My 1998 4.6 runs absolutely perfect 99 percent of the time. The other one percent of the time when I am sitting at a red light, my idle will drop 400 rpm's real quick and then return to normal. It does not happen every time. It thru a code last week. I went to Auto Zone and the guy reset it and told me and I quote, it was a "bull **** code". I changed the gas cap, nothing else. Anybody else had this problem.

'Thanks Eric G

 

that happened to my 2008 4.6L AT when i fill a bad gas.. so i run a gas cleaner and that's it

 

is it misfiring? any lean codes? maybe iac issue? does it go to normal then randomly go down to 400 rpm then back to normal? is the a/c engaging when it happens?

 

No misfire, no lean codes, and yes it goes back to normal right away. A/C is off. It thru a code and the guy told that the code could mean bad gas, gas cap etc.

 

I hear a sound coming from the rear by the fuel cap. It sounds like air is being released for about 3 seconds. I just purchased a new cap from dealer.

 

I have a 1998 4.2 V6 and I had the same problem exept mine was idling bad all the time. We replaced the spark plugs/wires, and nothing changed. I took it to autozone and ran the codes and it came up with idle air control valve and temperature sensor malfunctions. We replaced both of those and still nothing changed. Then I took it to a shop. They couldn't figure it out either until they had some codes that they had never seen before come up on the computer. They had to call the ford dealership to figure out what they were. Turns out, it was two little "flappers" in the intake manifold that were getting stuck. The mechanic told us to pour two cans of "Sea-Foam" engine cleaner directly into the intake manifold via the vacuum lines while it was running and warm. He told us it would rattle and smoke but not to worry. We did just that. We poured it in and the engine started to rattle and we could here the carbon deposits breaking loose because of all the pinging and knocking. Alot of smoke came out of the exaust and after it cleared up and the engine smoothed out, it ran fine. We drove the truck all day and it idled bad only a couple times but in all, it really helped it. We're gonna put one more can in it and try to completely clear it up. We got the "Sea-Foam" at Advance Auto for about $9 a can but its better than paying several hundred dollars to pull the manifold out and replace the parts.

 

The guy is a jerk for not telling you the exact code.

 

My '01 4.6 started this same problem about 3 weeks ago also. Idles at a light at about 600 rpm then drops to 450 or 500 for 10 seconds or so, then back to normal. It will also do it in neutral. Seems as if something like the ac pump is engaging, but I've watched the engine when it starts to act up, and cant tell whats causing it to do it. But, I think the IAC valves been acting up for the same time period. The engine fires right up, then almost stalls before catching itself and idling normal. Two weeks ago it stalled 3 times in a row on start up. Got a feeling the two symptoms are related, but havent had the time to check the IAC. If I get time this weekend to check it out, I'll post back.

 

My truck has not stalled yet but I have the exact same symptoms as yours. Let me know after you have checked, thanks


Eric G

 

You've got to be kidding me. Go back there and tell the owner or someone higher up what fuzz nuts did. I'de be

One, -their not suppose to erase anything. That's if he did ,-and what you ment by "reset" ?

Two, - that's not up to him to decide! Who's truck is it ??? That code could of helped pinpoint the problem.

Three, - there isn't a "bullcrap" code - they all have definition and that's not one of them.

Next time you get it scanned , write down the numbers yourself. Make sure you do that, - and slap silly boy upside the head.
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You probabaly had an EVAP code of some kind.

P0442 Evaporative Emission System Leak Detected (Small Leak)
P0443 Evaporative Emission System Purge Control Valve Circuit
P0446 Evaporative Emission System Vent Control Circuit
P0451 Evaporative Emission System Pressure Sensor/Switch Range/Performance
P0452 Evaporative Emission System Pressure Sensor/Switch Low
P0453 Evaporative Emission System Pressure Sensor/Switch High
P0454 Evaporative Emission System Pressure Sensor/Switch Intermittent
P0455 Evaporative Emission System Leak Detected (Gross Leak/No Flow)
P0456 Evaporative Emission System Leak Detected (Very Small Leak)
P0457 Evaporative Emission System Leak Detected (Fuel Cap Loose/Off)

But who know's which one.

The EVAP system does not effect the operation of the vehicle. It is possible that a disconnected vacuum pipe beneath the vehicle in the Fuel Tank area could cause an unmetered leak in the Inlet system and cause poor idling.

The only way in which a driver may decide that the EVAP system is not working is when there is no hiss of vacuum as the fuel cap is released after the engine has run.
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Usually a defective IAC is more revealing @ cold start. Some kind of hint should be given - Is any sort of deflection @ cold start going on? Well, it wouldn't hurt to pull the throttle body elbow and give it a good cleaning (that includes the IAC).

 

iac....

 

Hey man thanks for the information. Now I wish I would have asked for the code.

 

Eric, you ever get the problem solved. Cleaned my IAC and elbow, really didn't need it, just did it abot a year and a half ago, but wanted to take a better look at the IAC. Ran fine for three days until today when it did it again. Had it scanned, and no codes stored. Dont know if you can test a IAC to see if it functioning properly. Kind of hate to just start throwing parts at the problem. Have you replaced anything, and did it work?

 

Yea I changed the IAC and the truck runs perfect again. Thanks for the reply and information.