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What would cause a 931 fuel pump fuse to fail?
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chuck21401  



Joined: 20 Feb 2005
Posts: 402
Location: Annapolis, MD

PostPosted: Tue Sep 22, 2020 4:14 am    Post subject: Reply with quote

Mike9311 wrote:
When you are checking at the pump, do you check both sides of the terminal? Wire side and pump side of the connection to check if power is truly getting to the pump


I think I understand what you mean. I attached the alligator clips to the nuts on the fuel pump rather than to the terminals at the ends of the wires. The car does run, so the pump is getting power...just not 12V +.

I tested for voltage drop on the positive side again: 1.90V

I suppose positive wire (black/green) between the fuse box and the fuel pump could be part of the problem. How can I test that? Disconnect the positive wire from the fuel pump...pull the fuse from the fuse box...and measure the resistance?
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!tom  



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PostPosted: Tue Sep 22, 2020 4:48 am    Post subject: Reply with quote

What voltage drop are you looking for?

There will always be voltage drop whenever significant currents are involved, such as running the fuel pump. There are several connections, and many feet of fairly small wire running from the fuel pump relay to the fuel pump. If the fuel pump is pumping sufficient fuel, isn't making too much noise, and isn't blowing fuses (once bad connections that cause overheating have been resolved), then what does it matter if you're getting the expected 0.5 volts drop (for perfect wiring) or a bit more?

I'm not exactly sure where you are measuring the voltage across, but there's always checking the ground for the fuel pump as well...
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chuck21401  



Joined: 20 Feb 2005
Posts: 402
Location: Annapolis, MD

PostPosted: Tue Sep 22, 2020 5:46 am    Post subject: Reply with quote

!tom wrote:
What voltage drop are you looking for?


Well I'm not sure exactly...but nearly 2V seems like too much.

Quote:

There will always be voltage drop whenever significant currents are involved, such as running the fuel pump. There are several connections, and many feet of fairly small wire running from the fuel pump relay to the fuel pump. If the fuel pump is pumping sufficient fuel, isn't making too much noise, and isn't blowing fuses (once bad connections that cause overheating have been resolved), then what does it matter if you're getting the expected 0.5 volts drop (for perfect wiring) or a bit more?


I'm seeing 13.41V at the battery and 11.38V at the fuel pump. Shouldn't the voltage at the fuel pump be around 12V? If not, the pump will pull more amps...and in turn heat up the wiring/blow fuses?

When I run the car for short periods of time the fuel pump fuse and fuse board heat up quickly. So doesn't that indicate too much resistance in the circuit? I suppose I could keep the car running for awhile to see if the fuse blows/melts again. All of the connections look OK to me.

Quote:
I'm not exactly sure where you are measuring the voltage across, but there's always checking the ground for the fuel pump as well...


I traced the ground wire from the pump to the trunk and cleaned up the contacts. The first voltage drop test on the negative circuit was .59V. I'll try that again. I didn't test it after hitting the contacts with a file. I could also run a temporary new ground wire from the pump to the ground point to see if that makes a difference.

The Haynes manual suggests checking current consumption and that "a current consumption of no more than 8.5 amps should be seen." I'll try that test next.

Thanks for your help.
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chuck21401  



Joined: 20 Feb 2005
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PostPosted: Tue Sep 22, 2020 11:26 am    Post subject: Reply with quote

Tested for current at the fuel pump. Hovered between 8.81 and 8.85A. Doesn't seem abnormally high?







I was looking at the wiring diagram for the fuel pump. Apparently there T2 is a connector between the additional fuse board and the fuel pump. The diagram indicates that it is near the fuse/relay board, though I don't remember seeing one. I'll take another look.


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chuck21401  



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PostPosted: Sat Oct 03, 2020 7:58 am    Post subject: Reply with quote

So this is the wrong wiring diagram.
Quote:


I pulled down the additional fuse board -- and found more wires connected than I expected.

The turbo diagram is different now that I look at Haynes. And there should be a 25A fuse (blue) not a 16A fuse (red)...

Here is the turbo current flow diagram. This circuit runs the fuel pump, supplementary air valve, warm up regulator and the frequency valve (show on next page -- see the other black/green wire).


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Mike9311  



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PostPosted: Sat Oct 03, 2020 10:51 am    Post subject: Reply with quote

Did you switch it out yet?
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chuck21401  



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PostPosted: Sat Oct 03, 2020 11:16 am    Post subject: Reply with quote

Mike9311 wrote:
Did you switch it out yet?


Almost done. Remounted the fuse panels. Need to bolt on the steering wheel and reconnect the fuel pump. And of course the battery.

I checked the manual again, I remember verifying 16A fuse for the fuel pump.




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chuck21401  



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PostPosted: Sat Oct 03, 2020 12:02 pm    Post subject: Reply with quote

With 25A fuse for the fuel pump, 11.56V at the fuel pump



Haynes calls for at least 11.5V



Going for a ride to see what happens.



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Mike9311  



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PostPosted: Sat Oct 03, 2020 12:14 pm    Post subject: Reply with quote

chuck21401 wrote:
With 25A fuse for the fuel pump, 11.56V at the fuel pump

Going for a ride to see what happens.




Voltage should be higher once you are driving so enjoy
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1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
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1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car
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chuck21401  



Joined: 20 Feb 2005
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PostPosted: Sat Oct 03, 2020 1:38 pm    Post subject: Reply with quote

chuck21401 wrote:

Going for a ride to see what happens.


