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e p slick's Avatar
 
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Direct wire fuel pumps or not?

Bought an old 1979 930 and have discovered that the fuel pumps have been directly wired to run when the key is in ON position before start-up. It was a home made job and needs to be re-done/cleaned up. My question is: What overwhelming reason is there NOT to leave the pumps directly wired. The car runs fine so I do not see a reason to alter the set up - other than cleaning up the prior owner's sloppy job. Any thoughts?
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Old 03-08-2007, 04:17 AM
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The only argument I can suggest against having the fuel pumps run without the car actually running, is that you run a greater risk of filling the combustion chambers with raw fuel. If you ever just switched on the key to listen to the radio for instance and forgot that the pumps are running - this would be a bad thing the next time you tried to start it and that fuel hadn't evaporated...you could actually hydro-lock the engine. However, I know there are many owners out there that like this setup. If you keep it just continue to be cognizant.
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back in the saddle: '95 993 - just another black C2
*SOLD*: '87 930 GP White - heroin would have been a cheaper addiction...
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Old 03-08-2007, 05:15 AM
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FWIW, I only disconnect this "safety feature", when I need to connect my CIS pressure gauges to test something, and I need the fuel pumps to be running.
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-jeff
back in the saddle: '95 993 - just another black C2
*SOLD*: '87 930 GP White - heroin would have been a cheaper addiction...
"Ladies and Gentlemen, from Boston Massachusetts, we are Morphine, at your service..." - Mark Sandman (RIP)
Old 03-08-2007, 05:20 AM
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One other instace where having this feature connected saved me, was back when my CDI ignition box had it's electrical harness come loose. The car continued to crank and crank and crank but would never start! Had my fuel pumps been running that entire time, I think I would have been in trouble!
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-jeff
back in the saddle: '95 993 - just another black C2
*SOLD*: '87 930 GP White - heroin would have been a cheaper addiction...
"Ladies and Gentlemen, from Boston Massachusetts, we are Morphine, at your service..." - Mark Sandman (RIP)
Old 03-08-2007, 07:28 AM
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The injectors will not fire until the air meter plate moves.

I have my fuel pumps wired that way for tuning purposes, to verify the system fuel presure. Another positive for that method of wiring is that it allows you to presurize the fuel system before you attempt to start the car. Good for cars that hit the roads on weekends only.

As long as you are mindfull of the situation there should be no issues.

On the safety side I believe it could be a hazard in an accident situation, but I'm not certain that the stock configuration cuts the pumps with the key in the run position if the engine dies.
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Old 03-08-2007, 10:00 AM
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I have a switch for gas pumps and msd box in my radio delete panel .
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Old 03-08-2007, 01:49 PM
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My rear BOSCH 044 pump is wired this way... it's pretty loud and annoying when the ignition is switched to the on position. I'm thinking of wiring it to a relay, so it only turns on when the engine is cranking/running.
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Old 03-08-2007, 01:53 PM
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Mine is set up this way, but I have a inertia switch wired in case there is a shunt. Otherwise the fuel pump will keep the engine running.
Old 03-08-2007, 05:09 PM
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Quote:
Originally posted by RarlyL8
The injectors will not fire until the air meter plate moves.
Okay...I feel better now! I didn't realize this, but it makes sense. I had been told by a CIS "expert" years back that when the pumps are running...the engine is getting fuel (including the injectors)...even if it ain't running! I guess I can now refer to this "expert" as a jack-leg-shade-tree-mechanic! But as I think about it and look over some CIS diagrams, what Rarly says makes sense! Sorry for the confusion!
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back in the saddle: '95 993 - just another black C2
*SOLD*: '87 930 GP White - heroin would have been a cheaper addiction...
"Ladies and Gentlemen, from Boston Massachusetts, we are Morphine, at your service..." - Mark Sandman (RIP)

Last edited by sand_man; 03-10-2007 at 03:21 AM..
Old 03-09-2007, 10:30 AM
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Quote:
Originally posted by RarlyL8
The injectors will not fire until the air meter plate moves.
That is correct on a stock car, but maybe not if someone has fiddled with the wiring.
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

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Old 03-10-2007, 05:09 AM
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The wiring makes no difference. The meter plate is activated by air flow alone - nothing to do with wiring of the pumps.

I have many times run my pumps with the engine off for up to 10 minutes or so while checking system pressures and for leaky injectors. To check injector function you gently push down on the air metering plate.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 03-10-2007, 06:04 AM
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Quote:
Originally posted by RarlyL8
The wiring makes no difference. The meter plate is activated by air flow alone - nothing to do with wiring of the pumps.

I have many times run my pumps with the engine off for up to 10 minutes or so while checking system pressures and for leaky injectors. To check injector function you gently push down on the air metering plate.
Yep...understood. That explains why having my gauges connected, the fuel pumps running for several minutes to check fuel pressures all with the engine not running, didn't lead to a flooded situation.
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back in the saddle: '95 993 - just another black C2
*SOLD*: '87 930 GP White - heroin would have been a cheaper addiction...
"Ladies and Gentlemen, from Boston Massachusetts, we are Morphine, at your service..." - Mark Sandman (RIP)
Old 03-10-2007, 07:45 AM
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Cool. I didn't think CIS was that advanced
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg
Old 03-11-2007, 01:32 AM
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