Hi,

To summarise first
Jaguar XJS 4.0l
Celebration model 1996
24,000 miles
UK car.

Ok, I've been suffering from a high idle condition. Car idles at 1600 rpm all the time.

Fitted a new idle control valve. But it made no difference.
Carried out manual base idle reset. Oddly it reset the idle to about 900 rpm but then slowly starts to creep back up again. The car is not moving.

The one part that really puzzles me and that is why I'm on here.

If I out the car in neutral, and let it roll down a slope (no power, no brakes) the revs go crazy up to 3,500 rpm. As soon as I stop they return back to 1,600 rpm again.

Is there a speed sensor or wheel speed sensor that could be causing this?

The brake warning light has been on for years. Could the ecu be revving the engine to get brake pressure? They feel fine. Although I am planning to get them looked at in a few weeks time. Previously it was abs sensor cables.

I really want to try and get an understanding in this. As putting the gearbox into drive at 1600 rpm is making a terrible jump through the transmission and I don't want to damage something.

One last thing that might help, about two months before this all started I did a cross european drive. About 3,000 miles in 7 days. I live in guernsey so the car normally does about 1,000 miles a year. And never gets above 25mph. Could the long journey knocked some ecu settings out of sync?

Many thanks.

 

I had a similar problem with my 96. Had to take it to a shop that had the PDU computer to reset the parameters.

$150 later, the idle hangs where it should.

Short of that, please check your throttle body. It is probably caked. It shouldn't take you long to get it out and clean it.

Don't spray carb cleaner with it in place, because you may destroy the TPS at the bottom of the throttle body.

 

Hi Vee,

Yeah I read some of your posts on a similar issue.

Gonna try and get the car booked in next week and see how it goes.

My only concern is the dealer in Guernsey not only does Jaguar but about five other makes and I don't think they have things like the PDU.

but I'll let you know how I get on.

Thanks

Andrew.

 

As Vee has stated, check the throttle body to make sure that the plate is not sticking. There are some sensors that also will affect the idle strategy.

Crankshaft Position sensor measures engine speed and can often cause problems because it is either dirty on going on the fritz. It is mounted above the crankshaft pulley.

Coolant Temperature sensor mounted on the thermostat housing.

Throttle Position Sensor mounted under the throttle body. (be careful here because removing this and fiddling with it will require a PDU to get everything synced up.)

Any halfway decent mechanic will have no problem checking these items.

Cheers,

Allan

 

Hi Allan,

Yep I cleaned out the throttle body a few weeks back. The car is fairly low mileage and there wasn't much build up as I was expecting.

I also cleaned out the elbow from the air filter as that had 10 years plus worth of gunk from the breather pipe.

New Air Filter fitted as well. Old one was due to be changed anyway.

I did wonder if the water temp sensor was on the blink. Maybe someone could answer for me. When the engine is at running temperature the gauge needle only reaches 1/3 across. Is that correct? Thought I remember it being always half way across but could be wrong. It does reach halfway when the car sit's in traffic for long lengths of time.

Crankshaft sensor is a possibility I suppose (but I'm not getting any error lights/codes). Would that cause the issue where I let it roll down the road?

I've had an issue in the past where it feels like the engine is not firing for a split second, it's very odd and difficult to describe. Almost like a spark plug doesn't fire on one cylinder; but just the once. No way could I demonstrate it because it's so random. Having said that though after the long drive across Europe it appears to have gone. So maybe I just cleared the cobwebs!

Driving at 20-30mph on a island probably annoys it! haha.

Many thanks for your help everyone.

 

Hi Andrew,

There are two temperature sensors and they are next to each other. One called the Coolant Temperature Transmitter supplies the temperature directly to the gauge on the dash only. The other is called the Coolant Temperature Sensor and sends the info to the ECM.

As your gauge is not getting to normal, I wonder if your thermostat is stuck open? Low coolant temp might also cause the engine to idle faster. Definitely worth checking the thermostat first.

Spent many a summer holiday on Jersey and Guernsey when I was a lad Nice places both but cruel and unusual punishment for an XJS


Cheers,

Allan