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Join Date: Feb 2014
Location: Westcoast Norway
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Porsche Crest Fuelpump constantly running

I've had some back and forth with my 930's fuel pumps. But now both of them is back to OEM standard. My current issue is that when the key is turned, the pump is constantly running (without motor running). Isn't supposed to stop when the pressure has built up?
Old 02-20-2014, 03:12 PM
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Yes, in stock configuration they should not run unless the engine is running and pulling air down across the metering plate, to deflect the arm and trip a micro switch which in turn energizes a relay and allows the pumps to run. It's for fire safety reasons in the event of a crash, to assure the pumps don't continue to pump fuel when the engine is down.

Face the engine, and with your left hand reach behind the left side of the fuel head to find the blue (or is it green) electrical plug back there. If it is not plugged in, the pumps will run as soon as you turn the key to the on position. Some people leave it unplugged because they want the auditory confirmation that the pumps are running before actually starting the engine.
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Old 02-20-2014, 03:18 PM
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What can be done to have the pump continuously running even with this plug on the fuel head plugged in?

On my '76 turbo this connection is plugged in yet the pumps always ran as soon as the ignition switched on and engine not running.
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Old 02-20-2014, 03:41 PM
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I would check the wiring for the red relays in the fuse box. You can put black relays in those spots temporarily to rule out stuck relays.

You didn't mention if this 930 has been in your possession just recently. There is some bad CIS tuner's out there and they may have defeated the pump switch to mask some other problem, like hard starting cold starts. That is why I ask if you owned this 930 long.
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Old 02-20-2014, 04:16 PM
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The sensor plate switch is hooked up to the yellow relay on the rear relay console. Some older cars have a red relay there instead. It disconnects the grounds to the fuel pump relays in the trunk when the sensor plate is at rest. Disconnect the green plug on the sensor plate switch to disable that and have the pumps come on with the ignition.
A bad rubber socket for the yellow relay or a bad yellow relay can do weird things to that ancient system too. Expand the split pins on all the fuel pump relays with a knife blade and they will make better contact in the rubber relay sockets.
Old 02-20-2014, 05:34 PM
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Jim's right on. Help me with this, it seems at one time I traced the circuit and found that the micro switch is normally closed and opens to interrupt ground - but I may have it backwards. I don't have my wiring diagrams handy to check it (everything has been hauled away to be cleaned of smoke-funk).

It could be as simple as a poor connection at the plug, which would mimic it being unplugged. That, or the relays Jim mentioned.
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Old 02-20-2014, 07:32 PM
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Thanks for good suggestions gentlemen.
I could possibly elaborate a bit more.
The car was brought over to Norway in 2011, and right away there was some funky with the fuel injection (to lean on high revs). The problems was non functioning front pump. Both pumps where changed for Bosch 044 pumps, not the best move since the fuse started to break constantly. The relay had a small melt-down by it self. After some google'ing (splitting up the line to the pumps with two fuses)and getting new pumps (original ones) and new relay's. The car was running as normal, but the pump is now constantly running..
Old 02-21-2014, 02:36 PM
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I think the metering plate micro-switch is wired in the normally closed (NC) position. If you use an ohm meter across the pins of the female socket on the fuel distributor, you should see '0' ohms with the metering plate fully closed and 'infinity' with the metering plate depressed.

I think JFairman may be onto something with the engine compartment relay.

Be careful with the '044' Bosch pumps. Porsche wiring for the fuel pumps was just adequate for the amperage draw for the original pumps. The '044's draw a little more amperage. Measure voltage at the battery source and then at the fuel pump terminals. The difference between the two readings is the amount of power wasted through the wiring and relays.
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'85 930 Factory Special Wishes Flachbau
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Old 02-22-2014, 04:45 AM
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