I have a 1999 Civic 1.6L w/ 186,000 mi. It runs great and idles perfectly, EXCEPT when it is started (cold or hot) the idle sputters, drops, and almost dies for about five minutes (sitting OR driving), then it runs fine. There is also a fuel smell while this is happening. The IDLE AIR CONTROL VALVE is fine, the plugs, wires cap & rotor are new. Fuel pump is fine, vacuum hoses are fine. When I remove the vacuum hose from the fuel pressure regulator, the idle rises as expected. I am baffled by this car and am almost un-willing to take it to a dealer, as I only paid $400 for it and do not want to spend the same to make it run better.
“…X1 is fine…X2 is fine…X3…” Sometimes people say stuff like that and later you find out that they said, for example, that the fuel pump was fine b/c they poked the valve & fuel squirted out or that the vacuum system is fine b/c they didn’t “see” any leaks or whatever. I’d esp wonder how you know that the IAC is fine.
That aside, check the coolant temperature sensor. In addition, if you haven’t actually put a fuel pressure gauge on it then I would, and I would watch it when first started up hot & cold. Of course, you have to know the actual specs for the car.
Just for a hoot, disconnect the EGR valve and see what happens.
Is the check engine light on?
If you want to have someone look at it you don’t need to take it to a dealer. Just find a local, independent mechanic with a decent reputation.
Absoulutely check the computer engine coolant temp sensor. The engine seems to want to run rich warm or cold at start. Two things control this. Air temp sensor and coolant temp sensor.