Stefan,
I'm afraid it sounds like you mechanic doesn't know what he is doing. The % throttle opening at idle should be about 12-13%. 20% is far too high. If the throittle were really that far open then I would expect a very high idle speed. The engine ECU certainly wouldn't consider 20% to be a close throttle and therefore would make no attempt to try to control the idle speed to the correct value by closing the idle speed control valve. There are various reasons that the throttle potentiometer is reading 20%

1) incorrectly adjusted cable (too tight) between pedal and traction control actuator
2) incorrectly adjusted cable (too tight) from traction control actuator to throttle body
3) throttle butterfly gets stuck in throttle body due to buildup of dirt
4) some form of electrical failure opf the TPS - never actually seen this myself though

80% is a very low % for full throttle if the pedal is pressed hard into the kickdown switch. I would expect to see over 90%, maybe even 95%. The most likely causes for a low reading is too much slack in either of the throttle cables to or from the traction control actuator. This is potentially at odds with the high percentage at idle. The kickdown switch is mounted on a piece of studding, so it is possible to screw the switch clockwise to allow more pedal travel. The electrcial connection to the switch should be disconnected before trying to rotate the switch otherwise the cable will just get wrapped round it.

Anyway a decent Jaguar mechanic would know all this. If you have now bought a new TPS it is ABSOLUTELY ESSENTIAL that after it is fitted that the idle position is adapted using the throttle adaption routine. This can be done with WDS or IDS Jaguar service diagnostic equipment or good after market equipment such as Autologic. Sine these cars are now at least 15 years old, Jaguar dealers don't see them very often, so you need to check that the technicians know how to perform this process, even if they have the kit to do it. The closed throttle position IS STORED IN NON-VOLATILE RANDOM ACCESS MEMORY (NVRAM). This value will not normally be lost just by disconnecting the battery for a few minutes. Having said that the ECU in my car now seems to lose its adaption when left in storage with the battery disconnected for 6 months.

There is no reason why it should not be possible to fix your car, provided the work is done by someone who really understands how these cars work. I drive my car 60 miles to a Jaguar specialist and take a train home just to make sure it is worked on by somebody who know what they are doing.

 

Thanks for your input, Andy.

Did I mention the idle drops to 8-900 when selecting 'D'? Back in 'P' it returns to 1200-1400 or now, after the 'repair', even 1600 rpm. The highest ever. Don't know what to make of the 'D' vs. 'P' idle?

It frustrates me that most work have to be done by a mechanic with the proper tools, as I am unable to find one who knows his way around the X300. When even replacing a TPS needs jag software, how much is there left for the home mechanic? As you may remember, my back is in rather bad condition, so on heavier work I have accepted that a hired mechanic is the only way to go. But when I can't find one I really trust, even a small issue like a high idle can be a huge and extremely expensive thing to TRY to fix.

I have finally received an original TPS, at a total of £195. The mechanic didn't order after all and he forgot to tell me (!), so I went to an online UK store and ordered on monday. Thursday evening I cancelled the order, because nothing happened, nothing was sent and no information given. Instead I had one overnighted from britishparts.co.uk.

I find it rather interesting that the TPS is spring loaded. Is that normal for jag? Don't remember that from any BMW I have worked on.

If the new TPS and recalibration doesn't work, I think the throttle cable slack is the next thing on the list. But that should have been checked/fixed, when the mechanic had the throttle body removed for cleaning and tensioning of the spring.

Biggest issue with the car right now is the trans issue. Can't even drive 100 meters without the risk of the service light coming on and the shifts get VERY hard. In exactly three weeks I am driving the family on summer vacation, provided the car gets fixed. If not, this is the first time in my life a car has let me down so badly and then there is no way I'll be keeping it. If it doesn't drive, what's the point? I must admit I am close to giving up on Jaguar and returning to BMW. Not because Jaguar is a lesser car, which I don't think it is, but because no one knows how to fix even these minor issues. Scares me to think of what would happen, if something major broke.

 

The engine speed drops when you select drive simply because the transmission puts more load on the engine. If the ECU was actually trying to control the engine speed then this drop would be limited to 100rpm.

