The temperature gauge needle heeled hard over into the red. ‘Not again,’ I said to Oscar, sitting beside me on his booster seat. Sunday evening, and I was returning him to his mother after having him to stay for the weekend. The distance door to door is around 20 miles: about 40 minutes along country roads. After ten miles, a steep uphill climb proved disastrous. Steam was pouring in through the heater vents and fogging up the inside of the windscreen. I tried the demist blower on maximum heat but only cold air came out. So I drove the car with one hand and wiped the steam from the glass with a sodden little tissue, searching for somewhere to pull over.

Oscar was as calm as an adobe wall in the moonlight, as Raymond Chandler once put it. He never knows what sort of car his grandfather will turn up in next, and this 1.3 Fiesta automatic is the smallest and slowest one he’s ridden in yet. But he is accustomed to automotive fiascos because whatever the make and model of his grandfather’s latest, it is invariably old and clapped-out.

I swung over into the entrance to a muddy lane and cut the engine. Then I got out and propped open the bonnet. The coolant reservoir was empty: all the water had turned to steam. I refilled it with our emergency supply and went a little way up the lane to take a leak. Oscar decided he’d get out and have one too.

Lying in the grass was a magnificent dog fox. He was stretched out on his side as though sleeping. He must have been hit by a car. The only sign of injury was congealed blood around the nostrils. I prodded him with my toe. The corpse was ramrod stiff. Oscar and I contemplated him as we urinated on either side of him into the hedge. We paid him the tribute of aiming away from his corpse. Then I went to the car to fetch my penknife from the seat pocket and cut off his beautiful brush.

Twice more in the last ten miles I had to stop and refill the coolant reservoir. But after I’d dropped Oscar at his mum’s, the temperature gauge needle remained in the normal zone for the entire return journey and I made it back in one go. The overheating problem was a mystery.

I couldn’t afford to have it looked at by a mechanic, so I went online and searched the Fiesta 1.3 Zetec chat rooms for discussions on engine overheating. They are a contumacious online community, Fiesta 1.3 owners, and much given to sarcasm. After reading a score of posts, I narrowed my problem down to three possibilities: a stuck thermostat, a blown head gasket, or a problem with the heater coil unit. I looked at YouTube videos made by home mechanics in Arkansas, Texas and Russia showing where the thermostat is located and how to replace it. The Russian guy had taken his engine out of the car and mounted it on a pivoting swivel to demonstrate the operation as clearly as possible, but I would have to remove one of the headlights and the alternator drive belt to access mine. The Russian mechanic showed me what to do in the most methodical manner imaginable. He assumed no prior knowledge in his audience. But when I tried to put his lesson into practice, I couldn’t even get the headlight off.

There seemed to be no rhyme or reason to the overheating. Sometimes I could go for miles with no problem. At other times the car would overheat after five miles. Sometimes a steep hill would sent the temperature soaring into the red, at other times a steep hill would send the temperature back down to normal. Sometimes the temperature gauge needle roved back and forth as though it had taken leave of its senses. When I went to pick up Oscar the following weekend, it took 40 minutes to get there and an hour and a half to get back.

After that I surrendered, and drove the Fiesta to my local car mechanics — out of interest as much as anything else. The workshop is four miles away. The car was feverish that day and I arrived in the mechanics yard with steam pouring out of the bonnet and coming in through the heater vents. Johnny the chief mechanic happened to be standing in the yard and witnessed my entrance. And my entire point is, I think, that whereas I had got nowhere in spite of spending hours online trying to diagnose the problem, Johnny knew instantly, and without even looking under the bonnet, that it was a shot plastic connector to the heating coil.

‘It’s always a problem with these Fiestas,’ he said. It took about five minutes to replace and cost hardly anything.