I have a b16a in my 93 civic coupe. Lately, when the car is fully warmed up (about 30 min. driving) the vtec will not engauge. It will work fine until it is fully warmed up tho. The check engine light will also come on, but when i try to pull the code it doesnt flash. I checked my oil today and it was a little low so i added some but i still have the same problem. Also, for about the past two weeks the weather has been really hot (today, 94 degrees) and very humid (90%) so maybe that has something to do with it?

 

sounds like a oil pressure issue. When engine is cold and pressure remains high due to thicker oil vtec will engage, although you should never rev high when cold. As the oil heats up and thins out the pressure drops. I would install an oil pressure gauge this will let you check this out. The vtec pressure solenoid will not let the vtec engage if pressure is not high enough.