Fuel pump works, but won't turn on

#1

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. :S

#2

Maybe I missed it,replace the relay?

RP

#3

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

#4

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.

#5

I’ll double check that tonight, jules!

#6

+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.

#7

+2

RP

#8

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

#9

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.

#10

irecto[quote=“kingofpoptart” post=67761]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).[/quote]

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

#11

So I checked the CPS, and it looks ok from the outside (but like you said it could be cooked on the inside). Are there any tests you guys recommend before I try replacing it to make sure that’s the fault? I also tried switching where it plugs in with the connector next to it and that didn’t help either.

:pinch: stumped.

#12

Still having the same issue - as far as I can tell the CPS is ok - there’s continuity in the wires (540 ohm). Any other ideas for tests?

#13

Lots of us have serviceable used CPS’s just borrow one from someone. No moving parts but wiring problems can be hidden by the sheath. I’d send you one but I’m out of town.

I assume you’ve already test the FP relay socket for continuity to the DME?

A couple years ago a guy had these symptoms and the problem turned out to be the FP relay’s wiring socket.

W/o an oscilloscope there’s no test to confirm for absolute certain that DME is receiving square wave from CPS. You can certainly confirm tho that that FP relay is getting signal from DME. At FP relay socket put probes in rear hole and inboard hole. If it doesn’t go hot when starter is run you know FP relay isn’t getting triggered.

#14

Sorry for the late reply, the Summer has been hectic!

So I got some time to work on the car today and made some discoveries.

  1. The fuel pump turns on when the engine is trying to turn over (when the key is in the “START” position). The relay is successfully turning the fuel pump on when the car is trying to start
  2. We’re seeing spark at the spark plugs (The spark plugs are all new)
  3. We removed a spark plug, tried starting the car, and could smell gas in the cylinder
  4. We tried a different CPS that was known to be working on the old engine and the still car wouldn’t start
  5. I’ve tried 3 different ECU’s (one of which wasn’t stock - had the Mark D chip in it)

The car has been sitting since about May with only a quarter tank of fuel. Could it be too low on fuel - fuel that may have evaporated and broken down some in the past few months? I don’t know if that’s an avenue worth going down. I’m not sure what else there is to try. If anyone has any ideas/suggestions I’d sure appreciate them!

#15

I had this problem last week and then I remembered to plug my injector harness back in.

#16

[quote=“kingofpoptart” post=68124]Sorry for the late reply, the Summer has been hectic!

So I got some time to work on the car today and made some discoveries.

  1. The fuel pump turns on when the engine is trying to turn over (when the key is in the “START” position). The relay is successfully turning the fuel pump on when the car is trying to start
  2. We’re seeing spark at the spark plugs (The spark plugs are all new)
  3. We removed a spark plug, tried starting the car, and could smell gas in the cylinder
  4. We tried a different CPS that was known to be working on the old engine and the still car wouldn’t start
  5. I’ve tried 3 different ECU’s (one of which wasn’t stock - had the Mark D chip in it)

The car has been sitting since about May with only a quarter tank of fuel. Could it be too low on fuel - fuel that may have evaporated and broken down some in the past few months? I don’t know if that’s an avenue worth going down. I’m not sure what else there is to try. If anyone has any ideas/suggestions I’d sure appreciate them![/quote]
Check the rail pressure. Just because the pump is running doesn’t mean that the rail pressure is sufficient.

#17

What Jim said. Check the fuel pressure where the supply line fits onto the fuel rail. You can “borrow” a fuel pressure gauge at Autozone or Advance. Aftermarket FPRs are easily had from same type of stores for ~ $50.

Down load the Bentley manual. It has clear instruction on how to check your injectors and relays from the ECU connector. Basically you’ll be supplying power to individual components and make them click. Fuel injectors are bank fire (1-3-5, 2-4-6). You can check them one at a time by disconnecting the other 2.

There’s a little white 2 or 3 wire connector that breaks out of the main harness right at the ECU connector. If it’s not connected to it’s mate, the car won’t start.

KB

#18

Where does the white 2/3 wire connector plug into? (I’m not near the car, and trying to think of what that is)

#19

The connector mentioned (C104) only applies to a car (or engine harness) built before 9/87. It is located near the DME connector. But on an engine harness that has C104, the DME won’t run if that connector is not connected to the plug from the car’s wiring. And if that applies there won’t be spark when cranking the engine.

What car do you have, or more specifically what harness?

#20

Ah, my car is an 88 and the engine is from a '91 convertible. The DME is from the 88, but I also have a DME #0 261 200 173 which I got off EBay - neither one make a difference.