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chuck21401
Joined: 20 Feb 2005
Posts: 526
Location: Annapolis, MD
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 Posted: Sat Sep 05, 2020 11:13 pm Post subject: What would cause a 931 fuel pump fuse to fail? |
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I've been driving my 931 all summer. My daily driver (VW JETTA 1.8T sport) hasn't really moved since I've been working from home because of COVID. Yesterday I went for a drive -- no problems. Last night I started the car to take another drive....the engine ran for 10 seconds and then shut off. Wasn't able to restart it....didn't seem like it was getting fuel. It was dark so didn't try to troubleshoot it. Started reading up on what the problem might be.
This morning I started with the simple stuff...and found that the fuse had gone bad. Interesting that the metal ribbon didn't fail...the ceramic on one end seemed to crumble.
I replaced the fuse with a new 16A fuse and the car started right up. I haven't run it for long. I think I'll let the car idle for awhile before going for a ride...not sure that I can trust it.
So here's the question...why did the fuse go bad?
I believe that the in-tank fuel pump was removed and the car is only running one pump.
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1981 Porsche 931 w/S1 engine & g31 transmission. Water-cooled intercooler
Last edited by chuck21401 on Sun Sep 06, 2020 8:44 am; edited 1 time in total |
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morghen
Joined: 21 Jan 2005
Posts: 8879
Location: Romania
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| Posted: Sun Sep 06, 2020 12:40 am Post subject: |
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Bad wiring...insufficient ground...wrong pump...clogged pump?
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Carrera RSR
Joined: 08 Jan 2010
Posts: 2309
Location: Somerset, UK
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| Posted: Sun Sep 06, 2020 5:13 am Post subject: |
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The wrong pump working too hard due to no in tank pump. Get a Bosch 044. Put the in tank pump back in.
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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Then www.924board.org/viewtopic.php?t=31252 |
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chuck21401
Joined: 20 Feb 2005
Posts: 526
Location: Annapolis, MD
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| Posted: Sun Sep 06, 2020 8:35 am Post subject: |
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| Carrera RSR wrote: | | The wrong pump working too hard due to no in tank pump. Get a Bosch 044. Put the in tank pump back in. |
Thanks. Yeah, I'd have to figure all of that out. This is how it was set up from the previous owner. I went back and looked...the car was resurrected around June 2017...and now the gremlin(s) emerge.
My local dealer has the in tank pump (928-608-013-01) available for $702. Ouch. Another thread on this board mentions a problem fitting the new pumps...apparently they don't fit? http://www.924board.org/viewtopic.php?t=38783. Another thread suggests running one pump: http://www.924board.org/viewtopic.php?t=42062. Anyone else running one pump on these cars?
I need to get under the car again and see what kind of pump is there now. I think it might be a Walbro.
I'm curious to know how many amps the fuel pump is pulling. I ran the car for 3 minutes this morning and the fuse heated up quick. Shoot. Well the good news is that I found the problem in the driveway...rather than going down the highway at 80 mph.
From this thread http://www.924board.org/viewtopic.php?t=41592&highlight=intank it looks like I should check the voltage at various points...starting with the fuel pump.
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1981 Porsche 931 w/S1 engine & g31 transmission. Water-cooled intercooler
Last edited by chuck21401 on Mon Sep 07, 2020 12:41 am; edited 2 times in total |
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chuck21401
Joined: 20 Feb 2005
Posts: 526
Location: Annapolis, MD
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| Posted: Sun Sep 06, 2020 8:35 am Post subject: |
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| morghen wrote: | | Bad wiring...insufficient ground...wrong pump...clogged pump? |
Yes. 
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1981 Porsche 931 w/S1 engine & g31 transmission. Water-cooled intercooler |
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924RACR
Joined: 29 Jul 2001
Posts: 8803
Location: Royal Oak, MI, USA
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| Posted: Mon Sep 07, 2020 8:23 pm Post subject: |
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It seems the in-tank pump is no longer needed; there's a 928 part many have swapped in with success - not a pump, just a filter and pickup. But it's certainly quite possible the pickup on the in-tank pump is badly clogged.
Doesn't hurt to ensure good contacts on the electricals too.
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'79 924 #77 SCCA H Prod racecar
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TJC
Joined: 04 Apr 2010
Posts: 828
Location: Central-ish Arizona, USA
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| Posted: Tue Sep 08, 2020 12:56 am Post subject: |
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| 924RACR wrote: | It seems the in-tank pump is no longer needed; there's a 928 part many have swapped in with success - not a pump, just a filter and pickup. But it's certainly quite possible the pickup on the in-tank pump is badly clogged.
Doesn't hurt to ensure good contacts on the electricals too. |
Very good point regarding the 928 bit and of course making sure there are no restrictions.
Also as an alternative to running an expensive in tank pump, there is no reason a clever person couldn't devise a way to run two external pumps if needed, is there? We've been doing it on race cars for years to assure adequate fuel volume and pressure delivery in a myriad of applications. And there is no law stating that one must stay with a Porsche pump considering all of the applications available. It may take a fair amount of research and design, but it is a possible solution.
As OEM designs become NLA, thinking outside the box is they key! 
