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Fuel pump works, but won't turn on
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TOPIC: Fuel pump works, but won't turn on

Fuel pump works, but won't turn on 2 years ago #67761

Hey guys,
I'm trying to get the car started and I'm running into a bit of a head scratcher. We haven't got the car started up after installing the new engine. The battery has plenty of charge and there's about 1/4 tank of gas in the car. The problem is that when I turn the key to "on" the fuel pump does not activate.

However, we checked the power at the pump and at the relay and we're getting battery power for sure. We can also jump the fuel pump (at the relay) and it will turn on and start pumping fuel (we have a puddle of gas on the floor of the garage as proof). Since the fuel pump CAN work, this led us to believe that the ECU was the culprit and maybe something was wrong with the ECU sending the signal to turn on the fuel pump, but we checked the ECU and were able to complete the circuit manually as described by the Bentley manual. We also checked to make sure the grounds in the ECU were ok and nothing was wrong.

So in conclusion - it seems like the fuel pump and fuel pump wiring is ok, the ECU seems to be ok, but when we turn the key the fuel pump doesn't turn on. When we try and start the car, the car cranks, but will not turn over (since there is no fuel being pumped).

Any ideas where we should check next? I'm stumped.

Re: Fuel pump works, but won't turn on 2 years ago #67762

  • Patton
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Maybe I missed it,replace the relay?

RP

Re: Fuel pump works, but won't turn on 2 years ago #67763

Forgot to mention that - we already replaced the fuel pump relay and the main relay right next to it.

Re: Fuel pump works, but won't turn on 2 years ago #67765

  • tpr jules
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We had the same problem recently when we did a motor swap. Turns out that I crossed up the 2 wiring connectors (I labeled incorrectly) under the round test port up front (one for the CPS and the other going to the spark plug wires). Car wouldn't start and fuel pump wasn't activating... Swapped the 2 connectors around and car started. Hope this helps.

Re: Fuel pump works, but won't turn on 2 years ago #67767

I'll double check that tonight, jules!
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Re: Fuel pump works, but won't turn on 2 years ago #67768

  • Ranger
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tpr jules wrote:
We had the same problem recently when we did a motor swap. Turns out that I crossed up the 2 wiring connectors (I labeled incorrectly) under the round test port up front (one for the CPS and the other going to the spark plug wires). Car wouldn't start and fuel pump wasn't activating... Swapped the 2 connectors around and car started. Hope this helps.

+1. Look hard at the CPS. If the DME doesn't sense the signal from the CPS, the DME won't turn the fuel pump on. The CPS wiring can look ok on the outside, and still be hosed up on the inside. That happened on Jon Stroup's engine 2 weeks ago.
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Re: Fuel pump works, but won't turn on 2 years ago #67771

  • Patton
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+2

RP

Re: Fuel pump works, but won't turn on 2 years ago #67786

Besides checking the connections, check the CPS wire insulation. The wire routing/protection is something to be desired and it is really easy for the wire to get chewed up by your water pump pulley.

KB
Kevin Butler
Mobetta Autosport #612

Re: Fuel pump works, but won't turn on 2 years ago #67788

Thanks guys, I'm gonna check out the CPS when I get a chance. Hopefully work is smooth enough that I can get to it tomorrow.

Re: Fuel pump works, but won't turn on 2 years ago #67789

irectokingofpoptart wrote:
So in conclusion - it seems like the fuel pump and fuel pump wiring is ok, the ECU seems to be ok, but when we turn the key the fuel pump doesn't turn on. When we try and start the car, the car cranks, but will not turn over (since there is no fuel being pumped).


My car's doing the same thing right now. However, I've got a bastard fuel pump configuration in the tank. The original '86 pump frame grounded to the chasis in 2 places: 1- the external pump connector had a tab soldered to the top of the pump plate. 2 - The ground wire from the pump in the tank was soldered to the return pipe in the tank. This was factory configuration and it seems to me the whole pump circuit grounded to the entire chassis via the tank.

My present pump configuration is a 1991 318i pump frame with an Walbro pump attached. The pump ground wire in the tank is soldered directly to the pump assy connector. I'm thinking the ground wire in the harness doesn't provide enough potential for adequate current flow. Haven't tried additional grounding to the chassis to validate this theory yet.

Could it be possible you've made some update the fuel pump system that requires more current flow?

And one more question/observation - are you sure you haven't reversed the fuel flow direction? Do you get fuel pressure at the rail when you run the pump directly off the battery? If not, you got some hoses backwards or major fuel line blockage somewhere.

KB
Kevin Butler
Mobetta Autosport #612
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