Should be easy to diagnose, but there are many variables. So read carefully.
Problem happens only on hot days. On cool nights, runs great.
Symptoms: coolant level drops, car overheats and #3 cyl will misfire for 20 secs or so after warm startup. Only happens on hot days.
On a hot day, I start up car. Runs great. Drive for 1/2 hour and stop. Restart immediately, no problem with misfiring, but coolant level has dropped slightly and temp gauge will be normal. But if I wait 10 mins before restarting, #3 cyl will misfire for about 20 secs and then fire normally.
Drive again for another 1/2 hour or so and temp gauge starts to go up above the 1/2 way mark. Stop car. The coolant reservoir is now at max and the radiator level is down. I just pull the reservoir and dump it into the radiator. I feel this is ok since the coolant is new. Just a few days old.
Start up car again with temp gauge at normal operating temp. Cyl 3 misfires again for 20 secs, then runs fine.
Drive another 1/2 hour, same thing happens. Only this time, just recycling the coolant from the reservoir doesn't fill the radiator completely, so I have to add a cup or two of fresh coolant.
Drive again. By this time, the sun has gone down and we get nice cool air here in Los Angeles.
No more problems. The coolant doesn't drop, the misfires never happen.
Next day, the whole cycle begins again.
Things that have been replaced in order:
Car started overheating over a year ago.
First thing to go were hoses getting blown. Hoses replaced and new thermostat and new Honda radiator cap.
Next to go was the gasket at the water pump. So new water pump and timing belt.
Then the radiator blew out new radiator.
Then the heater core blew out. So now that's bypassed. No heat.
Then the gasket at the IAC valve went out, spilling coolant on the ground.
Replaced the IAC valve.
Then started getting codes for insufficient emission flow. So I cleaned out the EGR passage. Code went away. Brief misfire at #3cyl started happening after EGR passage cleaning. I also ran injector cleaner through all 4 injectors at this time.
New radiator fan. This thing was probably the cause of many overheating issues as it seems to have been running only sometimes. Like on hot days, with temp gauge high, it would sometimes be off. Flick it with your finger and it would spin on, but only about half speed.
The car really seems to love its new radiator fan, but on hot days, 75 and above, bad things happen.
There are no noticeable coolant leaks.
Have I blown a head gasket and coolant is getting into the #3cyl and burning off? Is injector 3 getting fouled briefly then clearing out? This possibility wouldn't explain the coolant level problem though.
Car is 2000 Accord LX 2.3 L, 4 cyl with 270,000 miles.

 

Symptoms (missing on warm startup) are similar to what I had on my 94EX when head gasket was bad.

I pulled plugs and turned engine over to get a coolant geyser on #2 cylinder.

A leak test of cooling system is another way to confirm. If no external leaks and pressure keeps dropping steadily, a head gasket leak is highly likely.

good luck

 

Thank you. I was afraid of that.