Rough Idle during Warming Up

The engine should not take 20~30 minutes to warm up. Check the coolant temperature sender to see if it is operating properly. Also check to see if the thermostat is stuck open and check EGR operation.

Thanks, Ray. The engine temperature gauge seems working properly. It only took a few minutes to reach the middle. I meant it took about 20-30 minutes for the problem to disappear, at which point it seems the engine compartment is warmed up and the engine is "hot" (in the sense of normal hot). I'll start checking those items.

OK, it seems like the engine is actually warming up. Check the black plastic intake snorkel between the air filter housing and the throttle body for cracks, holes, leaks, etc. If there are any holes, use duct tape to seal them up.

I don't remember what kind of air flow sensor the car has, but I think it is the MAF type. The MAF sensor will be located on the air filter housing and will have a tube that sticks out into the air flowing into the snorkel. Inside the tube is a thin wire that gets coated with gunk over time. Carefully clean the wire with rubbing alcohol or carburetor cleaner and a Q-tip. The MAF sensor is not supposed to be serviceable, so you have nothing to lose by cleaning it. If the air flow meter is the flap type, make sure the flap moves freely without hanging up.

One other possibility is the idle air control valve on the throttle body may be hanging up. Carefully clean it with carb cleaner.

It looks good and there is no cracks or holes.

It's the flap type, and it's working properly.

I did more testing and remember I had similar "surge" on highway doing 60-70 MPH. It happened on local road too recently. So it looks like it will happen even after completely warmed up. It's just less often than during warming up and just warmed up.

I also googled this problem. Most points to EGR Valve. Can the EGR be cleaned somehow or it has to be replaced?

Thanks again!

The EGR can sometimes be cleaned although replacement is usually more effective. To see if it is the EGR valve, temporarily disconnect the vacuum hoses from the EGR valve and plug the hoses with a golf tee or screw and see if the problem clears up. If the problem does clear up, either the EGR valve is hung up in the open position or the vacuum switching circuit is not working correctly. You will need a repair manual to check the various vacuum switching control valves. You can remove the EGR valve and see if you can see any buildup around the valve sealing surface.

I tested the EGR system and it's working fine.

It turns out it's actually the distributor cap and rotor. They were worn out. I recently replaced the distributor, but not the cap and rotor because they looked like new, even though there were replaced long time ago. I replaced the plugs, wires, cap and rotor, and it's working fine now.

I was mislead by the fact the engine would "surge" from time to time. It's strange that the engine would surge (instead stumble) on this car if the ignition system misfires.

Thanks for the help!

You're welcome, although I didn't contribute much to the solution!