OSUjustin313

My 99 GT V8 auto randomly cut out one day and it got worse as time went on. now, its at a point where I'll start it, and it'll shut down after 5 minutes. Injectors are fine, alternator has been tested, battery tested, grounds tested, fuses tested, CCRMs replaced/tested...seems to be the computer...anybody know what the odds are of finding one at a decent price? also, does it do any good to attempt to tune it first??

DeathRattle

Any codes? If the ECU was bad you would probably get a code.

OSUjustin313

it gave a code for IAC, all 8 cylinders misfiring, fuel pump, and I can't remember the other one but it gave it for that too...also shut the A/C unit down...I have since replaced the A/C, I tested the fuel pump, all 8 cylinders are fine, the injectors are fine and clean, and the IAC came on after I took it to Ford in Columbus, OH and those guys are useless...they said it was a clogged intake when it was fine...then they charged $400 for a job that took 1 hour..almost had to threaten to take them to court before they agreed on a much lower price for their useless work.

DeathRattle

Ok yea you might have a bad ECU then. It is located behind the kick panel on the right side of the passenger footwell. You should be able to find a replacement one for cheap on Craigslist. Look under "Mustang Parts Car", or "Mustang Parts", or just "Mustang" under the auto parts section. You may also find a parts car in the Cars/Trucks sales section. Make sure it is a 99-04 GT ECU. A quicker way to search through all of craigslist is Searchtempest.com ... give it a whirl.

OSUjustin313

thank you for the feedback. I've never had to do something like this before. I have been told that you should buy a tuner, attempt to tune it without buying anew ECU and that you don't need any software necessarily. sound right?

cliffyk

Short answer, no...

If the PCM (Powertrain Control Module, Ford's name for the ECU) has failed electronically--or if the strategy EPROM has been scrambled) it will have to be repaired or replaced.

However if the problem is that the tuning data (held in flash EEPROM) is scrambled then all that will be needed is to force feed it with a good copy of your stock tune. Ford dealers and shops with professional tuning systems can do this; inexpensive ($400) handheld tuners cannot.

Ford dealers also have diagnostic systems that can fully test the PCM, the problem is finding a dealer that has someone competent to do it.

One additional complication is that in the PATS anti-theft system the PCM and the PATS control module (in the instrument cluster) are "married" via a shared security ID. If either assembly is replaced they engine will not start until the security ID is reset (only Ford can do this), or the PATS is disabled in the tune.

I would first replace the crankshaft position sensor, which can cause the issues you describe, and also consider replacing the generator.

It is common on older/high mileage GTs that the generator's output becomes electrically "noisy", creating all sorts of EMI and RFI spikes in the vehicle's electrical system that can "glitch" the PCM. It may still be charging the battery and test "OK" if that is it was tested for; an oscillscope is needed to test for excessive EMI/RFI...

OSUjustin313

I am buying a tuner tomorrow that will disable the PATS and wipe the ECM clean to start over. Hopefully that solves the problem. If it doesn't, I will buy a new ECM for about $200 since I need the tuner anyway. the trouble will be buying the software: I hear that runs upwards of $600. Ford doesn't neccessarily have to do it; someone who knows what they're doing ( not those monkeys at Ricart Ford in Columbus for sure) can do it. This will be my first time going through this and I will attempt to keep everyone informed of the progress ( or lack thereof) in the coming days/weeks. I hope that my experience can teach someone in the future so they don't sink a bunch of time/ money into the same problem. Thanks everyone for their responses: any other info is certainly appreciated!

cliffyk

By all means let us know how this approach works out. I woild like to know more about PCMs than just how to tune and bench flash them:

DeathRattle

If your ecu is bad then flashing a tune is not gonna solve anything. And if you do get a tune flashed in, you might have trouble getting through state inspection with it in there. As far as tuners go, you might be better off just buying a flash tuner along with a used ecu from a different GT. You can find GT ECUs for like around $100 if you look around.

Chromeshadow

A scope in the best tool to look for noise, but you can sometimes find problems with a good quality DMM. You use the lowest AC range across the battery or alternator and compare the reading with a known good car with the same meter and same voltage range. Ideally there will be no AC voltage in either location. This test may show that you have a problem, but a low reading on a DMM does not prove that your alternator is noise free.
Btw, SL 500s Rock!