SOLUTION- (you may not want to hear this) ditch the returnless setup and go return. You absolutely will not regret it. I wasted a lot of time and money fiddling with mine untill I took others advice and went return.

 

Return is overkill at this power level. Much more expensive than it is worth.

As for the issues you're seeing, you've got a couple problems. One is you need more FPDM capability, the other is you need more pump. Either get a modified FPDM, or a dual FPDM. Seeing FPDC at 95% is also a bad sign. I would want more margin than that.

That said, I was running dual FPDMs with dual GT pumps with the wiring upgrade for ~15psi of boost from an Eaton M112.

The dual FPDM harness is pretty easy. There's a few nice writeups on it, and if you can solder, you can make it yourself. Buy two pigtails, a spare FPDM, some wire and a relay, and go to town. Pretty cheap and easy in my opinion.

 

I haven't had a chance to retune mine yet, but I recently installed a second pump and dual FPDM.

I previously lost 1 psi of fuel pressure at 5800 RPM, so the system was more or less taxed (I think it was the plumbing, not the pump personally).

After installing the second pump and FPDM (prior to a retune mind you), I'm getting 9.3% fuel pump duty cycle and 90 psi of fuel pressure. This is with two AEM 320lph pumps.

I got my harness from West Coast Mustangs- very professional, great directions, good communication, and I would highly reccomend (I'm not an electronics guy).

 

OP, you have the stock cobra pumps, correct?

A single FPDM can run those pumps and a lot of guys with your combo with a BAP and stock Cobra dual pumps have been good up to 600 rwhp so I think you are having some other problem.

I would try a wiring upgrade. Right now you are running all the pump current through both the stock wiring and the inertia switch. The wiring upgrade only runs the relay coil current through the stock wiring and switch, eliminating any voltage drop from those.

In the mean time I would do the following though.

Cycle your inertia switch on/off several times to clean its contacts.

Disconnect the connector that goes to the pumps, its right above the top of the back of the tank, you can see it if you get low down under the rear of the bumper.

Look for any corrosion on both the mail and female pins in the connector and clean them with WD40 and a small screwdriver or mini-file to clean the mail pins and a paper clip to clean out the female pins.

On my connector there was a greenish corrosion, it took a while but I was able to clean it all off.

I would also check the connector at the FPDM and clean it. If doing all this doesn't fix the problem I would try a new FPDM. You shouldn't need dual FPDM's at your current HP level.

 

Thanks for the input guys. My tuner had dropped the tank and checked all the wiring at the pumps so that is good. The connector at the FPDM is also clean. I've ordered a new FPDM and picked up a relay and some 10ga wire and will be powering the FPDM direct from the battery via the relay and an inline fuse just to make sure no voltage drop issues. Should the inertia switch also get on the new 10ga wire? Also, how do you cycle the inertia switch? It is all the way down and when I push on it it doesn't move up or down. If all this fails I will go to the dual FPDM mod as a precaution.

 

You don't need the 10 ga wire going to the intertia switch, keep the stock wiring for that and use that power feed to power the relay coil that turns on the direct power feed to the FPDM.

There's a lot of threads on here with examples of doing the wiring upgrade, search "wiring upgrade" and you'll probably hit them.

My memory is you just push on the inertia switch and it pops up then you push it back down. If its not doing that it could be the sign of your problem, it could have poor continuity and overheated and got stuck.

 

I run dual FPDMS. I built my own setup with a wiring upgrade to 8 gauge, relay and FPDMS for 50 bucks... that includes plug and play wiring harness I built, 2 FPDMs, 8 gauge feed wire, 50 amp relay.... everything cost me 45 or 50 dollars..... My wiring upgrade is 8 gauge from battery which is trunk mounted to the FPDMS, 8 gauge from the FPDMs to the dual pump setup and 14g wiring upgrade through the hat to each pump connector. - 45 or 50 bucks total

 

found the issue with the dying. Loose ground wire front passenger side. All sorts of quirky symptons that I never would have guessed all led back to this one loose ground.