Fuel Pump Cuts out while driving

Tiny
MACDOJ17
  • MEMBER
  • PORSCHE
I have a 1977 Porsche 911 Targa. Ever since I bought it I have been having issues with the car dying. Typically it happens more so when I first get the car going, not as much if I get it going later on in a long drive.

What I have found is if it dies while I'm moving I can turn the ignition off, and turn the key back over just to where the electrical turns back on, then the pump kicks back on and everything runs great.

So I read a little on my 101 Projects for your Porsche book and thought changing out the fuel pump would be my best bet. So this weekend I replaced the fuel pump and fuel filter. However, I still had the same problem. So now I have a hunch that it is something electrical. The alternator light always is glowing on the car. Sometimes it is brighter than other times. I don't think the battery is ever low, because the car always turns over. However, I think that it might not be a steady flow of power and maybe that is causing the issue. I'm not sure. This light problem was documented back in '98 by the previous owner as he took it to a repair shop.

Anyway I'm looking for any suggestions on this. Is there a way I can check the flow of power to the fuel pump? Or are there any other thoughts.

Thanks.

Jesse
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Sunday, September 23rd, 2007 AT 9:20 PM

11 Replies

Tiny
KEN
  • ADMIN
One of two things are happening, either the fuel pump relay is shorting or the main controller ECM that controls the relay is shutting down. I would try a relay first. Please let me know
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Saturday, September 29th, 2007 AT 4:06 PM
Tiny
MACDOJ17
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So far I went and pulled out the relay for the fuel pump. It was clear this relay had seen better days. In fact it was very loosely in there and upon further examination I found it probably was shorting out a bit. It was black inside on a spot and had melted away some of the plastic on the cover of the relay and on the outside of the relay (where you plug it into).

So I got a new relay, put it in and nothing. I even went back to the old relay. Nothing again. The fuel pump doesn't even kick in now. I haven't been able to spend much more time troubleshooting it yet, but I wanted to provide this update.
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Tuesday, October 2nd, 2007 AT 6:08 PM
Tiny
KEN
  • ADMIN
I think you found the source of the problem, or the result, check all connection at the relay make sure the connection are all pushed up into the relay holder and reassemble to recheck, if ok, try supplying 12 volt the fuel pump to see if that doesn't work for you. You might have a shorted fuel pump causing the problem.
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Thursday, October 4th, 2007 AT 6:12 PM
Tiny
MACDOJ17
  • MEMBER
Just to update I have a few more notes on the fuel pump.

I came across a 12v circuit tester. It seems as if the relay has good connections to it. Although in doing testing it only clicks sometimes now, not everytime I turn the key. There wasn't any power going to the fuel pump. So far all of the fuses check out. So I'm looking over the wires to make sure I don't see anything that is loose. One thing that was interesting was one of the empty fuse areas had power going to it somehow. There are no wires going to it but yet power was going to it (as checked by the circuit tester on the metal contacts for the fuse).

Hopefully this weekend I'll have more time to find out more about this.
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Thursday, October 11th, 2007 AT 9:23 PM
Tiny
MACDOJ17
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I confirmed that the fuel pump does in fact work. I ran a direct line from the battery to the pump and it fired up right away. So I'm looking to trace the wires back to see what the problem could possibly be.
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Saturday, October 13th, 2007 AT 4:01 PM
Tiny
KEN
  • ADMIN
Do you have a wiring schematic? if not you can find one hear. http://www.2carpros.com/kpages/auto_repair_manuals.htm
please let me know.
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Monday, October 15th, 2007 AT 10:34 AM
Tiny
MACDOJ17
  • MEMBER
I already had the Haynes manual for this car. It included the wiring diagram. I had someone help me a little with learning how to read these. I have done my best. (He suggested I make copies so I can write on them and make into one large document instead of flipping through 5 or more pages, that worked out well).

I have found out a few things. 1) The wiring from the relay to the fuel pump checks out. 2) The wiring outside the ignition switch to the relay checks out.

Also the same wire (yellow wire) off of the ignition switch splits off and goes to the starter. When I turn the key the starter turns (as it should). If I run direct power to this particular wire the starter turns, but the fuel pump does not turn on. (I have directly wired the fuel pump and it does work).

Now of course the car did work (not great) until I replaced the relay. Then the fuel pump refused to kick in at all.

I went to the local car place and got the new relay. It is identical (or seems to be) except for two things. 1) The number on the original is: 911.615.108.00 and the new one is 911.615.108.01. 2) The only other difference is the new one is made in China.

The other relays in the car have the same number as the new one. However, they are covered in a black plastic casing instead of a red one.

The only other information I have is that I have run the fuel pump by direct wire (then disconnected) and thought that I should then be able to get the car to fire, even if just for a split second. However, the starter turns the engine and turns and turns, but it never actually fires and then dies.

If anyone has any thoughts let me know. Thanks.
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Sunday, November 11th, 2007 AT 10:09 PM
Tiny
KEN
  • ADMIN
The car runs with the fuel pump relay jumped we know that, so the relay is not engaging, we need to trouble shoot the computer to see why the ecm is not turning the system on. Does the car have a mass air flow sensor? If so the computer must see activity through the sensor to start the fuel pump relay. Is it possible to scan the computer in that car?
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Tuesday, November 13th, 2007 AT 9:52 AM
Tiny
MACDOJ17
  • MEMBER
I looked over the book tonight and don't see anything on an air flow sensor. However, I don't have the best in the manuals (as those look to be over $500). I don't see anything on the ECM either. The only thing I see in the wiring diagram that might be of interest is the warm up regulator as this looks to be connected to the fuel pump relay in some sort of fashion. At this point I'm beyond my or my relatives understanding of the electrical system.

As far as hooking this up to a computer I'm not sure. I think if I did that I would just take it in and get this problem fixed. Which could end up being my choice. I also was looking around and found something (maybe this is where the computer hooks up? Not sure, looks like something should plug into this thing, I'll attach the picture.

Anyway I'll probably keep looking things over. If you have any thoughts or ideas let me know. I appreciate all of your help so far.


http://www.2carpros.com/forum/automotive_pictures/102027_carplug_1.jpg

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Tuesday, November 13th, 2007 AT 10:38 PM
Tiny
KEN
  • ADMIN
Yep, with porsche there are some specialized test equipment that needs to be purchaced to trouble shoot electrical problems. Let me know what the shop finds.
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Wednesday, November 14th, 2007 AT 9:42 AM
Tiny
MACDOJ17
  • MEMBER
I got the car back from the shop. They replaced he fuse box as the back metal portion on the back was no longer one solid piece. Because of this there was not a good connection to the fuse that got the fuel pump and ignition going. I believe they also did some rewiring to bypass the fuel pump relay wire to the air meter. There was some other wiring cleanup they had to do as well.

So this is the end of the fuel pump issue.
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Thursday, December 6th, 2007 AT 8:40 PM

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