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jazz guy
Joined: 26 Nov 2002
Posts: 434
Location: Colorado
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| Posted: Tue Sep 08, 2020 3:09 am Post subject: |
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| TJC wrote: | | ...as an alternative to running an expensive in tank pump... there is no law stating that one must stay with a Porsche pump... |
+1 The Porsche pump is ridiculously expensive. I replaced a bad one years ago and it was quite expensive for the time. If I have to replace another one, I'll figure out how to make it work without the in-tank unit. You might want to pull the in-tank pump to check the intake, I remember that mine was quite clogged.
| morghen wrote: | | Bad wiring...insufficient ground... |
The wiring in our old cars would be high on my list of suspects too. I've had a couple of wires overheat due to high resistance.
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!tom
Joined: 28 Aug 2006
Posts: 1931
Location: Victoria, BC Canada
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| Posted: Wed Sep 09, 2020 4:10 am Post subject: |
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Poor electrical contact on the end of the fuse that is burnt. Clean that contact, or your new fuse will likely suffer the same fate.
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Fasteddie313
Joined: 29 Sep 2013
Posts: 2596
Location: MI
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| Posted: Fri Sep 11, 2020 2:36 pm Post subject: |
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Agree with Tom...
I cleaned the contacts for the fuses in the fuse box by hand spinning a drill bit against them..
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chuck21401
Joined: 20 Feb 2005
Posts: 526
Location: Annapolis, MD
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| Posted: Sun Sep 13, 2020 2:22 am Post subject: |
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I was finally able to get into the garage last night. Checked the voltage in a couple of places.
Voltage at the battery - 13.41 V
Voltage at the fuel pump fuse - 12.30 V
Voltage at the fuel pump - wiring for in tank pump 11.17V
Voltage at the external fuel pump 11.06V
What should the voltage be? 11.06 seems low.
I'll clean the contacts at the fuel pump fuse today...good suggestion.
Here are the connections at the fuel pump. Probably would be a good idea to clean those up as well. The boot for the negative connection was full of dirt when I went to connect the multimeter to it.
So after cleaning up the contacts...next step would be to check the voltage drop in the fuel system? http://econtent.autozone.com:24999/znetcs/psb/en_US/2/0/78/ar1tsb_0307_01_atx.pdf
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1981 Porsche 931 w/S1 engine & g31 transmission. Water-cooled intercooler |
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chuck21401
Joined: 20 Feb 2005
Posts: 526
Location: Annapolis, MD
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| Posted: Tue Sep 15, 2020 11:18 am Post subject: |
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I cleaned the contacts at the fuel pump fuse and the ground in the luggage compartment.
I ran voltage drop tests on the positive and negative sides of the circuit.
.59V on the negative circuit
1.92V on the positive circuit
I found another relevant thread in my searches and reminds me to check the Haynes manual. Needless to say 11.06 volts at the pump is too low. Annoying...but it's good to have more clues. http://www.924board.org/viewtopic.php?t=41592.
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1981 Porsche 931 w/S1 engine & g31 transmission. Water-cooled intercooler |
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Eliza
Joined: 17 Aug 2015
Posts: 174
Location: Prov. Utrecht, The Netherlands
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| Posted: Wed Sep 16, 2020 12:24 am Post subject: Plastic bullet fuses... |
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Just to add my own 0.02...
The original Bosch / bullet fuses were made of Porcelain not plastic.
I found out the hard way ( with just that, car not starting after a run because of the fuse body melting and the tension of the fusible strip falling away ) that the pastic fuses are just a hair longer than the Porcelain ones, and also have rounded ends rather than the more prominently conical ends of the Porcelain ones.
The plastic fuses therefore spread the tangs of the fuse holders a bit farther apart, sit less securely, fuse body eventually touches the metal holder and starts melting. Presto.  No more go.
Electrical resistance and contact corrosion don't help of course, but since I threw all my plastic fuses into the bin and replaced them with Porcelain, I've been good. For a few years now.
See if you can find the Porcelain fuses anywhere at all...
Good luck,
Eliza
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1985 924 NA RoW
1981 931 S2 RoW |
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chuck21401
Joined: 20 Feb 2005
Posts: 526
Location: Annapolis, MD
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| Posted: Mon Sep 21, 2020 6:44 am Post subject: |
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Today I cleaned the contacts for the fuel pump. I used a file on the positive and negative terminals. Did the same thing with the ground terminals in the trunk.
While the fuel pump was disconnected I checked the resistance across the positive and negative terminals. The result was .4 ohms. Is that low? I checked it several times...hovered between .4 and 5. ohms.
I pulled the steering wheel and put my head up under the dash. I'm 6'4" tall so there were some acrobatics involved.  I removed the two screws for the 2nd fuse panel and pulled it down. I saw where the previous fuse melted but nothing else looked unusual. I cleaned up the contacts at the fuse pump fuse, again with a file. I put everything back together and took more readings with the car running.
Connected the multimeter to the fuel pump. 11.38V.
Connected the multimeter to one side of the fuel pump fuse 12.60V.
Connected the multimeter to the other side of the fuel pump fuse 12.40V.
I'm not sure what to do next. I'd run a new wire between the fuse panel and the fuel pump if that would help. I found some new black wire with a green stripe...it's 14 gauge so not as thick as the original wire. The wiring diagram indicates that the original wire is "2,5 mm." Is that closer to 10 gauge or 12 gauge wiring?
Voltage at the fuel pump
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1981 Porsche 931 w/S1 engine & g31 transmission. Water-cooled intercooler |
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Mike9311
Joined: 14 Dec 2004
Posts: 1678
Location: Chicago-ish
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| Posted: Mon Sep 21, 2020 10:55 pm Post subject: |
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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
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