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Fuel pump question 1977 911s 2.7Lt

Wondering if I should be able to hear the fuel pump buzzing prior to cranking the engine, on both hot and cold starts? Currently, I can hear the pump buzzing only occasionally prior to cranking.
I've had some ignition issues in the past, however, thinking a cold start issue may be fuel pressure related. p.s Takes a long time to get to drivable ideal and power seems to be down
Old 10-11-2018, 11:45 PM
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no it does not run, only when engine is running and cranking.(have not looked at 77 wiring t6o verify that).

if you have problems after it is started, that is probably causing your cold start issues too.

if you suspect a fuel pump problem you need to check fuel flow volume and pressure.
cold control pressure can also effect hard starting and bad running when cold.

air leaks will also effect both of those issues.

first, fix your ignition issues.
check and fix air leaks
check fuel pressures, system, cold and warm control pressures.
check residual fuel pressures.
check the mixture

you can also check the inlet screen to the WUR. if clogged it will raise your CP and make cold starts hard.

you can remove the electrical connector on the top rear of the AFM. that will run the pumps with the key on and engine off, just as a test to see if fuel pressure is an issue.
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Old 10-12-2018, 03:52 AM
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My 75 the pump runs when key is turned with the starter not engaged. I think 2.7l engines all function in this manner. 78s have a switch on the air flow sensor that starts the pump when the motor is cranked
Old 10-12-2018, 07:21 PM
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Fuel pump operation .........

Quote:
Originally Posted by 47silver View Post
My 75 the pump runs when key is turned with the starter not engaged. I think 2.7l engines all function in this manner. 78s have a switch on the air flow sensor that starts the pump when the motor is cranked

FP operation for CIS model years (73.5~75) is different from 76~77. The MY (76~77) operates similar to 78~83 because they all have the AFS switch and FP relay.

Tony
Old 10-12-2018, 07:50 PM
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Thanks, however, I'm still confused. Can't hear the pump today prior to cranking but yesterday I could. The car still starts at operates as previously described. I don't have the proper tools to measure the fuel pressure, however, I do have a new fuel pump I purchased a few years back. Might be best to use the opportunity to clean the fuel tank and replace the pump.
Thanks again.
Old 10-12-2018, 11:24 PM
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pull the connector and see if it runs.

FP relay

spin the fuse
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 10-15-2018, 03:55 AM
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Fuel pump test.........

Quote:
Originally Posted by 77 911 View Post
Thanks, however, I'm still confused. Can't hear the pump today prior to cranking but yesterday I could. The car still starts at operates as previously described. I don't have the proper tools to measure the fuel pressure, however, I do have a new fuel pump I purchased a few years back. Might be best to use the opportunity to clean the fuel tank and replace the pump.
Thanks again.


A running fuel pump is only half of the story. If you don’t have a pressure gauge, use volumetric flow rate. Test run the FP and measure the fuel flow delivered by the FP per min. You need at least a liter per 30 sec. (or 2 liters per min.). Keep us posted.

Tony
Old 10-15-2018, 04:22 AM
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