Recently I went on a road trip with my 94 xj6, the temp was about 70 degrees F outside. There was a lot of traffic and the needle went to the first mark after N. I listened for cooling fans but nothing, so I turned on heater to cool it down but I was not blowing hot air at all, this just made it overheat more. What could be causing this?
First thing I would do is check the cooling fans are working and kicking in at the correct temp.
I don’t know why you couldn’t get any hot air from the heater unless the heater valve is stuck in the closed position ? Then I would check/replace the engine thermostat, it may be stuck in a half open/closed position.
Nicola, next time when your car’s engine is hot, turn on the heater for a few minutes to let the coolant circulate in the heating core. Then shut off the engine and remove the left side knee bolster and steel plate (if your car is left hand drive). You can see the inlet and outlet pipes of the heater core. Check their temperature with a digital thermometer if you have or can borrow one or try to touch them with your fingers but be careful in case they’re hot. If they’re cold then most likely the heater valve (attached to the left rear corner of the cyl. head) is stuck in a closed position as Casso mentioned.
If you determine your cooling fans are not working you may want to check out technical Service Bulletin (TSB) 86-140, available at jagrepair.com reproduced piecemeal herein:
Were you low on coolant by any chance?