NEED HELP ASAP!!!!

[Hulksvt99]

Hello everyone, i have a 99 svt cobra and iam having issues with it, first off the car ran great until about a week ago. Here is the issue. When driving the car it goes alright until going at around speeds at 40 plus and i come to a stop or down shift the car will drop RPMs and die. Now i have replaced the IAC which was a recommendation but that doesnt seem to do the job. I tried the unhook the negative terminal computer reset and it seemed to run ok but during shifts it likes to hang high. Now i ran through all of the hoses and mass air cleaning and well the basics. So the question is what in the world do i do next to try and get this to settle out? I also had a guy hook it up and it read as computer issue. The guy before me has a tune in it but i didnt have any issues within the three months ive owned the car. Any help will be appreciated!!!

 

[wmburns]

You may not believe this but your problem could be something as simple as a bad battery or loose battery terminal/cable.

Sudden quitting for no known reason but easily restarting is a prime symptom of an alternator drop out.

Don't be "that guy" that changed a bunch of parts only to find out it was something simple.

Even if this isn't the solution to your problem at least you can do additional trouble shooting having covered the basics.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

 

[Hulksvt99]

so even though it shows im not dropping on the in car meter it can still be this issue. hense i went to a gas station the other night it idled really high and dropped of instantly. also when the car would die during driving i never had an issue starting right back up. Also after i unhooked to do the battery reset i hooked it back up and didnt have a problem with a restart.

 

[I only know what I know]

wmburns is right about the potential effects of low and also high voltage. If it is a voltage problem don't forget to check the belt. You say downshift but do you have an auto or man trans? If it's an automatic have your torque converter checked. It probably isn't unlocking. I'd also check the fuel pressure. If it's fuel pressure the source of the problem could be a lot of things including the fuel filter. Is it choking fuel delivery forcing the pump to work harder which conflicts with what the PCM expects to do when slowing down? The problem could also be something as simple as a bad gas cap. It's part of your evap emission system so actually any bad seal could cause this problem. I'm not familiar with your specific car but I believe it has a return fuel system? If so, a leak in the evap system is more problematic then in a non-return system. If it is a return system you should check the vacuum line to the fuel regulator. Beyond the regulator check all the vacuum lines. Is there a vacuum pump on your car? Check the vacuum control valve, the vapor canister valves and the solenoid or motor on the EGR valve along with the hoses and sensors on the EGR valve. What this all gets to is when you rpm goes down the vacuum should be going up and if it's not there are a number of things that aren't going to serve their intended function which could stall the car. There are text books on vacuum and there is no way cover it all here.

As you can see, there isn't a quick answer to your problem. Ford has a Powertrain Control\Emissions Diagnosis Manual for every year of car\trucks that will tell you how to check all of this without buying anything until you track the problem down. You will need some tools like a VOM, vacuum hand pump, etc.

 

[wmburns]

so even though it shows im not dropping on the in car meter it can still be this issue.

The dash volt meter can not be used to trouble shoot an issue such as this. Why?

• It's not accurate enough. Try it sometimes. Disconnect the alternator control wire and see just how little the meter moves.
• It's too slow to change. The needle is highly dampened. The needle would never even move except for a voltage change that lasted several seconds.

The best way to monitor voltage inside the car on a budget is to get a "add a fuse" (sometimes called "add a circuit). Connect a VOM to the pigtail of the add a fuse. This will give an easy way to monitor the VOM while driving.

The next best way is with a data logging ODB2 scanner. Log the system voltage PID.

Personally I would start by having my local auto parts store test the battery and alternator. Then a visual inspection of all connections. Clean, clean, clean.

 

[Neuron]

I would also check ALL vacuum lines closely.