A compression test is a simple way of showing how well your car engine’s pistons are sealing

centre with both valves closed. A coupling is then fitted into the head and compressed air is fed in.

The reading is then taken and the amount of leakage noted. On an engine in excellent condition, a reading of only five to ten percent will be shown. Up to 20 percent is no cause for concern, but start worrying if it tops 30 percent.

Put your hearing aid in. A leaky exhaust valve will bring a hiss from the exhaust while a leaking inlet valve would emit air from the carburettor or throttle body.

If the rings and/or bores are badly worn, air will be heard coming from the oil-filler cap or PCV valve.

Mind you, if low compression on all cylinders is shown, but air leakage is very slight, valve timing is a natural suspect. We had a car in recently with just this fault.

The owner had worked on the engine and refitted the timing belt incorrectly. A belt or even a chain that slips a tooth may cause such a problem.

Of course, a misfiring cylinder or one that proves to be weak in a bal- ance test could be due to many more things than leakage.

If compression is up to spec and leakage minimal, then a look at the ignition or fuel delivery is called for. Easiest to check are plugs and HT leads, followed by a look at the injector to determine if fuel delivery is as it should be.

On another subject, I got a call from a friend whose car engine had slowed and died just as if it had run out of petrol on a main road fairly near to my home.

But before offering to go and check the car, I asked him if there was fuel in the tank.

Sounds silly, but it would not be the first time a fuel gauge proved deceptive. I told him to remove the petrol cap and try to restart the engine.

The engine could then be started. He refitted the cap, drove to my place where the blockage was cleared and his troubles were over.

When you suspect a blockage, there could be a noticeable intake of air as the cap is removed.