The most common cause of transmisison problems is the harness from the transmisison conrol unit to the gearbox. When the rear gearbox mount deteriorates it allows the gearbox to move more than usual relative to the body. This causes the harness to flex resulting in failure of the wiring in the harness or poor connections at the gearbox connectors themselves. The loss of electrical continuity can generate all manner of problems. However, it is still possible that simply having a new TPS that is properly adapted will fix your transmisison problem as well.

 

So this means, that the ecu is NOT creating the high idle. In fact it doesn't consider the idle AS idle. By that I conclude that it isn't the idle control valve that's been made to open too much, as the mechanic suggested, because then the ecu would just turn it down again. The ecu never sees any idle at all. Instead it thinks that I am constantly revving the engine, either because the TPS is sending a wrong signal as we know it is (P0123), or because the gas cable is adjusted too tight. I am ruling out a sticky throttle for now, as the throttle body was cleaned a few weeks ago.

Is this correct?

 

Yes Stefan,
If the throttle voltage measured by the ECU is higher than a value that it has previously "learnt" as corresponding to a closed throttle condition (ie idle) then the ECU will not try to control the idle speed by adjusting the position of the idle speed control valve. The actual idle speed of the engine could be higher or lower than the target speed depending on a wide variety of factors, such as coolant temp, oil temp, oil grade, engine mileage, altitude, whether drive or neutral is selected, whether there are any air leaks). It will also depend on the actual position of the throttle butterfly as along with the ISCV, this controls how much air is flowing into the engine. If the butterfly sticks slightly open then this will cause the voltage to be to higher than is acceptable to register an idle condition and will let too much air flow into the engine. The same effect would occur if there was not enough slack in either section of the two cables, so in fact the throttle was effectively being held open , even when your foot was off the accelerator pedal.

You need to get enough slack so that the throttle percentage is about 12-13% when you foot is off the pedal. You don't want to much slack or too high a kickdown switch setting to stop you getting at least 90% throttle opening, when you foot is hard on the accelerator pedal. You need the throttle adaption routine to be run after you have these other conditions set. Setting the throttle needs diagnostic equipment and someone who knows how to use it, otherwise you are wasting your time and money.

 

Yesterday I took the car and my new TPS, an original JLM 12074, to the mechanic. Was hoping it would be a quick job that could be done from underneath the car, but that was impossible. Had a talk with the mechanic and we considered removing the oil filter console to gain some working space, but gave up on the idea. Instead he pulled everything apart again, spending about four hours on that. 30 seconds of that was replacing the actual TPS...

Before I reveal the result, let me tell you about the last two drives. A week ago I got about 100 meters, before the trans light came on and late + harsh shifting followed. Already when I turned on the engine, the idle was very nervous, as if I blipped the throttle repeatedly very fast. The idle was below 1.000 but not by much. Gave up and took the car home where it stayed until yesterday, when I drove to the mechanic. Yesterday it fired up and hit 1.500 rpm very fast, but then dropped to about 800. On the way to the mechanic everything seemed fine. And this is the frustrating part, because sometimes it was almost perfect, while the next drive could be pure hell...

So the new TPS was installed and the engine was fired up. It straight away hit just below 700 rpm and ran perfectly smooth without any of the misses, that's been haunting the car since I got it. The ignition was turned off and then the car was restarted. Repeated a few times and everything was perfect. The computer was attached and software, which I believe to be IDS, was ran. The TPS adaption was performed and no errors found.

The kick down test was on the high side. I believe the pedal should hit the kick down switch at 66%, but it travelled all the way up to 71%. After a total of 5 hours at the shop, I decided to have that fixed another day. The important thing was that the idle was ok and the trans failure hopefully fixed.

Btw, can I adjust the kick down switch to be a bit higher myself, or is wds/ids a must for this job?

Talked a bit with the mechanic and the boss. Got in the car and it started and ran so smooth. Drove home through downtown traffic at rush hour. At every stop the idle settled at just above 600 rpm straight away while in 'D' and the drive was perfectly relaxed. When at home I shifted into 'P' and the idle rose just above 700 rpm. The car seems transformed and I now feel I finally drive a Jaguar.

