Hi All,

I have an on-going issue with my XJR6. Starts on fast idle as expected and drives fine, but when you press then release the accelerator on many occasions the revs do not drop to the usual idle level and remain just above 1,000 rpm and the car mildly drives itself. If you "blip" the throttle by quickly pressing the pedal the revs go down to normal idle. Can anyone kindly give me some guidance on troubleshooting this? So far the mechanic and I have done the following:

1. Checked the butterfly in the throttle body shuts properly - the throttle body was reconditioned and mildly bored by MaxBore in the past and has a new spring, etc.
2. Stepper motor has been replaced.
3. Tensioning of the throttle cables (from pedal to traction control unit and from traction control to throttle body) have been readjusted in accordance with instructions kindly uploaded by people on this forum.

Could the steeper motor be sticking? It is an aftermarket part with a red dome, not the original one which has a slimmer brass plunger. Could the stepper motor be receiving incorrect signals from the ECU?

Any pointers would be much appreciated.

Thanks,
Nick

 

I drove an XJR/6 for many years and had various idle speed problems...and cures. Cable adjustment problems, throttle body problems. But it was long enough ago that my memory is getting a bit blurry.

However......

Towards the end it seems that I had an idle speed problem, intermittent but persistent, that I was unable to solve. I think it came down to the traction control unit itself. I think I determined that it was sticking or erratic in some fashion.

Others with more recent experience will chime in with better advice and information, but you might want to consider the TC unit itself, especially since you've covered so many other bases

Cheers
DD

 

Hi Doug,

Many thanks for that. I am really at the end of ideas on this one, I might consider fitting a used traction control unit to see if it has any impact.

Thanks and any other ideas would be most welcome

Nick

 

I had a similar problem with my XJR6. it turned out the throttle body end of the accelerator cable was very frayed and jamming. in addition, the frayed end wouldn't go up the cable surround, so was limiting max throttle opening to 55% on the OBDII readout.
My mechanic had a local aircraft engineer make me a new cable (the distal half of the cable from traction control to throttle body) and hey presto.
Also I got a whole lot of free horsepower all of a sudden, which happened to coincide with the fitting of the Powerhouse crankshaft drive pulley. I reckon I went from about 250bhp to about 340 all at once (it will now do a genuine 5 sec 0-60mph - manual, Andy's reprogrammed EPROMs, Powerhouse 110% supercharger drive pulley)

 

Hi Al,

You might be on to something here. I tested the maximum throttle opening using my ODBII scanner and it goes to just over 75% with the pedal to the floor. If I do the same on my X100 I get 100%. The cable might be the issue as you describe, but do you think the traction control unit could also limit this if defective?

Thanks,
Nick

 

we eliminated the traction unit by putting car on hoist, me in car pushing gas pedal, and reading %max opening, and mechanic under the car accessing the throttle body.
i think you will have to look at the throttle body end of cable to know.

 

I went through all of the agonies with tickover speed on my XJR, most of which are chronicled on here. The throttle position sensor was the eventual solution.

 

This is a textbook symptom of a dirty throttle body for a naturally aspirated AJ16. I'm not sure why the supercharger would prevent that from happening.

The oil blowby coming from the camcover deposits a sticky residue on the butterfly and causes this particular issue. Because the TPS is located at the bottom of the spindle, it's not unheard of for this residue to hurt the TPS. It's usually a lazy tech that decides to clean the throttle body out, in situ, allowing the carb cleaner chemicals to make its way into the TPS. That damages the contacts and ends the useful life of the TPS.

I've successfully bench tested a TPS, believing it was still good. After chasing my tail, and throwing parts at the car, it was only a TPS replacement that resolved a rich fuelling issue I had for years. I don't think you have that problem, unless you check your LTFT and it comes in high, at about 10%.

I know it's a royal pain to get at that throttle body, so I would recommend having a new TPS on hand, just in case, as countyjag recommended.

 

It is well worth taking some time to categorically eliminate the cable issues mentioned by Doug, remembering there are two cables, one to the traction control unit, and one from there to the throttle body. This shouldnt be too difficult or take much time, and if it solves your problem, great.
I fear however it wont, and removal of the throttle body would seem like your next course of action. There are various threads on here describing how to go about it, and as you will have gathered it is a pain, but doable. I would suggest disconnecting the battery before you start, or at least refrain from turning the ignition on with the TPS disconnected to minimise the risk of your TPS needing recalibrated, which is another pain! The TPS is remarkably expensive, around £200 here, and certainly above my normal "fit it and hope" threshold, but you might want to reflect on whether you fancy taking the throttle body off again once you have it in your hand.....
Well worth the effort to fix, as these cars are no fun to drive once the tickover gets much above normal.

