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General Forum : Fuel pump testing

- BMW E21 Community
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      - Fuel pump testing
DP57   Posted Friday, Apr 6th 10:59pm [Edit] [Quote] [IMS] [View car]
Junior Member
Post nr. 21

'82 320i, '94 318i
What's a good way to test a fuel pump? I've got my motor back in and cranking good, but I've got no fuel. I'm trying to figure out for sure if one or both pumps are bad before I go an order new ones. I just want to make sure that I'm not missing something in the wiring first. I haven't had anything on for three years, so I could have easily missed something. I've got some current at the actual pump under the car, but I can't figure anything at the sending unit in the tank...any ideas? Should I just drop the whole tank and have a peek anyway?

melloh   Posted Friday, Apr 6th 11:21pm [Edit] [Quote] [IMS] [View car]
Post nr. 527
California, USA (San Jose)
1980 320is, 1991 318is
I just did some testing on my fuel system a little back, but my memory sucks. I think you can do a quick check by jumping pins 87 & 15 on your fuel pump relay (bosch manual suggests it have an 8 amp fuse... I dont have one so I didn't) and turning the car to the on position. You can check to see if fuel is pumped by either removing the injector or the banjo bolt to the distributor. At the "fuel distributor return line", if you run the pump for 30 seconds, 750 CCs should come out (you'll need a measuring beaker of some sort). 1 cc = 0.001 liter.

This is according to the Bosch manual.

If this don't get you going, then you'll have to determine if it is electric (no power to pump...) or a mechanical (clogged filter, clogged in-tank filter, clogged lines...) prob. If you got it jumped and nothing comming out, take out the rear seat bottom, and on the passenger side listen to whether or not the pump is going or test w/ multimeter.

HOpe this is enough to get you started (and that someone will chime in w/ more details or, i hope not, corrections).

Much luck, and keep us posted!

DP57   Posted Saturday, Apr 7th 1:14am [Edit] [Quote] [IMS] [View car]
Junior Member
Post nr. 21

'82 320i, '94 318i
I've got the feed line to the distributor off and it's just full of air. When I turn on the ignition I hear no hum anywhere. I'm going to go pick up a multimeter this weekend and see what kind of numbers I'm getting from the sender/pumps and I'll try jumping 87 & 15 as well to see if that helps...I'll be sure to update, thanks.

By the way, anyone know what kind of voltage drop we should be seeing across the pump or resistance or anything like that?

melloh   Posted Saturday, Apr 7th 9:05am [Edit] [Quote] [IMS] [View car]
Post nr. 529
California, USA (San Jose)
1980 320is, 1991 318is
My understanding is that you should be getting ~12v at the pump. If you take it out or somehow directly energize it w/ 12v (be sure its grounded) it should run.

Oh... i know this is lame, but you did make sure there's fuel in the tank, right?

DP57   Posted Sunday, Apr 8th 11:58pm [Edit] [Quote] [IMS] [View car]
Junior Member
Post nr. 21

'82 320i, '94 318i
I'm just checking them (and fuel) now...should there be 12V at both the in-tank sending and at the external pump?

melloh   Posted Monday, Apr 9th 9:02am [Edit] [Quote] [IMS] [View car]
Post nr. 537
California, USA (San Jose)
1980 320is, 1991 318is
You know.. I honestly don't know a thing about the external pump. I don't even know where mine is. Is it the one next to the fuel filter? Anyway, I'm not sure. Hopefully someone else can answer this b/c last time I tried to check voltage, I got 0, yet running a fuel pressure test show at least one of them was running. I obviously did something wrong.

Greg323i   Posted Monday, Apr 9th 7:05pm [Edit] [Quote] [IMS] [View car]
Post nr. 318
Melbourne, Australia
1982 Hennaröt 323i
There will only be voltage at the pump when the engine is running or turning over or the relay is jumpered, but yes, it will be whatever the battery voltage is: 12.6V-14.4V.

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