Any ideas?

I’ve got a 99 Ford Escort wagon that’s occasionally running on 3 cylinders. It’s idling fine, under load when the cylinder cuts out, it is usually preceded by a loud pop. So far I have replaced the coil pack, plugs, gas filter, and plug wires which hasn’t helped. My mechanic checked the new wires yesterday, and pulled out the injector wires while it was running and all the injectors are working - maybe one is weak? I’m stumped and don’t know where to look next - the ends of the plugs look fine. Any ideas???

Is it engine temperature-related?

Possibly a warped valve that is hanging open when it gets hot enough. If you shut the car off while it is running on 3 cylinders, and a minute or two later you hear a “tink” as the valve cools and pops shut, that is the problem. However, just because you don’t hear a “tink” does not rule out a warped valve.

I don’t think so - we let it idle last night until the fan came on - I’ve also had the cylinder kick out on start-up in the morning when the engine was cold.