Electrical 01 V6 wont stay running

Hey everyone. Good to be here finally! Been a user of the forums for a while. Thanks in advance for the help now and what's to come!
2001 V6 Mustang Automatic. Car wants to stall and idles rough like the battery is loseing charge and the alternator is dead. When I went to my buddies house to look at it I had forgot my multimeter so I did the diag without it. Here hows it went. Battery was dead, pulled it and charged it. Battery is from 5/19 so it should be ok. Started the car and it started ok but then ran for a little bit and stumbled and died. Then went to restart it and it had nothing. No lights, dings from the key, nothing at all. Charged the battery again it started. This time I took the battery out of the loop and let the alternator run the car alone. It stayed running just fine, until I started putting load on it. With load and in gear everytime the a/c compressor would kick it would almost stall. The battery gauge would flatline and rpms would drop. Then come back up. So we changed out the alternator. Now itll handle load just fine. But when you reconnect the battery it wants to stumble and die and then act like it has nothing. My thoughts were that it would have a really bad ground somewhere but I'm not to familiar with ford electrical systems. There is no corrosion on the terminals either. So where would I look next. Thanks again!

I am currently driving a mechanical restoration in process. A 93 fox 4cyl. Been great owning it and it alot easier to work on then most things. Gunna be doing a clutch soon.

I appreciate the response wmburns! Wont be until sunday until I can get out there to test. Thanks for the info!

Did not notice a voltage drop at all. We did notice with the ignition off and the lights on the battery discharged really fast. Took it up the have it checked and charged. Also noticed that it was spitting and sputtering before it died. Almost like it was loseing spark. Car has good fuel pressure when running. Just put a pump in it 4 months ago.

Should also note, the battery rebounded from 4.57 to 11.7 after the lights were cut off. At that point the vehicle failed to crank.

Fair enough on the battery being a big draw on the system. I still haven't been able to get ahold of my buddy to find out the battery results from the testing. I just never really thought about it.being a draw because for atleast a few minutes it doesnt pull the rpms or the alternator down. Had 14.3 at the battery and about the same at the alt case when running. As soon as I hear from him I can proceed.

Battery checked out fine, cells were good and it held a charge. Come to find out they started driving the car again. Reported it still wants to stall but if they keep the rpms up by doing the neutral shuffle itll stay running. About a week of then doing that and it wont stay running again. Going to be going over there tomorrow and checking fuel pressure and spark. I'm almost certain the fuel pressure will be fine. Anything particular I should look for or check for spark dropping out?

wmburns said:

Here's some information on how to trouble shoot idle/IAC issues.

Troubleshoot IAC idle problems 1996-2004

Troubleshoot IAC idle problems 1996-2004

Back to the basics. Check for blown fuses. Especially fuse F2.2 and F2.8 in the Central Junction Box (CJB 1999-2004 MY). Confirm key on power using a known good ground. 1999-2004 MY fuse panel schedule...

www.stangnet.com

Click to expand...

So after a little time has passed and life getting in the way I am back at this car. It's now at my house. Since previous conversations we have been able to identify three issues. First being the alternator not charging. Figured that out. Next being a crossfire condition. New coil, plugs and wires to knock that out. Now after all that I went back to basics. I think in previously stayed the fuel pump had been changed. Decided to check pressure at the rail anyways. Fuel pump primes just fine. When cranking it tanks at about 20psi then comes back up. Now, my gauge is accurate. But, when this thing comes to an idle my gauge starts reading erratically. Then it will start to drop pressure little by little until the car starts to sputter. Give it throttle and it leans out and tries to stall. If I unplug the vac line it will come back up but still sputter and read erratically. By that I mean it vibrates that gauge with a variance of almost 5 psi either way. What are your thoughts on this? Cant be a bad filter as it does get good pressure until start up. Also it passes a leak down test. If you let the car continue to run it will eventually stall. It will start back up. With the same condition.

wmburns said:

Do you have an ODB2 scanner that will monitor what the PCM "thinks" the fuel pressure is?

Consider that IF the fuel rail pressure sensor (FRPS) is sending false information to the PCM, the PCM will send commands to the fuel pump driver module (FPDM) in an attempt to adjust fuel pressure to maintain about 40 PSI.

So the issue is to figure out is the fuel pressure so low because the fuel pump can't keep up or because the PCM isn't getting good data to adjust to.

Are you positive that the CORRECT fuel pump has been used for this application? There is a difference between a fuel pump used in a return verses a return-less system. Regarding the question about the fuel filter. IMO it's very short sighted to replace a fuel pump and not replace the fuel filter.

Have you doubled checked the voltage at the trunk mounted IFS switch to be sure there is full system voltage available at all times? How about the ground in the trunk? Can it carry a real ground current?

Here's some information on an affordable Windows based ODB2 scanner. Note, most any generic ODB2 scanner (not a simple code reader) will also read fuel pressure.

ForScan ODB2 scanner w ELM327 USB

ForScan ODB2 scanner w ELM327 USB

While working on your car have you ever wanted: to find an ODB2 operational PID value (say fuel pressure or MAF)? How about graph a PID value over time? Or compare multiple PID's over time? Access a bi-directional PCM control such as test a...

www.stangnet.com

Click to expand...

I have good voltage going to all circuits front the ccrm all the way to the inertia cutoff and beyond. The weird thing is the car is not storing codes. As long as it has had this issue it's never even flashed the check engine light. I do have a decent scanner and use it often lol
I am sure it's the correct fuel pump as it was running perfectly fine for almost 5 months on that pump with no issues at all.
As for the filter, I suggested it strongly, she does not have much money to spend and always just replaces the exact thing wrong, regardless of recommendations.
I will check out the frps today sometime. Got some PM work of my own to do today first.

Okay so update. I have a running parts car here at the house. Found out my scanner does not do snapshot data unless and fault is occurring. So I went with the next approach. Swapped a known good unit into the car and had no change. I am at a loss with this thing and I'm about to tell her to take to a shop cause its apparently out of my skill range. Talk about feeling defeated.

Update 2: pulled the trunk apart and double checked voltage at inertia switch and pigtail for the pump. Voltage is good at both. When running the voltage is also good. When the car starts to sputter the voltage remains constant. This would indicate the pump is no good correct?