Fuel Pump Roulette
 

I have an Aeromotive 340 fuel pump installed with a separate relay for power. Please see the schematic below to see how I have it wired. I installed the new pump a few months ahead of installing a supercharger.

The day I put it the new pump in, the car would not start as quickly as it did with the original pump. With the original pump, I would press the start button and the engine would fire up immediately. With the new pump, the engine would turnover for an extra second or two before starting.

After I installed the supercharger, I would also sometimes hear the pump prime and sometimes it wouldn't prime. If it did prime, the engine would crank for a second or two and start. If it didn't prime, the engine would crank and crank and would sputter for another four or five seconds and then finally start.

This morning I turned the car on and I didn't hear the fuel pump prime and the engine just kept cranking. Tried it three times and no luck. I tested the relay I installed and it seemed to work fine. I thought the fuel pump was bad. With my relay installed, I am briefly getting 12V to the pump for about a half a second after I hit the start button but then it goes down to 0V. However, the pump does not prime in that time that it is getting 12V.

Before I went to go pull the fuel pump out, I reconnected the original fuel pump power wires back together. The fuel pump primed and the engine started just as quickly as it had with the original pump. I tried it again and the pump primed and started immediately again.

Does anyone know when the ECU sends power to the fuel pump to prime? Is it supposed to happen every time you start the engine? Is there any reason why the ECU pump relay would not provide power when it is connected to my relay (how would it even know)?

Last question: The Stillen supercharger instructions simply tell you to swap out the OEM pump with the one provided by Stillen (I don't know the size they provide) but they do not say anything about adding a new relay or heavier gauge wire. My multimeter shows 11A draw to the Aeromotive fuel pump. The circuit is protected by a 15A fuse. Has anyone driven around with a larger pump on the original wire?

My understanding is the ecu turns on the pump when you hit the ignition switch to the “on” position, foot off the brake. You should try this by pressing once for ACC, and a second time for “on” - foot off the brake.

At the on position, the pump will cycle for a second to raise the fuel pressure for starting. It should raise the pressure to factory spec, like 50 psi. Once the driver presses the start button a third time - foot on brake, the pump will come on and stay on while the engine is running.

Two relays in series, may not work well, because you have two coils in series, there may be varable resistance and heat. I would try the OEM relay only with the Aero pump and just keep an eye on the 15 amp fuse.

^^ Correct, the fuel pump only primes when you tell it to, by turning the ignition to full ON without activating the engine starter. Otherwise the fuel pump relay is just energized basically at the same time the starter is activated.

The reason your car starts harder after upgrading the fuel pump is because you will now have a spike in fuel pressure at engine start and idle. The fuel pump module in a 370z uses regulator waste fuel to create a pressure differential in a pair of venturi action jet pumps for transferring fuel inside the tank. The orifices of these jet pumps behind the regulator outlet are "tuned" for the amount of waste fuel there is with the stock fuel pump. With a larger fuel pump, the amount of fuel passing through the regulator is much higher, and the orifices are too restrictive, and they backup the pressure regulator causing a fuel pressure spike. One of them can be removed, enlarged, and reinstalled to fix the pressure spike.

So by reading this .
Im starting my car wrong ..

As I hop in, clutch in and push the button and 99.5% of the time car fires up, the other .5% is starts then dies ..

I should be hopping in, no feet on pedals, push the button twice .. .Then clutch in and fire up . .then engine is primed , as the non start days I put down to a little back draining in the fuel system thus lack of fuel

Just like I do with a car with a key , turn on for a few sec's before firing up ..

My 1st car with no key start

In typical use, there is no need to force a prime cycle before starting. Just hop in, clutch down, press button once; and you're off. It wasnt meant to be something people had to consciously do before starting the engine. A healthy fuel system will reach operating pressure in an INSTANT, quicker than the starter can have the crankshaft making meaningful revolutions.