Slow throttle return when rolling along.

[duncanwoolgar]

Hello. I have a 3L manual year 2000. I am interested if anyone else has experienced this problem especially on a manual as the nature of the beast makes the fault more obvious. When stationary the throttle behaves as you would expect with sharp response up and down the rev range. If you are moving along the road the revs take far too long to drop, making changing up painful unless you wait a long time for the revs to drop. In the high rev range if you are driving with gusto the revs drop sufficiently quickly for a smooth gear change. At low speeds it is a real pain, if you coast in traffic sometimes the revs can be 1500-2000 rpm until one comes to a standstill and then the revs drop to an expected tick over. Car is well serviced regular oil changes etc. Throttle body clean, Maf sensor clean, air filter clean, no fault codes. Regards Duncan, West Sussex

[J44EAG]

My first reaction Duncan, when reading your post was to think that the throttle place might be slow returning to the shut position even though you say you have cleaned the throttle body. The throttle plate should snap shut with a resounding "pop" if it is working correctly. You might check that before going on with the diagnosis.

The throttle is all drive by wire/electronics. Actuation is by way of a stepper motor which is controlled via a feed back pot on the end of the throttle plate spindle (used for referencing throttle plate angle) and the position of the potentiometer on the throttle pedal unit. Pressing on the throttle pedal signals the ECU to operate the stepper motor using the actual angle given by the pot on the pedal and use of the throttle plate feed back pot to determine if the command requires the stepper motor to open or close the throttle. It is a very precise and usually reliable system.

I think my suspect would be to look for air leaks around the induction system. Well known for failure are two small rubber air pipe elbows which are notorious for splitting and collapsing in service. They tend to give elevated throttle speeds so this may perhaps be where your problem lays. The inlet manifold will need to be removed for replacement. One set of plugs is easier to replace than the other.

Many V6 owners have learned that the two air elbows are best replaced as a matter of preventative maintenance at each and every spark plug change interval. (I recall each 60k service event). Near side plugs can be changed easily but the inlet manifold must be removed to gain access to the off side bank. It seems worth investigating when the last plug change took place and whether the elbows were also replaced. If in doubt, then you really do need to change those elbows as they may never have been replaced since the car left the factory.

I hope that helps.

Mike K

[duncanwoolgar]

Thanks Mike. Thank you for your prompt and clear reply. Very helpful indeed. I shall change those L pipes asap and let you know how it goes. Regards Duncan

[jeff1954]

I've heard of this problem before on Ford engines, and the fault could be the throttle position sensor, if I'm not mistaken the 2.5 and 3.0 ltr units are Ford units. However Jaguar may not use the same system's.

Jeff

[J44EAG]

Thye TPS unit is common to V6 X-type/S-type and many other V8 power units. I once took a V6 pot from a breaker and put it on my V8 4 litre. From memory the cost is around £30 new and is possibly Denso manufactured. Most of the elctronic gear on our cars of the period is of Denso manufacture.

Mike

[jeff1954]

OK Mike we shall await duncanwoolgar's reply.

[duncanwoolgar]

Hi everyone, sorry for the delayed reply and thank you for all your interest and help so far. It has taken me a while to acquire various parts and a free weekend. Cutting a long story short, I have just replaced the L shaped pipe under the fuel inlet gubbins. Definitely needed it, the old one was not visably leaking but it was very deformed and squashed. Also changed the straight bit at the top and L bend. Changed upper and lower gaskets on the fuel inlet gubbins. (Obviously all cleaned well)
(Normal service items are in good order plugs and filters etc changed Inc fuel filter)
I thought I'd cracked it, but after a few miles the slow throttle return, returned. I also acquired an apparently good s/h throttle body and even the pedal. I have fitted the replacement throttle body (new gasket) and pedal and all is exactly the same. No fault codes showing. A drive down to Devon returned 35mpg according to the trip computer which is usually about 1mpg out, so even with a pinch of salt that's good. Exhaust is always black though. Runs smoothly. I will update again as and when more happens. Regards Duncan

[J44EAG]

These engines do use a bit of oil and this can frequently be seen as blackness on the exhaust tips, Duncan. It is nothing unusual.

I wonder if you might have a brake servo vac pipe leak or an issue with the servo itself. Worth investigating?

Mike

[Tintin]

When stationary the throttle behaves as you would expect with sharp response up and down the rev range. If you are moving along the road the revs take far too long to drop, making changing up painful unless you wait a long time for the revs to drop

I had similar symptoms some years ago - the cause was a broken/disconnected induction hose; the effect of moving forwards forced air into the inlet manifold and increased revs. When sitting in traffic at idle the pressure wave from oncoming passing vehicles had a similar effect, causing revs to rise and fall automatically.

Robbie

[jeff1954]

It does sound like it's drawing air from somewhere. There is a product on the market that you spray around the induction system ie vacuum pipes ect, and if a leak is detected the engine note or revs alter. This probably sounds a bit vague as I have not used it myself, but it could be worth a try.

Jeff

[duncanwoolgar]

I think this issue is finally cured. The car has been driving like a dream for a couple of weeks now. Around Christmas time there was a particularly cold morning which finally finished the obviously deteriorating battery off. I fitted a new 5yr guarantee Bosch heavy duty battery. The car drove horrible for day or so until the ECU re learnt everything. Now it is driving better than ever which I am putting down to the healthy voltage, the throttle and associated sensors are all electronic. I am not sure of the exact reason but it is definitely linked. Fingers crossed it stays like it. One can pull up now without the engine racing like the choke had been left pulled out.

[J44EAG]

Well thats the first time I think we have heard of battery voltage affecting throttle RPM. Nothing is impossible!

Usually its air leaks or throttle pot or the throttle pedal itself. It could well be that removing the old battery and installing the new one could have cleared out a rake of deep seated fault codes. I soon learned that an accumulated fault history if not cleared off could generally affect the way the car ran. Disconnecting the battery performs what is known as an ecu memory "hard dump". Sometimes that course of action is really the only way to clear obstinate fault codes. Perhaps you hit the issue square on the head.

Glad you got the car running properly. Even with just one fault, a car can become miserable to operate.

Mike K

[davidr]

Does the idle valve use a stepper motor? (Sorry - not as familiar with these engines as some others!)
Could it be that the stepper motor was affected by low voltage?

[J44EAG]

As I recall, certainly on the 4.0 V8, there is an idle valve with solenoid control. Essentially it is a chamber with sliding ported piston. A dodgy connection on the plug and socket or a sticking piston could cause idle issues. Thats worth a ganda.

Mike

[duncanwoolgar]

Well it's not really right. I've never had a car before that won't tick over when you take your foot of the throttle. Im beginning to believe that it is the way it is designed, some sort of anti stall programme? I would like very much to speak to anybody with a manual transmission model jaguar with the V6 engine to see how it drives. I am convinced that there are no air leaks on the engine side of the throttle body. I've changed most things baring the inlet manifold casting itself.