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Tach vasilates and blows fuel pump fuse

On my Bone stock 1987 930 with just 30K miles. It started when at a car show it would not start. My son drove it there for the first time since winter storage which I drove it from a week before, No problems. After a long cranking it started and he drove it home carefully. Next day I start it and drive it up the road and noticed when reaching 2300 RPM the tach jumped to 5K. So i tried it again and it jumped again and then shut down with a smell of burnt fuse. Sure enough the fuel pump fuse was gone. Drove it slowly home after switching with a good fuse. I changed the pump relays still the same. I remembered years ago a tach jump problem with the cdi box. I changed it with a spare and drive a bit. All good until it warmed up then the tach jumps again starting as low as 1200 rpm. Let it sit overnight , drive it and revs up to 4-5K again no blown fuses and tach steady until warm. Then tach jumps from just about any RPM and can smell slight fuse melt but no complete burn through. I hate to take it to a shop as it is show clean and the shop is driving over 25 miles through a marginal part of town. However my hunch is a bad pump I f so how to safely drain the tank if it is the front pump as I have a hoist in my garage? Has the old CDI box been weakened by what ever is causing this problem?

Last edited by voitureltd; 06-23-2008 at 08:04 PM..
Old 06-22-2008, 03:47 PM
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sound like bad voltage regulator to me..also check liquid level in battery
Old 06-23-2008, 07:52 AM
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Yes, hope you checked the Voltage? I have a nasty feeling your alternator might be producing too many volts and burning everything...

IIRC 13.8V is all you want, no more.
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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

Last edited by NathanUK; 06-23-2008 at 12:48 PM..
Old 06-23-2008, 12:38 PM
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Quote:
Originally Posted by NathanUK View Post
Yes, hope you checked the Voltage? I have a nasty feeling your alternator might be producing too many volts and burning everything...

IIRC 13.8V is all you want, no more.
NathanUK, Murf, Thanks for the suggestion. That you both agree is a good sign. I will get a reading on the voltage output tomorrow. In case the test is 13.8 or less, anything else I should check for? At the show when it would not start it acted like a bad fuel pump,or leaky F P check valve as it cranked for close to two minutes before starting. Then it drove home 10 miles. That is when, after sitting for a few minutes test drove it up the block and the tach went way up and blew the fuel pump fuse. In past experiences the tach jumping has generally been a faulty CDI box. After I switched it with another CDI it now only has the tach jump after 10 min. of driving and does not blow the fuse even though it blew the fuse the second time the tach jumped with the original CDI box. That is why I asked if the original CDI box was damaged from whatever as they now display different faults. Also this may lead to a more complicated or multi causal part if this is contemplated with the various effects, or someone has experienced the same situation? I will definitely check the voltage output first. Also of note the last fuel pump relay gets a lot warmer than the one toward the front of the car. I have changed out them both since this problem started and noticed the heat difference between front and rear mounted relays. With the new relays the heat differential is the same as with the originals.

Last edited by voitureltd; 06-23-2008 at 08:05 PM..
Old 06-23-2008, 07:18 PM
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I think they are spot on. if you do need to drain the tank use the filter/drain at the bottom of the tank pretty easy as long as you have a big enough tank. use good ventalation
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Old 06-23-2008, 07:22 PM
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Yup that was it , Voltage regulater 14.3-6 on start up then after 10 min of run tach starts to go up and front fuel pump pitch vacillating much higher. Connect meter and wala 17.2 to 18+ when rev ed up. Pesky intermittent Lucas type gremlins somehow got into the Porsche suppliers food chain.

Last edited by voitureltd; 06-24-2008 at 11:09 PM..
Old 06-24-2008, 03:30 PM
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Glad you found it. I'm not 100% sure what the max voltage is meant to be. I usually just check another car for clarification. I know you have plenty
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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 06-25-2008, 12:23 PM
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