I have seen many posts on this subject but cannot find one difinitive fix to try.
99 4.6 (106K miles) will stumble / miss when at 35-50 mph under a mid throttle load (just before reaching the downshift point). Has been doing this since I bought the truck at 70K miles.
Computer has no codes, engine runs fine under all other circumstances, and gets a decent ~20 MPG on the highway.
Done the standard tune up items (plugs / motocraft) and cleaned MAF, replaced TPS, tranny flush/ filter. Replaced the left O2 sensor 2 months ago, but it dumped a code and was really noticeable when it went out.

I am about ready to dive off and replace the MAF, but at $100 a shot would like other alternatives to try first. I am really convinced the engine is leaning out and missfiring under this load.
Any hints to look at first?
Thanks

 

Have you changed the fuel filter? Just shots in the dark...

 

idk i would try cleaning the MAF agian or replace the air filter

 

I would do that. It is cheap and easy to do and is regular maintenance anyways.

-Matt

 

OK
Bought a new MAF, swapped it out and no change. Guess I just threw away some bucks, but at least I know what is NOT the problem.

Did some really intensive drive testing and refined the trouble somewhat. Seems the magic spot is between 40-45 mph after a deceleration.

Thought it might be EGR related as it seemed to be there after a deceleration so I disconnected the vacuum line to EGR valve. Again no change.

Next I switched off the Overdrive. This seems to have done the trick. Looks like the transmission is hunting between 3rd and 4th gears and not smart enough to go ahead and lock into 3rd.

So, at this point, are we looking at a VSS issue or does anyone know if there was a TSB with a EEC re-flash on this issue?
Any help appreciated.

Dialtone

 

just remember to use motocraft wires and plugs. I had the same issue and replacing with ford brand parts fixed my problem.

 

Is it that or torque converter chatter?

 

You discribed the classic lean miss condition.

This kind of condition most often does not set a code. It can't because the soft fault comes and goes to quickly for a threshold to be met, for code and CEL.
Coils!

 

dialtone, my truck seems to do just about the same thing as yours, i've got a 99 5.4L. mine stumbles under a load only with the converter locked with engine under a load, i've been chasing it for a long time(about a year). have replaced 2 coils on the pass. side rear of the engine it drives me crazy too. it will also stumble off idle under a load, then all 8 kick in and it feels like its flying. does anyone know of a way to test the coils on this truck.

 

That is what I thought too (classic lean miss). I think I have eliminated this by deselecting OD and it is gone.
Torque converter shutter is not likely, as it locks up at a lower speed than my problem. As for coils, this engine/year combo does not use the individual coil packs, so I am ruling them out for now.
I searched the TSB's in the forum and none refer to gear hunting, but I am almost positive that is what I am seeing. When I get some more time, I am going to build a break out box for the 3-4 shift wiring and see if I can "see" the shift solenoids being switched in rapid succession.
I am tempted to just buy a re-programmer to let me modify the shift points. My bet is this would also eliminate the problem.

 

I have a 01 4.6 Supercrew with the same problem. I took it to the dealer two years ago and they couldn't reproduce the problem. Yeah, they thought I was crazy. Now I pull a 401 code daily. I have replaced the EGR, Plugs, and three coils with no fix at all. I have had the fuel system cleaned twice and always replace my filters. It is very annoying and you are right when you shut off the overdrive it goes away. Somebody must have found the fix for this. Anybody?
I have 104,000 miles and this is my ONLY problem. What about DPFE, P401 is an emissons error?

 

bdski, you have bad coils.
I did go thru this, found all the reasons, the work that affects this, and the final fix..
Have all the coils stress tested and get the bad ones out.
All you have with ignition, is plugs and coils.

If it makes any difference, here is the picture.
When you decell in OD, the fuel goes lean by design intent, the ignition goes advanced.
During this lean condition which is considerable and with EGR invoked, makes the mix hard to fire by the plugs. The reason this is so, is because the cylinder mix has a much higher dielectric constant (less molecules in the gap for breakdown).
If one or more coils have shorted turns , it reduces the voltage available to fire a lean mix until the mix goes more rich lowereing the dielectric constant, then the available voltage is sufficient to fire the richer mix. THIS becomes a margnal condition that you go in and out of.
It most often happens in a just off idle throttle position under light load.
I don't expect a dealer tech to know this unless they have been officially informed by Ford.
A good tech will pursue your fault using all his available means and find it anyways, if he even half tries.
By the way, this kind of condition ,because it is a (soft) fault, won't set a code.

EGR: Sometime try this, put a "good" timing light on #1 cylinder (clamp-on around the coil harness is enough) and find the reference down at the crank (there is one there). Yes it can be done!
Begin to pull the EGR open with a hand vac pump and watch how fast the ignition advances. This becomes part of the stumble (leaness and advanced timing) the coils can't fire. Plugs can and do add to the difficulty if they are old and the gaps are wide.
For you who don't believe in this, go on be bullheaded and have a good time with your stumbles.
All I can do is offer the reasons and the fixes.

 

yes i have a 4.6 had the same thing replaced coils with high out put ,no more miss ,skip. slippinout99

 

How do you stress test a coil??? Did you use Ford coils of or another brand?

 

There is a reflash for that exact concern. Pretty common concern with customers. I usually just reflash PCM and redrive it. 90 percent of the time it is gone.