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Thread: 2003 Cobra Electrical Gremlin
06-15-2019, 08:05 PM #1
2003 Cobra Electrical Gremlin
I run a shop in my hometown and although I do own a 2003 Cobra and through the years we have built a few we have never experienced something like this before. Also we haven't done many in the last years, but we been working on this particular vehicle for quite some time now. Decided to ask first, because maybe someone has experienced something similar and would point me in the right direction instead of doing a lot of troubleshooting and testing.
The car is an 2003 with rebuilt engine with stock compression 2.8H KB, small Comp Cams, headers, exhaust and was previously running dual Ford GT pumps with dual FPDM. Let me give you some background, he rebuilt the engine because around 5 years ago he lost it due to a bad CCRM. It went bad, there were no fans, overheated and then started many problems that prompted a full rebuilt. It overheated a few times until the previous shop found the issue with the CCRM. When replaced the fans worked fine, but it was already toasted. Fixed the engine and since that time he has been having some electrical problems, like eating alternators easily, stock, modded, rebuilt ones, PA, etc. All of them die...
Fast forward to me working on the car... he came first for a tune. We did the tune and although I noticed some funky stuff with the voltage and the fuel pressure and AFR we were able to give it back and it ran fine for a while. Then another alternator die, he changed it but the car was inconsistent. Like from run to run it was loosing 4-6 tenths and 4-6 mph for no particular reason. Just like that from run to run it was gaining and loosing. Told him to bring it back. First few runs on the dyno and it was fine, yes it had the usual returnless issues when trying to push it hard like unstable fuel pressure and it looked like the alternator was dying again as it was loosing voltage and when loosing voltage the fuel pressure went like crazy and the same for the AFR.
Decided to do a full return fuel system and installed new alternator and thought that everything was going to be fine now... wrong! Even with the fuel pressure steady and in place and voltage from the alternator and the battery in check, the car is doing the same crap as before with the AFR all over the place... even without making any changes on the tune the AFR is different from run to run on my dyno wideband and on the one in the vehicle as well. It can go really rich, then somewhat lean, the oscillate all over the place, plus the datalog shows a voltage loss, even when at the alternator and the battery it stays over 13.5 on the whole run and the fuel pressure is good and steady. Also the gauges go like crazy, the datalog quits at around 3700 rpms and there the last view you can see the voltage dropping like to 10.8-11.3 volts and the AFR goes all over the place. It can go to lean or rich when this happens.
Any idea of why the voltage at the battery and alternator is fine but low at the ECU? I know this is why the gauges go all over the place and datalog quits. I tried a run with no datalogger running and the gauges don't go like crazy but the afr goes all over the place after 3700 suggesting that it probably is still droppjng at the ECU level.
Anyone has dealt with something similar or has any pointers on where to check?
06-16-2019, 11:28 AM #2
That certainly is a pack of gremlins, Charlie. A couple of thoughts ...
The ECU's ability to maintain proper fueling depends on a reliable and relatively consistent supply of power. The variations in voltage that you see where the ECU voltage roams around, with the battery and alternator relatively stable, makes me suspicious of the grounds for the ECU. If you have not already, I would check them out to make sure they all have a good connection. I suspect the voltage variations could be a result of one or more grounds that have deteriorated, for whatever reason.
I am not suggesting you consider converting your customer back to a returnless system, however there is an excellent write up on returnless fuel systems and their mischievous pressure spiking behavior on the site here, that was done by Jerry over at Sutton Performance in 2005. Jerry addresses the returnless system pressure oscillations and how to properly control them in the tune. It involves using a pump pressure PID to clip the pressure surges. Excellent Sherlock Holmes work and fix to the problem. Here is a link to their fix => Returnless Fuel System Hesitation Discussion
A little more on the fuel system. The Ford ECU monitors the differential pressure across the injector (using the FRPS) and adjusts fuel rail pressure (by adjusting pump speed) to maintain a 39.15 psi pressure drop across the injector. At idle that means the vacuum in the intake manifold will cause the fuel pump driver logic to command a reduced fuel pressure to maintain that 39.15 psi differential across the injector. A 9 psi idle vacuum in the intake will cause the ECU to reduce fuel rail pressure from the 39.15 psi standard to a 30.15 psi level. Reduced rail pressure not withstanding the differentia pressure across the injector is still 39.15 psi. If the fuel pumps can not exactly match the fuel rail pressure to hit the 39.15 psi differential the ECU wants then it uses the measured differential pressure from the FRPS to modify the injector pulse width to deliver the correct fuel charge to the cylinders.
When a fuel system is converted to a return style system some folks will remove the FRPS thinking it is a returnless component no longer needed. The fueling logic in the ECU will still try to look at the FRPS for differential pressure information. Even in a return style system it will use the FRPS information to calculate correct injector pulse widths. The reason this is still important in a return style system is because the mechanical pressure regulator can not respond as quickly as the engine's fuel demand can escalate. The FRPS insures the correct injector pulse width is used to meet commanded lambda or AFR during rapid changes in engine fuel demand. If the FRPS is disconnected or removed, reinstalling it will mitigate some of the problem and also provide better fueling.
All the fuel system science stuff aside, I am still suspicious of one or more faulty grounds introducing the power gremlins you are fighting. The Helms printed Ford Wiring Diagrams Manual for the 2003 Mustang will be the best source for all things electrical, including the ECU, on the car.
06-16-2019, 11:12 PM #3
- Join Date
- Mar 2010
I would look at the grounding.
In particular the ground of the ECU.
It's ground wire is terminated at the negative pole on the battery.
There is a connector hidden behind the battery.
06-17-2019, 03:29 PM #4
We will concentrate now on the electrical part and will report back.
- - - Updated - - -
Will work on that now. Thanks!
06-19-2019, 05:43 PM #5
- Join Date
- Mar 2007
Check alternator pulley. Chances are it's too small for the application. Symptoms are of the alternator being overspun. This will result in voltage drop at a certain RPM along with reduced power to the electrical system.
Repeating the process will eventually lead to premature alternator burn-out.
This is most likely your culprit.
Switch to 3.2" or larger alt pulley
06-19-2019, 09:20 PM #6
It is improbable that the alternator is the problem. As you review Charlie's post you will notice he reports the alternator and battery voltage voltage as steady throughout the engine operating range. If the alternator were the problem the alternator voltage would not be steady throughout the engine operating range.