1993 BMW 325 Fuel Pump

BABYBOY4U4684

MEMBER
1993 BMW 325
6 CYL
2WD
AUTOMATIC
125,000 MILES

Hey I have a 93 bmw 325 is and I just recenly purchased a fuel pump for it cause the last one was defective now when I replaced the fuel pump and I put the brand new one in it I noticed when I tried starting the car it would not start it is trying to turning over but I noticed I am not hearing anything from the fuel pump so I figured maybe the fuel pump was defecting and when I tested it to def make sure it was defective by taking the fuel pump out and jumping it off of a battery well the fuel pump kicked on and worked so I tried putting it back in my car but it does not kick on when it is in the car do you have any idea on what this could be please let me know you can email me a babyboy4u4684@gmail. Com. Thanks again and I hope you can help me resolve this issue iam facing

thanks rob

1 Reply

DR LOOT

MECHANIC
2,311 POSTS

You've established that the pump operates if given power, so the question is: why isn't it getting power?

Here is a simple test that will help isolate where the problem is. Pull the fuel pump relay and jumper it so that power goes to the fuel pump. If you hear the pump turn on, then your problem is related to the control of the relay which comes from the DME (the engine computer). If the pump doesn't turn on, somewhere between the relay and the pump, you have a bad connection. According to Bentley( a book publisher of technical manuals for the BMW), the relay is in the number one position on the diagram shown.

When you pull the relay, you will find one pin that is pointing in a different direction than the others. When you look at the connector the relay came from, imagine that that pin is pointing to 12 o'clock. The correct pins to jumper will be at 6 and 9 o'clock. I use a paper clip suitably reconfigured.

If the fuel pump doesn't turn on when you jumper the relay, then either you have misread my instructions and jumpered the wrong connections, or your wiring connections to the pump have a problem. Start by rechecking the ones you removed when you replaced the pump.

On the other hand, if the pump does turn on, then you have a problem in the control of the relay. Likely causes in this case are:

1. the fuse is blown (but, I assume you checked that first)
2. the DME is bad
3. the crank position sensor is malfunctioning, because the DME won't turn on the fuel pump unless the position sensor sends it a signal.

To test for number 3, pull the coilover for a sparkplug and have a friend put his finger in it while you crank the engine. If he jumps and swears at you, the position sensor is working. If you determine that there is no spark, then likely the crank sensor is bad. I've seen this happen several times. Start by replacing it because it's cheaper than replacing the DME. If it's not bad, then the DME probably is.

Good Luck
Dr. Loot's Assistant

EDIT: ok, it's probably better to find a different way to check for spark... don't have your friend hold the coilover while you crank the engine.. Put a sparkplug in it and see it is firing.