Engine missfires when idling, but only when hot

Question

I have a 1988 Toyota Corolla EE80 that I bought recently, 1.3L carburetor engine with distributor ignition.

The engine idles normally at around 1500rpm idle when it's cold. But the moment the coolant approaches normal operating temperatures the engine starts to miss-fire and the idle speed stays around 1000~1100rpm.

I've checked and re-checked everything in the ignition system. took apart the distributor, checked all the cables and plugs.

The miss-fire gets more aggressive, to the point that the engine stalls it self if I have the vacuum advance on, even with the ignition fully retarded.

What can cause a miss-fire when hot? Valves? rich or lean condition?

Answer 1

If the problem is entirely related to coolant temperature, it is probably actually a head gasket problem. The head gasket may seal properly when cold, but when the engine heats up it no longer seals. Coolant entering the combustion chamber can cause a misfire.

You can test other components individually by allowing the engine to cool completely. Then heat the component with a heatgun for an extended period of time and start the car. If it misfires at that time, you know the component you just heated is the issue.

Answer 2

After suspecting for a worn out ignition coil and changing the distributor, the problem persisted. Ended up fiddling with the idle mixture screw and the problem disappeared.

The distributor change was not in vain because the older one was already very worn out and used to get the governor stuck with advanced timing after a few revs.

Note: My Corolla (1.3L 2E) has a characteristic Variable Venturi Carburetor that is famous for being unreliable with it's water temp based idle speed compensator and vacuum lines nest. Only installed on early EE90 models. They later came with a simpler 2 stage carb.