I have a 98 supercab 4WD with the 4.6L V8. The truck has 105K on it. I just had to replace the alternator and battery last week. I did it myself and it went fine. Now, when the truck is cold or wet it is missing as I accelerate on the highway. It idles fine and holds any speed without missing. In the rain the other day it was much worse. Check engine light is now on constant and flashes when missing. I did do one other really stupid thing. I poured some gas out of my snowmobile which I had just gotten from storage. The gas had stabilizer in it and should have been fine except I forgot it was from the sled I had the engine rebuilt in last year and it had break-in oil in it. I'm now through that tank and put in 93 octane. It did run fine for most of the oily gas tankful, until I changed the alternator. I'm thinking that it's sucking in air from the outside somewhere, but can't find the leak. Any suggestions (please don't pile on for the oily gas incedent, I feel like enough of an a-hole).

 

When was the last time you changed your plugs and wires. It could very well be those. I would check them. If they aren't your problem then get a diagnostic. I think autozone gives them for free. I just went and bought my own diagnostic computer for like 140 bucks. Its a real good investment.

 

I'm getting the same issue on my '97... it's going in thursday for the diag... but the mechanic tells me there's been an issue with a lot of the f150s where it has a flow return sensor that's invertly mounted against some vacuum return port, which tends to get wet and corrode. Anyway - it was missing BAD this morning, a lot of dew on the windshield, about 40 someodd degrees outside...

(more to "offroadmechanic") I was thinking about buying a code reader too, but he was telling me that some of the indi. readers can't get past a specific code set and need to go to Ford... just what I need, $80 an hour for them to tell me what's wrong... eek!

 

I had the plugs and wires replaced about 5 months ago with all Ford OEM replacements. Other people are coming forward with the same problem. I'm really beginning to think that I upset some part when I changed the alternator. Jkingman, mine is doing just what yours is. It's horrible when I start it in the morning, but on a nice sunny day like today it's fine. Alsmost wouldn't run the other day in the rain.

 

I just got through changing my plugs at 73k on my 97 4.6L and it ran fine. One week later the engine started missing badly any time it was under a load. Replaced the plug wires last Friday and it ran just great. Yesterday the Check Engine light came on. I have gone through everything I had either removed or touched in any way when I put in the OEM plug wires. I noticed a slightly loose plug wire but I am not sure if it was the cause of the Check Engine. Guess I will take it to a local shop (not a dealer). They have a good rep and have done good work for me before. If you have any success in your efforts to resolve your problem I would appreciate hearing about it.

 

I hate dealing with a miss. Everyone I've dated had some sort of issue or another.... JK, LOL.
Randall Phillip & others like this
If I was you I would be changing my fuel filter and running a good fuel system cleaner like Chevron Techron concentrate. Also, I'd almost bet it's a bad plug or wire on 1-2 cylinders. A flashing MIL indicates a misfire condition. Getting that DTC out of there will help you eliminate which cylinder is the problem. Before you go buying all new plugs and wires I would check each wire out closely. Just pull each one out, check it, regrease with dielectric grease, and FIRMLY reseat the wire and make sure it snaps down over the plug. The wires on my 4.6 like to sit in 1 direction only for best contact.

 

I have to agree with KYFordFreak.
These early 4.6 engines with the plug wires are really prone to miss-fire (mine included). I just replaced my wires with Taylor wires and this is the first time in a long time I have not had a problem.
Grab each wire and loosly jiggle/pull on it. If your lucky you will find one that is loose.
Good Luck...

 

I will do that. If you've ever changed an alternator on one of these you know that it's a tight fit to get it in and out. I easily could have loosened or even scored a wire with a shrp edge. I will post the results. It is defintely a moisture problem I am discovering. It missfires in the morning or when raining. The past to days in Rochester have been sunny and warm (imagine that) and it has run fine once the morning dew is gone.

 

Yes I did change my alternator and I know what you mean.
I think the moisture just creates an environment that promotes miss-fire by lowering the impedance and allowing the spark to jump easier to the block. Must be the difference needed to make that the path of lowest resistance...???
Sounds good anyway...

 

So get this. I went out at about 10:00pm last night and started the engine and looked under the hood. The plug wires all around the new alternator (where the wires come together and bridge over the alternator and under the air intake hose) are one solid arch of electricity. Now my theory is that I have a defective alternator. It never did this before and I had these plug wires on it. I think the alternator is overcharging for what the battery cable can handle and charge is bridging to the alternator body and then out to any metal nearby. With a charge flowing through the plug wires from the inside and from the alternator from the outside, it's too much for the insulation to handle.

 

That just don't sound right to me. The spark comes from the coils not the alternator. I bet the wires are bad and the poor performance of the old alternator did not allow for the full spark to show up the problem.
Start first with new wires and make sure they are good wires.
Again I recommend Taylor's

 

It's the plug wires. The voltage the alternator produces is essentially 0 (same as the block) compared to the voltage in the plug wires. It may have started with a single point and over time the arcing causes surrounding insulation to fail so it only gets worse.

 

I put new Bosch plug wires on last night. It turns out the problem was that the first plug wire (front) on the driver side wasn't seated properly on the plug. The gap was causing a misfire and the spark was finding its way back up the wire and then all over the place if it was wet. I had pushed all the wires down firmly in the past so I was surprised to find this. When I looked closer I saw that the wire for the next plug back had worked its way under the little lip on the front plug boot and was wedged between the boot and the head, not allowing the plug to seat properly. Now, I could have loosened the wire when I changed the alternator, but judging by how hard I had to pull to get the other boots loose, I find this improbably. What I think happened is that the mechanics at the garage never seated that boot properly but got it close enough to function on the old charge. Then when I was changing the alternator, because that plug boot wasn't all the way on (the plug wire was propping it up), it was a little loose and I did pop it the rest of the way off. At any rate, the old boot was fried, covered with carbon and wouldn't fire when I put it on properly. It maybe could have been cleaned, but opted for the new wires anyway. Better safe than sorry for $70. Thanks for everyone's help here.

 

I'm glad it worked out.