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RIP
RIP, Lead Technician
Category: BMW
Satisfied Customers: 5591
Experience:  BMW Factory Training, ASE Master Techinician, Advanced engine performance.
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88 bmw 735i. Bad fuel pump...changed pump.

Resolved Question:

88 bmw 735i. Bad fuel pump...changed pump.   pump wont run unless I jump the wires on the fuel pump relay. whats that mean?
Submitted: 6 years ago.
Category: BMW
Expert:  RIP replied 6 years ago.

Hi, welcome to Just Answer.

  1. Will the engine run with the jumper in place?
  2. How many miles are on this BMW?
  3. Have you replaced the relay yet?

Rip

Customer: replied 6 years ago.
Reply to RIP's Post: didnt try it with the jumper in, it shows 225,000 on odometer but I was told the engine and trans has only 25,000 on it. Didnt buy new relays but moved some of them around
Expert:  RIP replied 6 years ago.

With the jumper in place...see if the engine runs. Also, how did you determine the old fuel pump was faulty?

Rip

Customer: replied 6 years ago.
Reply to RIP's Post: tried it without the relay and with the jumper wire. pump pumps but car still wont fire. I took the pump out and bench tested it and it wouldnt run. Smacked it a couple times and it started so I put it back and it quit again. So I replaced it
Expert:  RIP replied 6 years ago.

Alright, you may have a bad computer main relay or the engine computer. The engine computer triggers the Main Relay, which in turn triggers the fuel pump relay. If the main relay is not the problem the engine computer might be. With the fuel pump jumpered, it should allow the engine to run. Switch out the Main Relay, if the engine still doesn't start...tap on the engine controller gently while someone cranks the engine and see if it fires than.

Main Relay Location.

graphic

RIP, Lead Technician
Category: BMW
Satisfied Customers: 5591
Experience: BMW Factory Training, ASE Master Techinician, Advanced engine performance.
RIP and 8 other BMW Specialists are ready to help you
Customer: replied 6 years ago.
Reply to RIP's Post: put a main relay in from my friends car and it didnt do much but that was before I put the pump in. where isthe engine controller? and does a crankshaft sensor sound like it could be a problem cause I heard that could make it not run.
Expert:  RIP replied 6 years ago.

The crankshaft sensor could be a possibility, but if you are not getting both spark and no trigger to the fuel pump relay, it further indicates a deeper problem. If you are getting good spark, than you can elliminate the CKP sensor. The engine computer is also in the "E" box with the fuel pump and main relay.

Rip

Customer: replied 6 years ago.
Reply to RIP's Post: sorry for the delay, I forgot to say that I tried a good shot of either and it started for a sec or two then stopped, wouldnt that mean that the engine com. is ok?
Expert:  RIP replied 6 years ago.

It doesn't mean that the engine computer is o.k., but it means that the ignition spark and engine compression is good. So either your problem still lies in just the fuel delivary or the fuel injection. Do you have access to a fuel pressure guage?

Rip

Customer: replied 6 years ago.
Reply to RIP's Post: no but when I hot wire the pump its not like the gas shoots out of the line it just flows out. is that a problem?
Expert:  RIP replied 6 years ago.

Well, that's difficult to interpret without a guage..it should shoot out. Did you replace the fuel filter? Are any of the hoses kinked? Did you install a NEW pump? Are you positive the fuel level is sufficient for the pump to pick up? You also need to be positive none of the lines at the pump are loose or cracked and sucking in air...if so that will reduce the pressure and flow.

 

Rip

Customer: replied 6 years ago.
fuel filter is new, pump is used. Also, where is the pressure regulator and is there any way to check it. and could that be the problem? I dont understand why either would start it but it wont start otherwise.
Expert:  RIP replied 6 years ago.
Fuel pressure regulator is on the fuel rail...it cannot be the problem. Sounds like a bad pump or air sucking in through a line. We need to get a pressure test at the fuel rail to be sure. If it starts on starting fluid, that means that the fuel is not entering the engine. If the pressure is low, it will not spray through the injectors...if the computer is bad, it will not trigger the injectors. I doubt it's a computer, becasue from your last description the original fuel pump seemed faulty, so it would be odd to have a bad pump and another problem as well. I would recheck the pump installation after a fuel pressure test, we're looking for around 47 psi of pressure at the rail. If you want to elliminate the fuel injector pulse, you can purchase a Noid Lamp and install it on a fuel injector connector. When the engine is cranked the lamp should pulse, if it does, than our only problem is the fuel pump pressure or flow. Be sure the fuel filter was not installed backwards, it can restrict flow. The filter should have lettering on it indicating "IN" and "OUT."

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