We have a 1992 Mitsubishi Eclipse with a perplexing issue.
We’ve had it to a reputable mechanic at least three times for an intermittent overheating problem. It has not be successfully diagnosed: the only answer we’ve been able to receive is that it “may” be the head gasket but no one is sure.
It will also not pass an ECheck. The NO is 3x allowable limit and the HC is 20 points over limit. We’ve done many things to get this car to pass: three years ago we got a waiver because we put $300+ in it of repairs.
Radiator has been flushed 3x: does not leak nor do any of the hoses. You cannot see water coming out of the engine anywhere and there is no steam unless it overheats. Water pump has not been replaced: has not given us any trouble though. It’s more frequent than not - you can drive it steady for a good distance but when you have to stop and start like city traffic, it overheats. Does better in winter than in summer (obviously). Engine runs smooth and has lots of power. There is no water in the oil. We have replaced the thermostat as well as the radiator cap.
Any insight that you could provide would be greatly appreciated. This has been puzzling us for four years!
“you can drive it steady for a good distance but when you have to stop and start like city traffic, it overheats”
Check the fan and relays. Also look at the thermostat if you haven’t already.
Edit: electric fan/s correct?
Elec fans (they are working) and already changed thermostat as stated above. 
I would have someone pressure check the cooling system.
It is an original radiator? It could be 17 years old. It may be partly clogged. Flushing may not be getting it. Fixing the cooling issue may help with the NO issue.
Let’s say the radiator fan relay, sometimes, doesn’t engage. There is no sure-fire way to test that supposition; so, swap the radiator fan relay for another in the relay box.
What are the symptoms of “over-heating”? We need those symptoms.
When the over-heating occurs, do you hear the radiator fan turn on? (Turn Car Talk volume down!).
We await your remarks. Errr, constructive remarks, preferred.
If this car has been overheating–even briefly each time that it occurs–for 4 years, I would assume that the head is badly warped by now, and that there are likely other kinds of internal damage to this engine.
Although you tell us that your mechanic is “reputable”, there are tests that can be done by any competent mechanic to determine if the head gasket is bad. If he is not aware of how to diagnose a bad head gasket, then I question that he is “reputable”.
All-in-all, I think that you need a new car, or a new mechanic, or both.