Overheating V8

[OllieG]

Hi, hope someone can help with this...

My 4.2 V8 overheated on the A3 the other day. I had done about 30 miles in free flowing traffic when I reached some congestion. In the slow moving traffic for 5-10 minutes and suddenly the temp gauge started rising from it's usual dead-centre position and within a minute or two an Overheat message was displayed.
So, I turned off the A3 onto a side road looking for somewhere to stop. Out of traffic and at about 30mph the temperature started dropping almost immediately and was back to normal in a couple of minutes.

I decided to carry on my journey and had another 20-odd miles of steady untroubled motoring. As I arrived in South London though I again got into slow moving traffic and as I reached my destination the car was starting to overheat again (Gauge in the red, steam from the bonnet as I parked up).

I've had my local guy look at it but after it had cooled down he hasn't been able to make it overheat again. He's replaced the Thermostat and the Water Pump which I wanted done as a precaution and both of which he says didn't have much life left in them anyway.
He also tells me that the Head Gasket is fine.

He reckons it's likely to be an intermittent problem with the Radiator Fan, and suggests replacing the Fan Control Module.
Annoyingly this looks like it only comes as part of the full fan assembly which is about £500, so I'm looking around for quotes from breakers.

Any advise on where to go from here would be greatly appreciated.

Thanks in advance,

Ollie

[J44EAG]

I`ve had several V8 apparent "spoof overheats" over the years with the dash needle into the red BUT a normal engine temperature to the touch. The issues occurred on both my old V8 4 litre S-type and my 4.2 XJR. Curiously, the covered mileages on both cars were almost identical at around 80k miles.

I got wise after the first instance on the S-type. The cylinder head temperature sensor failed, gave the red reading and upset cold start enrichment due to the failed sensor telling the Power Train Module that even when the engine was cold, the engine was in over-heat. The cure was replacing the sensor with a genuine Jaguar OE unit which in the case of both cars, cured the issue in one stroke. It is an easy job to do with just a 19mm spanner being needed to remove and replace the screw in sensor.

If you go to the S-type cooling section, look for my post entitled "V8 spoof over-heat condition" or similar title. That will explain more and you will see images of the repair job in progress.

Hope that helps.

Mike K

[OllieG]

Hi Mike,

Thanks very much for the reply.
Could I check though whether I've understood, is this cure is for a phantom overheat?

I ask because my S-Type is definitely actually overheating; it was throwing out steam from the expansion bottle when I parked up the second time.

What do you think?

Regards,

Ollie

[J44EAG]

My post is just for the spoof overheat as regards the sensor.

All Jag V8 models are VERY sensitive to thermostats and air locks. Only genuine thermostats should be fitted. Even then, just like the sensor, they are usually only reliable for 80k miles. I`ve never have either part last longer than that either on the S-type or X350R.

You will need to work reasonably hard to ensure you have ALL the air out of the coolant system. Sometimes air locks can be rather difficult to shift. Even a small lock can really screw up coolant flow.

I`d suggest you have your garage "sniff test" the coolant system and firstly ensure you don`t have a head gasket issue. The sniff test will detect C0 gas if present if a gasket has let go.

Mike K