So I hotwired my fuel pump and then.......

**92EuroDiamante** · 04-11-2011, 05:22 PM

Ok,
I just plugged back in the MFI, relay E8t07871, stock configuration and the car wont start at all.
It catches a bit but wont start.
Would the way to diagnose if it is the MFI or ECU be to wire the fuel pump to 12v and run a switch to turn it on right before I start the car?

**Steve '92ES** · 04-11-2011, 05:26 PM

Most people put the hotwire relay right by the fuel pump, and pull the Black/blue wire out of the fuel pump connector and use it to trigger the hotwire relay. You then replace it with a wire from the hotwire. That's the only modification to the stock wiring you need to do, other than to put a jumper in the hi/low relay if so equipped. (you of course still have to run a power wire to the hotwire relay, but that's in addition to the stock wiring).
Somehow I'm getting the impression you did something to the wiring at the relay. I don't know why I got that idea but I want to be sure. Did you do all you modifications at the fuel pump as I described, or somewhere else?

**Steve '92ES** · 04-11-2011, 05:28 PM

Would the way to diagnose if it is the MFI or ECU be to wire the fuel pump to 12v and run a switch to turn it on right before I start the car?

Do you have a voltmeter or test lamp you can use to check for power at the fuel pump?

**92EuroDiamante** · 04-11-2011, 05:35 PM

I have a voltmeter yes.

I put the aftermarket relay next to the stock relay. I did this because unlike the 3s the fuel tank cannot be accessed from inside the car.
Here is how I wired it.
The only wire I touched coming off the stock relay E8t07871 was the power wire to the fuel pump. I cut that several inches from the stock relay.
I connected that to the aftermarket relay. I connected the other wire of the aftermarket relay to the wire going to the fuel pump. The other two wires to the aftermarket relay went to the battery and ground.
just like this http://www.stealth316.com/images/fprw-schematic.gif

car started and ran fine several times. This happened right after I pegged the gas.

**Steve '92ES** · 04-11-2011, 06:02 PM

Hmmm. The point of a hotwire is to run a larger wire straight to the fuel pump from the battery to make sure the pump gets all the power it wants. By re-using the stock wire I don't see that you accomplish anything other than to add an extra relay that doesn't really bring much to the party. On the other hand I don't see that it would hurt anything either.

On to your current problem though. I was going to suggest you check for voltage at the fuel pump while you crank but you say the Diamante's pump isn't as accessible as the 3S. On the 3S there's a test wire for the fuel pump on the passenger side firewall, under the wiper motor. Do you have something like that?

**92EuroDiamante** · 04-11-2011, 06:37 PM

I didnt reuse the stock wire. I used 8g wire to the hotwire relay, and then 8g wire from the hotwire relay to the fuel pump. At this time the 8g wire to the fuel pump is hooked up to the stock MFI just to diagnose.

I made myself a little loop so I can tap for voltage

. I will do that.

**Steve '92ES** · 04-11-2011, 06:45 PM

Opps, sorry. I miss-understood that.
Do you get power on the wire going to the fuel pump while cranking?
(you have to be cranking, don't just turn the key on)

**92EuroDiamante** · 04-11-2011, 06:56 PM

Ok I just hooked up the voltmeter and cranked it. The MFI starts clicking like crazy. As it clicks I get intermittent voltage to the fuel pump. very short bursts, not enough for the fuel pump to build up pressure and volume. The car cranks and cranks and catches for a split second as the pump gets voltage. But since the voltage stops just as quick it dies.