Hey guys, I've got a nasty one for you. I've got a 5.4L with 94k on the odometer. When I come to idle, she starts vibrating really violently, then acts like she wants to die on me. I replaced the TPS (throttle positioning sensor) and cleaned a ton of crap out of the Throttle Body itself. No good. I ran 2 cans of fuel injector cleaner thru her, and still no luck. My check engine light has been on since I put on a Magnaflow muffler, and i'm getting a little moisture dripping from the tail pipe. I dont know if they're connected. I had it checked at an Auto Zone, and they said that it had to do with my O2 sensors. Has anyone ever encountered this problem? I'm really racking my brain trying to figure it out, but I'm comin up dry. Any help would be great!

 

My F150 has/had been rough at idle while at the stop light. 4.6l 102,000 miles.

At my friends shop he diagnosed the right bank front O2 sensor was causing a lean condition.

I replaced both front O2 sensorts yesterday, and today there was no vibration at idle while at stop lights.(Fingers crossed)

 

o2 sensors would be a good start. You'll need 2 unless they pinpointed which one was bad. They will cost you 50-90 bucks from Autozone though.

If you are at almost 100,000 miles, they need replaced regardless if they fix your problem. you'll probably notice a big improvement in fuel economy!

 

I had it but it was a bad coil pack.

 

Had a similar experience with my 98 Exp 5.4. Rough at idle only, OK with engine over say 1000 RPM. At its worst, would die when stopped at a light. Caused a check engine light, too. Turned out to be a simple fix. Cracked elbow on a vacuum hose. The connector is at the top rear of the engine. Easy to check.

 

Four good replies.
This is the type thread you want to bookmark.

 

3 days, and at least 150 miles of city driving, 300 total miles since O2 sensors. No rough idle.


By the way. Last week on Friday I had my diff done. I stayed with stock gears(3.55) With both repairs, I think my mileage has improved also.

 

My '98 Expedition (5.4) and my brother's '99 F150 (5.4) had idle problems. Mine went so far as the car starting, and then cutting out. Would have to power-brake it to keep the engine running.

Found threads hinting at the Idle Air Sensor on this site (I think specific to the F150, but since it has the same engine...). Took it off and cleaned it (it has a spring in it, and it was really gunked up). Used (I'm going off of memory here) throttle body cleaner, as it was recommened so that damage to the sensor wouldn't occur.

At any rate, after cleaning the sensor, I (and my brother) haven't had any problems with idling.

Something that might be worth noting is that we both have K&N air filters, which rely on oil to keep them lubricated & able to do their filtering job. We both wondered if this oil was the culprit (maybe we used too much? Dunno.).

Anyway, hope this helps somebody. Goodness knows these kinds of things are hard to figure out.

 

Maybe it's the IAC valve?

 

Actually, that's exactly what it was. I was thinking "sensor", and that's what I typed, but it was the IAC Valve, as per page 4-10 of Chilton's repair manual.

Steve

PS - the manual says not to clean it. Other posts said that it was possible to clean, as long as you used (again, going off of memory) throttle body cleaner. Anyway, I did clean mine, and so did my brother, and everything has been fine, for what it's worth.

 

I had a bad rough Idle with my 99 5.4L @ 85k miles. It would die out so to keep it running I had to adjust the throttle kick back set screw(i beleive that is what it is called) to keep it running at an idle. Only had to turn it two turns and it would stay running. Changed plugs, still had problems, had injectors cleaned, still rough, cleaned and lubricated IAC and changed coil pack, still ran rough ran better but still rough. Was really going crazy!!!!!!!!!! I finally gave up and took it to Ford to let them run a diagnostic on it. It turned out the coil pack was the fix but when I put the new one in I apparently just slightly bent one of the pins in the coil when putting on the connecter and it was not making good contact.

Just a thought. Wanted to let every one else know about the pin so if it happens to you after you change your packs.

 

I've got a 99 with the 5.4L (49k) and it has just started having idling problems intermittently. It usually clears up after the truck has run for a few minutes, but once it lasted most of the day. Its like something is getting stuck? Do they still have an automatic choke with EFI? Any advice is appreciated!

 

IAC valves seem to act up after 50k, but can be troublesome at anytime. When my IAC was going, the vehicle would have trouble starting or idling unless I was applying throttle. The problem would usually go away once the engine is warm.

I cleaned my IAC with throttle body cleaner, which temporarily solved the problem, but replaced it a few months later. +/- $70 and an easy DIY job.

 

Just got my truck back from the shop for a rough idle ('99 35K) My RPM's drop below 500 and the occasional stall at a light. The dignosis was dirty IAC and dirty Injectors. Both clean and truck is now idlling back a solid 680 (where it used to be).

 

Just wanted to say thanks guys! We had a cold snap that locked my IAC up for good. I swapped it out, 30 minute job, and I'm back in business! Doing the repair myself $aved me some money so thanks for your advice!