Engine Cuts Out When Warmed Up
 

Hello,


A strange one this. 1991 XJS Facelift 4.0L, AJ6 engine.


It starts ok from cold but if leave it idling just on tickover for 20 minutes until it is fully warmed up, the engine starts spluttering and cuts out. At first when it starts spluttering, if I rev it up to about 2000 rpm, the spluttering clears for about 30 seconds but then starts again and eventually gets worse before cutting out.


I think it started a few weeks out of the blue. I was cruising along at about 70 on the motorway (I'm in the UK), and the engine just spluttered and cut out then.


It was diagnosed with a faulty petrol pump which I then had replaced. Since then I have noticed it happens after the engine has been running for about 20 minutes and is fully warmed up.


I have replaced the distributor cap, the rotor arm and just yesterday, the crank position sensor. I have also cleaned and re-seated the maximum airflow sensor (but not actually replaced).


The engine doesn't overheat, the fan on the radiator is whirring, the oil pressure gauge shows about 38psi and there are no fault codes indicated on the on board computer.


Not sure where to go from here, and suggestions please? Thanks.

Get to the point where it starts to splutter and then unplug airflow sensor. I had the same thing on my x300 and it was the airflow sensor at fault.

Thanks, I'll give that a try hopefully later today after work.

Ok so here's the latest installment.

When I started the engine today it spluttered and struggled immediatly before cutting out on the first attempt. It started the second time and with a bit of revving on the gas it ran smoothly.

However, I noticed the cooling fan not spinning because the belt had been thrown off the alternator and the fan pulley itself. I think this must have been the juddering from the engine firing so roughly that first time.

I did let the engine warm up as much as I dare without the fan operating - the temperature gauge got to the normal position of about half way across - but the engine was not nearly hot enough to recreate the fault, so I had to switch off and leave it.

I will need to try it again after I have had the chance to put the fan belt back on and can try it safetly.