Everything seemed fine. Fuse panel didn't heat up like before. Drove around for about 30 minutes. Will see if the gremlin comes back.
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chuck21401  



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PostPosted: Sun Oct 11, 2020 1:16 pm    Post subject: Reply with quote

chuck21401 wrote:
Will see if the gremlin comes back.


Well that didn't take long. The following day I took the car for a 20 minute ride. Stopped by the beer store to pick up a 6 pack. Went to start the car and it was running rough...on 2 or 3 cylinders. Revved up the motor, not much improvement but was still running. Checked the fuel pump fuse - in place and not hot. Popped the hood...hoping to find a plug wire loose or something -- nothing obvious. Decided to try and limp home (2 miles). Seemed like the engine wasn't getting enough fuel. I was able to get into 3rd gear...then the engine came back to life! Pulled into the driveway...blipped the throttle...back to running on three cylinders. Shoot. Drank 6-pack and read through old posts about fuel pumps, fuses and relays.

The next day I popped the hood looking for obvious problems. Coincidently the engine stopped at top dead center (dot aligned with timing mark). Popped the distributor and the rotor was pointing to plug wire going to #1 plug. Pulled the wires going to the ignition module by the headlight, didn't see corrosion, also checked the grounding points. Checked the wiring going to the distributor (green wire maybe) and it was secure. Started engine, still not running right but listened to fuel pump, seemed fine...wasn't making odd sounds. Based on old threads I decided to get a new fuel filter. Ordered it from Rock Auto and had it two days later.

Pulled the old filter. A mix of brown and black sediment poured out. Ewww. But I thought that might be a good sign in terms of tracking down the problem. Installed the new filter. Cranked it a couple of times...engine came to life...and back to normal. Well that's good sign but made me wonder what else might be in the tank. Went for a 30 minute ride no problems.

The next morning I started it up to go for another ride. Was sitting there reprogramming the presets on the radio and the engine skipped a beat. Uh oh. Blipped the throttle -- back to running on 2 or 3 cylinders. Shoot. Turned off the car and walked away.

Just for fun I blew out the old fuel filter and run some carb cleaner through it. Not sure if you are supposed to do that. The old fuel filter didn't seem clogged so I put that one back in. Engine came back to life.

Next step will be to drain the tank and pull the tank screen (remember I'm only running one pump, previous owner replaced in-tank pump with the screen from a 944). Will be interesting to see what I find.

Was also thinking about adding a small in-line filter before the fuel pump. One of those inexpensive clear plastic filters. Thoughts on this idea? Found that idea from a fellow 931 owner: http://www.maxtakeoff.com/?p=542. Also want to replace the rubber fuel line from the tank to the pump. I've read that standard rubber line doesn't hold up well to ethanol. I plan to cut open the old line to check the condition.

Update: The line from tank to the pump is 944 part rather than generic rubber hose. Also seems to be larger than 3/8 ID. Rethinking the idea to add a filter here. Will see what the tank screen looks like first.




I'd really like to get to the bottom of this problem so that I can finally get around to installing the intercooler!
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chuck21401  



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PostPosted: Tue Oct 20, 2020 3:38 am    Post subject: Reply with quote

Well I don't think that there is a problem with the fuel filter or tank. The car started running rough again. I disconnected the negative battery terminal before starting the project to drain the tank. Just for fun, I reconnected the battery terminal...and tried again. The engine fired up just fine. Drove around the block a couple of times...runs fine. What in the world? I guess I need to have it fail again and run some tests, not sure how to find the gremlin when the car is running fine.

Any ideas? What can intermittently fail...and gets "reset" by reconnecting the battery?

One more oddity....the car radio powers on and cuts out when the car is running rough. Went I reset by disconnecting and reconnecting..the car radio works fine! The radio comes on with the key on....I believe that is the stock set up. So I'm thinking that this is some sort of clue.
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chuck21401  



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PostPosted: Tue Oct 20, 2020 3:41 am    Post subject: Reply with quote

I decided to drain the tank anyway since I have not done that before.

I put the rear of the car on ramps. The ramps are from Harbor Freight, so I left the floor jack under there, just in case.


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chuck21401  



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PostPosted: Tue Oct 20, 2020 3:48 am    Post subject: Reply with quote

Next I gathered everything I would need. Gas can, tools, hopefully not the fire extinguisher.



I bought these clamps from Harbor Freight for $9 to pinch the fuel line. I used the one marked 25mm (1").



Here is the fuel line from the tank to the pump



The clamp worked great. Only a small amount of fuel leaked out once I removed the hose clamp on the fuel pump. Caught that with a plastic pan and some rags.



Draining the tank into the can. Very simple. The tank was almost empty; probably drained 2 gallons.



Removed the hose. On my car the 2nd pump was replaced with a tank strainer (part number 928-201-081-04). I used a 17mm wrench to remove it; a 17mm deep socket also works.


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chuck21401  



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PostPosted: Tue Oct 20, 2020 3:51 am    Post subject: Reply with quote

Here is the tank strainer. It's hard to see, but there are some specs of rust. But not much and no holes in the screen.

I was able to pull some rust out of the bottom of the tank using the end of a rag. I was expecting a lot more thinking that my fuel filter was getting clogged...but I don't think that is my problem.



I put everything back together and went for a 30 minute ride; no problems.
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