Hi All,

I'm trying to understand why I'm not getting fuel delivered to the engine on my 1990 740 Turbo. Here is my problem:

I know both the in-tank and main fuel pumps work because when I jumper the fuel pump relay, both pumps run and fuel is delivered to the engine with good pressure. However, when I crank the engine (with the fuel pump relay in place), no fuel is delivered to the engine, and I don't hear the fuel pumps turning on. And yes, I have already replaced the fuel pump relay.

My real question is: Is there something that 'tells' the fuel pump relay to turn on during cranking? Why does jumpering the relay turn on the pumps but cranking the engine does not? Also, does the radio suppression relay give the fuel pump relay a signal to turn on?

Deeply appreciate your inputs!

-Kef

The usual suspect in a situation like yours would probably be the crank position sensor.

Have you ever replaced it? Over time they deteriorate. You might even find the insulation on the wire is breaking down.

It is located on the very top front center of the bellhousing. It has a connection about 18" up from the sensor. It is held in place by a bolt with a 10mm head.

Read additional information about it and its replacement in the FAQ's accessed through the drop down window in the upper right hand corner. Care must be taken when removing it so as not to turn into your worst nightmare.

The radio suppression relay provides a ground for the injectors allowing them to pulse (open and close)

Randy

Thanks for the prompt reply Randy!

The crankshaft position sensor is about a year old. I had also checked the resistance and found to be in spec (I don't recall the value now: 168 ohm??). Can you think of anything else?

-Kef

Sorry for the delay in getting back to you.

Electrical trouble shooting is not my strong suit. If the pumps aren't running when trying to start, but otherwise seem to be OK with the jumper in place I would assume you do not have spark. You need to check for that to be sure.

I am adding what one of our users (Lucid) calls his "canned post" on no starts. Hopefully he or others will jump in once you have more information to post back with. Good luck.

lucid wrote:
1) During starter cranking, the Crank Position Sensor sends timing pulses to Ignition Control Unit (ICU)

2-a) The ICU propagates these timing pulses to the Power Stage (aka Ignition Amplifier) to initiate spark at the coil.

2-b) The ICU also propagates these pulses to the FI ECU, to allow FI operation
(no ICU pulses means no FI operation).

3-a) The "System" relay (previously energized at Key On) powers the AMM, IAC, ECU, Injectors, and Fuel relay coil + side.

3-b) When ICU pulses are received, the FI ECU "energizes" the Fuel relay (completes the relay coil ground side) to run the fuel pumps.

When all these things work, the engine runs until the Ignition is switched off, which in turn shuts down the FI system.

Determining whether or not there is spark at the plugs (2a) —and whether or not the plugs are getting gas (3c)— will allow some analysis of this sequence.

For example, if 2a fails due to a bad Power Stage/Amplifier, there will be a no-start with gas-wet plugs. If 2b fails (practically never) symptoms will be Fuel-related: a no-start with spark at the plugs, but plugs remain dry.

Thanks for info, Randy.

I'm afraid Lucid's '2b' might my problem. Here is why I say that:

- I have sparks at the spark plug wires. I know this because I had someone crank the engine and can see sparks coming out of the No 1 spark plug wire tip.

- The fuel pumps can only run if I jumper the relay or pinch the contact. The other contact on the relay [for the radio suppression relay] comes on normally. The fuel line is so pressurized enough when I jumper the relay that when i try loosen the fuel line at the engine, fuel stars spraying.

-The fuel pumps do not go into prime mode at all. I don't hear them whirring with ignition ON.

-I tried this: Jumpered the relay (powered the pumps) and cranked the engine. No luck.

-Every once in a while, the engine whats to start and may be run for a second or two and dies.

I'm eagerly awaiting responses. Thanks in advance!

-Kef

Hi All,

The root cause of my problem turned out to be a bad ECU! As you know the original problem was that the fuel pump was not being turned on when cranking the engine or turning the ignition on.

Lesson learned: If the second switch (contact) in the fuel pump relay does not engage, the cause is most likely to be the ECU. The first contact is to engage the fuel injector relay (radio suppression relay). In my case, the first contact was working.

Thank you Randy for your help!

-Kef

Glad you finally got it sorted out and apparently are up and running again.

Randy