My 99 exp just started to not want to start or run without pushing on the gas peddle. If I keep my Rpms up by keeping my foot on gas it stays running. It will not start without pushing on gas peddle.I also notice that when I let off gasthe lights dimmed and heater motor went from running high to very low. I took it to shop and they checked alternator and battery andboth showed fine. Was told until it did it again they couldn't help me and had no suggestions. I put a bottle of injector cleaner in it after I filled up with gas but it is still doing it. However once the temp gotin the 30's it ran fine. But again last night when temp was low(17) it wouldn't start or run again.It is now 47 and started right up.Also noticed thatas it wasn't running very well the the gas needle wasbobbing a little. Have 133000 miles. I only drive it once or twice a week and to get totown is 15 miles so don't make any short trips.

 

It is probably the Idle Air Control Valve. It isunder the plastic cover on top of the engine. It is bolted to theback of the intake manifold. It controlsalot of your idle qualities. They get dirty and want to stickas well as getting very cold oout, they will stick. Should only costabout $40 bucks or so at your local parts store. The other solution is buy a can of electrical parts cleaner and remove the IAC and spray it with the cleaner and let it dry. Put it back on and see if it works ok then.