 

Nick,

The aftermarket IACV might also be your issue here, there are reports from R people here mentioning problems with them, they certainly work on NA cars but apparently can cause issues on R cars. That said stepper only works when TPS reports idle voltage, so whatever it shows when your RPM is around ~800. Check the voltage at 800, then check again at 1000 when it sticks, if they are the same = possible IACV issue, if they are different = your throttle sticks and TPS doesn't get to its base voltage and you'll need to look into that.

 

Hi katar83,

I think you might be on to something regarding the IACV. The symptoms are as if the plunger does not fall back. You rev the car, the revs do not full completely back. You rev it again they fall back down a little more. My mechanic also suggested this, I did actually order another one from a different supplier - in the picture it had the brass plunger but when it arrived it was the same red domed beast that is already on the car.

Annoyingly I think someone (read possibly me) might have thrown away the original one (arrgh), I might buy a £30 second hand one from eBay and try this next along with checking that no throttle cables are sticking. One thing this does not explain it why with foot to the floor my ODBII scanner sees throttle as around 75%.

Thanks to everyone for their advice so far - one day the car will be perfect !

Nick

 

In any diagnostic process it's best to think about how you can easily segregate the components to try and prove/eliminaate some.

In your case, I would thinking about disconnecting the IACV and see what impact that has. If the symptoms persist you can eliminate it from your worries. If the symptoms go away then you have probably found the problem. As always, it may not be definitive but it's extra information in the overall diagnosis.

You may have some side effects (like stalling at idle), but worth the short term hassle if you can narrow down the issue.

Also, do you have any diagnostic tools that can display Live Data? It would be worth monitoring data while you drive and look at some key data when Revs stick and when they don't (like TPS, MAF). There's a surprising amount of data available through OBD for the X300 given how old it is.

So as an example, if the throttle was sticking open (for cable issue or whatever) I'd expect MAF reading to be higher when revs stick V not sticking. If it was a TPS issue, I'd expect MAF to be consistent.

 

This could potentially worsen the problem. IACV parks in a specific position when you switch off ignition. As far as I remember it fully extends, then go back a notch to its park position so if you disconnect it completely, it would introduce a bit of extra air into the system at all times and in this case you want to eliminate it completely, otherwise you may get higher revs.

MAF readings on X300 show kinda incorrectly so sadly you wouldn't be able to tell the difference between 800 and ~1500RPM looking at MAF readings in OBD, as it has to be multiplied by a 100 I think to get actual readings. TPS on the other hand is super accurate, down to 0.01V and anything over 0.02V between the ECU base idle voltage and actual TPS reading would cause problems and high idle so best to use this for diagnostics.

I'm not entirely sure plunger would be an issue here, as they do work fine on NA engines and I think I've tried three different end caps now and all worked fine. Mind you they can be swapped around fairly easily if needed. I remember someone also mentioned socket orientation causing issues on their R.

This is an unrelated issue and its caused by stretching in that 25 year old cable. It doesn't affect your idle but it does certainly affect fuelling and overall performance at higher revs. Even on NA engines its a bit of a bast$rd to set it up I'm afraid as there are three components working together here and all of them have to be in spec for this to work. They are the base idle TPS voltage, throttle gap and TPS reading at wide open throttle as close to 100% as possible.
In a perfect setup, you need throttle gap when closed set to 0.002" with TPS showing approx ~13% or 0.6V(within 0.02V of what it shows when car idles fine) and then as close to 100% or 5V at WOT. Obviously adjusting TPS can affects gap and the other way around so when setting this all up, all three have to be taken into an account. You can get around the TPS base idle voltage by doing tps adaptation(if its considerably out of spec atm) but that comes with its own problems as not every garage has software to do it.
Where you wanna start with this is to have someone floor the pedal down and to check if you still can manually pull on the cable/throttle, that way you'll know if you have slack in the cable.
The good news is that when you get the TPS to show lets say 95% at WOT(think I've managed 96% on mine when I did it) it will feel like you got an extra cylinder back

 

I take your point if the IACV parks open. Maybe you can get around that by starting the engine and letting it idle, then disconnect the plug before driving as a trial.

The key is not necessarily to exclude all air via the IACV, but to exclude it as a variable. So even if there is air entering the system from the IACV causing higher revs, it would be constant if disconnected. So any intermittent variation in revs would not be due to that component.