I can't thank everyone participating in this thread enough. You, and those who gave information in other threads, have helped me figure this out and find a cure. It has been a long and rather expensive journey, but it looks like I have finally arrived. So THANK YOU!

What's next? A very scary time, always expecting the fix was just coincidental and the problems returning. I am definately a candidate for the thread titled 'How long does it take to gain trust?'

 

I only have manual cars and can't find any info in any of the service manauls that indicate the throttle percentage at which the kickdown switch should operate. However, 66% sound quite low. When the throttle spindle actually hits the end stop and the blade is at 90degrees to the bore, I would expect the percentage to be in the mid 90%s. If the kick down switch operates at 66% I can't imagine that the switch itself would have enough travel to allow the pedal to move fair enough to pull the cable enough to turn the spindle enough to get from 66% up to over 90%. Maybe someone who has any auto car which has a correctly set kickdown switch can advise you.

 

Congratulations! I was feeling your frustration as you tried to chase down your gremlins. It's always nice to see a thread like this one come to a happy conclusion. And always educational.

John

 

My tps/potentiometer reads 10.5 % at idle, this is new, and only 80%, this is also new, at wide open throttle. I recently replaced the the ported throttle body and placed the stock throttle body back, larger tb not good for stop and go city driving but very good for all out racing, my two cents. Do you think a reset will cure my problems or could it be a problem with the tps, tps is a few months old. I last had the tps reset with the larger tb in place and the closed throttle was at 13% or 14% if I remember correctly. I never had to adjust kickdown switch under pedal and i dont know why i would now, unless this was done by mechanic when i had tps reset last. I will check switch under pedal.

 

10.5% does sound low, but the software will adapt down to this value. 80% also sound low for full throittle. I think that it is possible to operate the throttle quadrant by hand when it is installed on the engine. If you look at you scan tool while you do this (the ignition needs to be on, but it will be best if the engine isn't running. You will be able to see if the percentage goes beyond 80% if you fully open the throttle by hand.

 

I have ditched the aftermarket tps and have now tried OEM jaguar, just because I am sick and tired of dealing with aftermarket junk.... but to my disappointment the OEM part has not made me happy. I have replaced the OEM tps twice and can only reach 90.5 percent throttle at best, the original tps on the car before it went bad was able to reach 100 percent. Do they just dont make them like they use to? ARG!!!!! Is this the best that I could expect?

 

Don't get too hung up about the percentage at wide open throttle. Whether the engine makes max performance is only really determined by whether the blade is fully open, not the throttle signal.

 

Hi Stefan, I've been reading your thread with interest and it's great to read that you may have resolved your car issues.
Although my problems do not mirror yours my car, like yours, is in a 'go today and not tomorrow' mode. I won't go into all the work that's been done on it but some improvement has ensued.
I read in your post that you used a OBD11 device and I'm thinking that one of these would be well worth having to help sort out my problems - hopefully. Do you mind me asking which make and model you have been using?

Many thanks,
RogerB
1994 XJ300 - 4 litre
132,000 miles

 

Sorry for the late reply, Roger.

I use a cheap chinese reader, found on eBay. The software is TouchScan. I don't think neither is top of the line, but they did the trick for me at a very low cost.

The hardware is the same as this:
OBD2 OBD II EOBD V1.5 V1.5a Diagnostic Interface Scanner Scan Tool | eBay

The software was purchased here:
OBD2 Diagnostic Software, OBD Software, Scan Tools | OBDSoftware.net

 

My 97 xj6 idle went to 2500 rpm in traffic. Try that on for size. Codes blamed the TPS but it turned out to be the Idle Air control. It came apart allowing air to bypass the butterfly through the orfice resulting in uncontrollable idle. Brakes alone will not hold that cat back, had to go to N to stop. Arch

 

If your brakes will not hold your XJ6 back, even at WOT, you better fix your brakes! Or, get on the stairstepper....