Hi, hope someone can help with this although similar issues have been covered on here before but this is slightly different!

I have an x300 with 80,000 miles which has the following issue, The car seems to often stall when it comes to a stop at traffic lights or junctions etc, sometimes the revs drop and recover slightly just before stalling but sometimes stalls but then restarts fine. If it doesn’t stall it still seems to idle to low.

I’ve also noticed that the miles per gallon seem to have got worse, mainly on short around town journeys can be around 15mpg and on long journeys where I’m heavy footed I’m getting about 21mpg, both lower than another x300 I used to own.



The car has been serviced Including oil, oil and fuel filter, new rocker gasket and plug well gaskets and spark plugs plus had the throttle body cleaned along with the stepper motor. Also had front and rear exhaust manifolds replaced as both were cracked. This was all done by a good local mechanic who is not a jag expert but just a good general mechanic, plus he said no codes were showing up.



I phoned a local independent to book it in for him to take a look and he said it sounds like an ECU issue, before he has a look can anyone suggest any ideas I could mention as it would be much appreciated, many thanks in advance for any help!

 

Has the EGR valve been gone through, cleaned and tested? Be sure to check the exhaust gas passages as well, that nothing is clogged up. This is notorious for getting clogged up at the intake, and in the passages from the exhaust. Then it needs to seat cleanly without leaking.

 

Check your Idle speed Control valve, which is attached to the throttle body. Easy to do on the normally aspirated cars. (An imperial PITA on the XJR!)
Its role, as its name suggests, is to control the idle speed, and in particular not to allow the throttle being closed quickly to cause the car to stall,,,,,
It is a fairly basic electro mechanical device, which can fail or become gunged up.

 

What do I have to take off the supercharged engine to get at the throttle?

 

The "official" method used by the dealers was to remove the throttle body from below the car. By removing the oil filter canister, space is created which allows access. to the various components, but given the amount of stretching, bending and general contortions required, in my view a lift or a pit is needed for this approach to work, and it needs a familiarity with the layout of the various components, as some of them are being approached "blind". I did it this way the third time I accessed the throttle......
The first time I attacked the throttle, I removed the inlet manifold from the car, which is undertaken predominantly from above the car. The procedure is relatively obvious, with the inlet fastenings being fiddly and needing backed off flat by flat in some cases, and the water pipe at the bulkhead being fiddly. Once everything is loose, the problem you have is that there is a wiring loom fed through between two of the inlet tracts which prevents the manifold being taken off the car without the connectors being unplugged and the loom fed back through the manifold. This might not sound like a big deal, but there are 7 or 8 connectors, and they mostly live in difficult to access spots and are best tackled from below.......
The second time I attacked the throttle, I removed all of the fastenings etc for the manifold, but left the loom in place. There was enough room, just, for me to lift the manifold to one side to let me at the throttle body.
With the benefit of hindsight, and having had an opportunity to see the layout of the throttle body and associated components with the manifold off, and having a lift, I would use the from below approach if I had to do it again.
Assuming you havent been there before, or dont have a lift or a pit, I would go the lifting the manifold to one side approach.
On a positive note, no special tools required, and when everything is right, she runs soooo smoothly, so it is worth the effort!!

 

Ok thanks CJ.
It is handy having a mechanic with a lift who lives across the road!
The car was fine until he bled the brakes; is there anything that he could have disturbed whilst doing the brakes or is it just coincidence?
I have ordered a new idle control valve, have a good used TPS to fit and was wondering if I can access the cruise system at the same time; I have already replaced the dump valve, brake switch and control unit but still no joy.

 

Sorry to join your thread so late.

Another issue that has been associated with stalling while slowing to stop or make a turn is low transmission fluid, so have your mechanic check that, and if it is low, top it up with a correct Dexron III equivalent such as Valvoline Dex/Merc (the conventional type in the blue bottle, not the synthetic type in the red bottle), Mobil 1 ATF or Redline Oil D4 ATF. I have found all of these to have the proper viscosity and to work well in the ZF 4HP24 gearbox. Even though Dexron VI is claimed to be backward compatible, its viscosity is significantly lower than Dexron III and it will not perform optimally in these 4-speed gearboxes.

The fluid should be checked hot after a good drive, parked on a level surface, after holding the brake pedal and shifting through all the gear positions, holding each position for at least 3 seconds to fill up all the passages in the valve body and gearbox. The fluid level should be exactly at the max hot line on the dipstick. Too high or too low can cause issues.

Cheers,

Don