Mustang with the stumbles

My 03 GT has an intermittent stumble. Can occur at basically any speed. When this happens, the speedometer and tach sort of freak out and the theft light starts to flash. Have replaced the upstream O2 sensors, spark plugs, coil packs, cleaned the fuel and induction systems. Dealer says they and Ford think it might be the alternator. According to the tech, there are a series of diodes on the alternator that help convert AC to DC and one of these might be bad, which might be causing the stumble. Dealer and Ford is not sure that replacing the alternator will cure the problem. They say replace it, see what happens. If it works fine, if not we'll do more tests. Anyone have this happen with their Mustang or 4.6? For what it's worth, there's 220k on the clock.

If that much stuff is happeneing, it's a core issue- whether it's a power spike or a major computer fault- it's much farther upstream than any of the engine sensors and actuators. Those won't cause the dash to freak out.

There's some sense to the alternator- since a power spike would freak out the computer. I'm sure that's not cheap at all.

Can you run it with the alternator disconnected to see if the issue goes away? It won't be charging, but it will run for a while on just battery.

I have an 05 GT, and I had a similar issue. I know, very different ponies, but here's my $.02 I started having a stumble here and there, usually at part throttle light cruise, but also on accell sometimes. It finally threw a code and got pretty bad. The codes was for a cam sensor, something that's not an easy job on the 3 valve. After some research in the webz (allfordmustangs.com in particular) I found out it was a common problem. One of the diodes in the alternator goes bad and cause weird voltage spikes that somehow make the diagnostics think it's a cam sensor.

I replaced the alternator ( a 15 min job, woohoo Ford!) and it's been right as rain since. I think I was around 80k miles when it happened.

So....I would say there is at least some evidence that an alternator diode issue can cause the problems you're having. Try searching the appropriate year model forums at allfordmustang as well, lots of good info in the noise.

BTW, most newer cars won't run with the alternator disconnected, in my experience. Wayy too many computery things going on. It's not as reliable a test as it once was.

Thanks for all the help guys. Will replace the alternator, hope it solves it.

A bad battery can also cause Ford PCMs to do weird things...

Yeah, I've had bad batteries (or bad connections) do strange things like that.

Check your grounds too.

My friend had an 03 GT that would totally shut down at highway speeds during normal cruise on a monthly basis. I had an 00 GT that would do the same thing but only once every 6 months. After several trips to the dealership with no results we both ended up selling them. Having your car die like that is scary as hell.

UPDATE: Replaced the alternator this morning. Took her for a test run of about 40 mi. Had a couple of stumbles, but were a lot milder than before. Ran a few errands then went to work. The stumbles came back and was almost to the point of wondering whether the Mustang would leave me stranded again. Here are a couple of things I've noticed: the stumbles don't occur when the engine is cold and they tend to occur between 1500-2000 rpm, which is where the engine spends most of its time. They don't occur at idle, but will come with a vengeance once the engine is warm and I'm taking off from a stop. With regards to Gearhead and Skyrender, the battery is less than six months old and the connections are good according to the dealer tech. I was going back through some old Facebook posts (don't ask me why I did that, I don't know) and ran across a post where I told a friend that the Mustang wouldn't start last year and the tech at the time told me the harness between the battery and the starter would need replacing (didn't have the money at the time and they did a band aid fix). I get paid Friday and will take it back to the dealer and have them do an electronic diagnostic test, will ask them about the wiring harness and see what they say. Cuda's right, the Mustang is getting scary to drive.