuming you got it all.

 

Turn the throttle stop screw a 1/4 turn and see if that fixes your issue.

The ALLEN screw is NOT metric. I forget what fraction of an inch the screw is?

You will likely need to cut off a small section of the ALLEN KEY to get it in the small space unless the throttle is OFF the engine.

 

Need a good picture of the idle stop screw

When I had my TB off the engine, I failed to notice it

Motorcarman / Bob certainly has more hands on experience then myself but my suggestion is to keep careful track of your idle stop screw adjustment so you can return to your original position

This relates to the throttle position sensor reading of the new idle position value ( 0.60 ish volts DC ) and idle range engine regulation fine tuning that everything is " together "

 

Here you go. This shows one end of that set screw. The manual clearly warns against making any adjustments to this screw to address idle issues.

Motorcarman knows way more about these cars than I do so I would listen to him, but please follow Parker7's advice about writing down the turn amount and the direction so you can put it back where it was if it doesn't solve your issue. Please write it down...you'll forget.

 

Thanks Vee ,

No wonder I did not notice it as my focus was inside the throat area

 

A very helpful photo from Vee, I too had never noticed the screw. The photo has set a couple of hares running with me; a) The mechanism around the spring and stop would seem to offer many opportunities for "sticktion", and cleaning the throttle body may have removed lubrication, which I suspect is needed, b) Could the rust on the screw be affecting the idle setting?
Where would we be without throttle bodies, coils, and crankshaft position sensors; there would be nothing to talk about on the X300!!
I remain envious of those who can actually access their throttle bodies with ease. I break into a cold sweat at the thought of the XJR set up.

 

1/4 turn on the setscrew will NOT cause any noticeable difference to the idle. (I promise)
Dozens and Dozens of cars were affected and we saw them on a regular basis at the dealer 'back-in-the-day'.

Some could just NOT be cured with the throttle spring and cleaning.
Some were so bad that the customer BENT the throttle spindle 'stomping' on the throttle pedal.(new throttle body assy.)

Keeping the throttle 'butterfly' from binding is what you want to accomplish. Adding a little more of a 'gap' when closed might be all you need. (unless the spindle is TWISTED!)

 

You'll apparently need a 7/32 allen key, which will probably have ot be trimmed down to make it fit in that narrow gap to access it.

Motorcarman, so he's gonna want to turn that setscrew by 1/4 turn to lengthen that exposed screw stop, or shorten it? I think you're saying to make it longer, so it ever so slightly reduces the travel of the butterfly?

 

Turn the screw IN. (tighten the screw to open the throttle SLIGHTLY)

 

That screw is only there to set up throttle butterfly gap at 0.002" when setting it up initially. First of all, cleaning throttle body with a soft cloth is a bit of a red herring. I had to clean a few of mine in the past few years and after 25 years they were caked with oil to say the least and just cleaning it with a soft cloth does absolutely nothing. I had to use a fine sandpaper to get that yellow layer of solidified and hard oil off the throttle body.
The whole process should go as follow.
1, Connect an ODB scanner while the car runs ok with correct idle and record TPS voltage or its position, depending on your OBD scanner it will either show in V at around ~0.61V or in % at around ~12%. This step is absolutely crucial, if you change anything later and that voltage goes up you'll end up with high idle.
2. Switch the car off with ignition in position 2(before starting the car) and floor the gas pedal all the way down and again record TPS voltage with, it should show as close to 5V or 100% as its possible with the foot down.
3. Take the throttle body off, clean everything down to clean metal, confirm that throttle body gap when closed it set to 0.002", if its not adjust it with the little screw.
4. Plug it back in but don't boltit all back yet, just confirm that TPS voltage is still what it was when you first checked it. If its not and its higher then loosen the TPS screws and turn it slightly to one side until TPS voltage returns to the one you first recorded. Recheck throttle gap and confirm all is still correct. There is enough slack in the TPS screws to adjust it by 0.08V which is enough to change idle by ~1500RPM.
5. Fully open the throttle with your hand and confirm that OBD shows TPS voltage at 100% or close to 5V.
6. Now you have throttle and TPS set up correctly, bolt to the intake.
7. Undo the top throttle plate, there are TWO springs there, bottom and top one.
8. You can replace the bottom one, Jaguar sells a revised version of it which is very slightly stronger.
9. Pay attention to the TOP spring, you can wind it up one extra turn to make the whole throttle body spring stronger then put everything together, adding some lubrication, like silicone or graphite spray or some grease to springs and shaft where the sit.
10. Put everything back together, connect the throttle cable and adjust it in a way where TPS voltage is as close to 5V or 100% as possible with the foot down but the throttle still closes easily and returns to that base idle TPS voltage.

That's all and should sort out your sticky throttle, also getting TPS and throttle cable adjusted should make the car run quicker as you'll bring it back to correct fuelling.

