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1999 Ford Expedition 4.6l

Vehicle sometimes idles low when stopped and occassionally shuts off, but starts backup. Service engine light is not on. Mass air flow sensor recently replaced.

Report on the state of maintenance - spark plugs, wires, filters. Check the fuel pressure. Clean the idle air control valve (IAC) and throttle body.

Classic symptom of a sticking IAC (or Idle Air Bypass Valve as Ford called them). It’s easy enough to replace. The replacement parts have some design enhancements to make them less likely to stick.

Look at the number 4(pass. side closest to firewall)plug and wire. I was having the same problem and it was caused by corossin inside the wire boot(i hope, just fixed this affternoon). Clean inside of wire boot with a .22 cal. gun barrel brush and see what happens,it seems to have worked for me. If it works for awile I will put on a new wireset,if it dosent back to square one. Hope this helps and good luck.

1999 4.6L Ford engines do not have any spark plug wires. They use COPS…Any misfire will cause the CEL to light up…Tardis is more likely to be correct…

I had IAC cleaned and air filter replaced. The problem still exist. The truck idles fine then all of a sudden it stumbles for about 5 seconds and starts to idle fine again. Minutes will pass before it stumbles again. Could it still be the IAC.

Is the check engine light still off? Sooner or later a code should get set and give you a direction.

In the meantime I would actually inspect & do basic maintenance on the EGR system and the evaporative emissions system. Here is why: as you describe it this is an intermittent stumble that will occur briefly and then correct itself. It almost sounds like a brief vacuum leak that opens & closes back up. The EGR & evap systems both have valves that periodically open & close to feed either exhaust gas or gas fumes into the intake. If there is a problem in one of those systems that the computer doesn’t yet “know” about these valve “purging” episodes may be producing a temporarily lean or rich condition.

Check engine lite is off. Can the egr valve be cleaned as well? My cousin cleaned the IAC and throttle body with throttle body cleaner. Thanks so much for your help.

Ok, I have clean the IAC, Egr Valve, and fuel pressure is 30psi and the check eng light is not on. I timed it and if sitting for some minutes it will stumble around 500rpms for about 5 seconds then idles fine again. What could be the problem.

Did you clean all of the EGR passages too? The lines & on the intake? The lines likely also run to a sensor.

Just as a diagnostic test, find where the evap line feeds the intake. Pull the evap line, put a vacuum cap on that inlet and see what happens. (This is likely to set a code for the evap system).

The EGR is back near the firewall in the middle, a big flying saucer sticking up. Yes, it can be removed and cleaned. Also, try to clean out the tubes that feed it exhaust gasses. They can plug up and throw a “EGR Low Flow” code…Be careful with the electrical connector. They get toasted and brittle…