 

Motorman, thanks for the idea of adjusting the throttle stop a quarter turn. That gave me an idea of what might be happening here, see what you think of this. If the throttle stop was a little too loose from the beginning, then over the miles the butterfly, which is made of a harder material, would slowly dig into the softer body and cause a ridge line. You wouldn’t feel it turning the throttle shaft by hand with the engine off, but with the engine running and with a small amount of movement due to shaft bearing wear, the butterfly is pulled by the manifold vacuum so it jams against the ridge and I have to kick the accelerator to get it free. Your idea of tightening the stop a quarter turn might move the butterfly past the ridge. Is that what you were thinking?
If tightening the stop doesn’t fix it then I guess I’ll have to replace the throttle body. I have a used one from eBay to try. Thanks for trying to help, I’ll look forward to hearing from you.

 

Yes Katar83 covered this excellently , but.....................hi Bob

The original problem description in the opening post may not involve the idle screw or butterfly sticking but may involve the throttle linkage through the traction control actuator that may be trying to retract the cable as a defective action

You may have the traction control in action light different then the traction control inop light as a clue

He seems to have intermittent but not constant problems advancing away from idle and no description of throttle relaxing back to the idle position

Traction control switched off or fuse # X pulled ?

 

Thanks everyone for the helpful thoughts. Just to clarify - this is solely a mechanical problem with the throttle body butterfly binding in the idle position with the engine running. When I check it by moving the throttle body shaft by hand, the cable to the traction control actuator relaxes so it's nothing to do with that, and the flexible link to the cruise control relaxes so it isn't that. Strengthening the shaft springs would be counterproductive. Once the butterfly is moved off idle by hand or by kicking the accelerator it moves freely and returns to idle freely but then binds again. When the engine isn't running there's no binding at all.

I tried tightening the adjustment screw (many thanks for the great photo of that) a quarter turn, it did not upset the idle and it made the binding a bit better, it doesn’t need as much kick to get it moving. Maybe another quarter turn would do the job.

Judging from everyone’s experience it seems likely that either I didn’t clean it adequately and need to try again with fine sandpaper, or the original dirt and kicking caused a distortion in the shaft although the shaft seems beefy enough. So for my next steps I’ll try a half turn, recleaning, an exchange unit, and if all else fails a new one. I’ll let you know.

 

Carburetor / Throttle Body icing

The 2 coolant lines installed ? but I would not expect icing at low throttle settings as the pressure drop across the butterfly is not as severe in a isothermic sense

 

Problem with this approach is, by screwing that little stopper in, you'll be changing base TPS voltage, quarter turn might be fine but if you go over 0.02V while turning in the screw, you might free up the butterfly permanently but you'll end up with high idle as ECU will think you've got your foot down on the gas pedal and it will switch off the IACV. That's what TPS at the bottom of throttle body does.
You also have no idea what's the butterfly gap is atm, screw goes further in and the gap goes bigger and like I said it suppose to be 0.002", if it's bigger, then you're further introducing air that shouldn't be there and the whole system is out of whack. ECU can only compensate a little.
The whole thing is relatively easy when you see it in front of you and get an idea what and how it's all controlled and connected together but changing one thing and forgetting everything else is just asking for trouble.
Replacing entire throttle body by the way, will introduce completely different TPS base voltage and there is 75% chance that new one won't match to your ECU so you'll end up with wrong idle so make sure to check and record current one so if you go that route you will be able to set TPS as per old throttle body. Only alternative is to do a TPS adaptation at a Jaguar garage/dealers that still have correct equipment to do it.

 

Katar83 - I greatly appreciate the details of how to set/reset the throttle body adjustments, particularly since it seems likely I'll have to replace the original. Two questions - first, is the.002 inch gap between the butterfly and the throat, or is it between the adjustment screw tip and the arm? At first glance there doesn't seem to be any gap between the butterfly and body. Second, re using an OBD scanner - I've just bought an Autel AP200 and am at a very early stage of leaning how to use it. Is there an easy way to guide me to the readings you describe?

 

In replacing the throttle body with the TPS adjustment disturbed by swapping or replacement it is important to record the present idle stop sensor value before disturbing

The ECU is electronically adjusted ( oriented ) to a null value to know if it is in agreement that the butterfly is at the idle stop , this effects your idle performance

This ECU orientation is hard to find some special equipment to do beyond the expense

The TPS recording can be done in 2 ways with a simple meter

You can read the return voltage signal to the ECU or a simple resistance check through the TPS

With a nick in the insulation on the middle Green / Yellow wire with the key in the run position but engine not running

You should see the 0.60 volts DC , it is important that you use a precise meter as 0.59 or 0.61 volts is what you may see and you have to match the original value

Or you can remove the ECU connector in the cabin and read the resistance through the TPS at socket Red 7 and Red 12

In both methods you would leave the connector on the TPS

Editing