Fuel Pump and Fuel Tank Filter Replacement

[Walter_Middie]

The pictures shown below are not from my car, they are from the post by P Moore:
Fuel Tank Restoration !! Many Pictures

(The P Moore tank is from a 1975 Targa – same year as my car)

Here’s my symptoms (1975 911S Coupe):

The Porsche was my daily driver for over 10 years and a couple of years ago began to run strange and was dying on right hand corners. I brought it to a local mechanic and amongst other small things, they replaced the fuel pump. This fixed my running issues, but the new pump always made a high pitched whine for a few seconds before quieting down. I thought that was just a feature of the new pump and did not worry about it.

Now my new daily driver (Jeep Grand Cherokee) decided that it would not run for more than a few miles before quitting in the middle of the road, so the Porsche has recently been pressed back into daily driving service. Friday at noon the car was making an uncharacteristic high pitched whine, but still running good, so I didn’t worry about it. Then on the way home, idling at a stop light, it briefly ran rough, then immediately died and would not restart. It would fire right up normally for about 1 second, then die. I checked for spark, and that was good – it must be fuel.

Now I usually use the Jeep if the car ever needs towing – which, in the past I have had to tow it to John Walkers Resort for wayward Porsches and their owners, but this time the Jeep was also broken. For the first time since I’ve owned the car (15 years or more) I had to call a tow truck. It’s never left me stranded by the side of the road before. I’ve had bad electrical connections, a drive shaft that came off and destroyed the throttle linkage, and a bad starter twice – but I’ve always been able to MacGyver a roadside fix and drive it home. Not this time.

Troubleshooting:
Once home, I disconnected the fuel line after the pump and found very little fuel would come out when running the pump. This eliminated the accumulator and the fuel filter as a source of the problem, and confirmed that a lack of fuel was the issue. Something is either blocking fuel from the tank to the pump – or the pump is bad again. I removed the pump and bench tested it. I could not get it to pump much fuel – only dribbles.

With the unbelievable cost of new mid-year fuel pumps, many owners are modifying their set-ups with universal pumps and moving them closer to the tank like later cars have. For those not familiar with the 1975 setup, the pump is located under the left rear of the car, just in front of the left rear tire. The pump runs continuously when the key is on (no air-flow plate switch like later cars and no fuel pump relay). A few years ago, a new pump was going for $600, then up to $800, and now they are no longer available. John Walker has a yearly Porsche swap meet here near Seattle, and a couple of years ago I picked up a used pump for $10 as a spare that I could carry on road trips. I bench tested it yesterday for the first time, and pumped fuel like crazy - turns out to be another great swap meet find.

I installed the pump, but could not get it to pull fuel from the tank. I could fill the fuel line from the pump to the tank with fuel, and the pump would work great until that fuel was gone, then nothing but air. I had run the car fairly low on fuel, but there are still a couple of gallons in it, so it should be fine. When I blew air through the fuel line from the pump to the tank, you could only hear air escaping into the tank – no bubbling or gurgling like I would expect if I was blowing air through gas in the tank. So it seems like the small amount of fuel left in the tank is not covering the tank outlet. Here’s what I think is happening:

1.) In the bottom of the fuel tank is a plastic swirl pot and a 3 inch tall fuel filter (note: these pictures are from P Moore, but the same 1975 style tank).





2.) In order for the fuel to leave the tank, it enters the swirl pot and then goes through the filter and out the bottom/side of the tank to the fuel line.
3.) What I think is happening to my car is that the 3 inch tall filter is plugged around the sides with debris but still open on the top. When I blow into the fuel line, air is escaping the top of the filter, above the low fuel level, but no fuel can get out the sides of the plugged filter.

I’m going to drain the tank today and remove the tank filter and see if my prognosis is correct. I’m hoping that I don’t have to remove and clean the entire tank.

I wonder if the filter has been partially plugged for years, and the pump with the high pitched whine for a few seconds was always cavitating at first, until it forced enough fuel through the clogged filter to run normally. This may have caused it to work too hard and fail after only a few years of service.

Some mid-year part numbers for future reference:

911-608-110-03-M244
Universal type replacement, minor modifications may be necessary for installation), 911 (1974-76)
$247.00

911-110-176-00-M14
Fuel Filter (Bosch 0 450 905 003), 911 (1974-76)
$13.00

901-504-932-20-M260
Fuel Tank Seal, 911/911 Turbo (1974-89)
$20.75

911-201-141-00-OEM
Fuel Tank Screen, 911/911 Turbo (1974-89)
$41.00

911-201-143-00-OEM
Fuel Tank Screen Sealing Ring, 911/911 Turbo (1974-89)
$5.00

[E Sully]

I would go along with your thinking that the in tank filter is clogged. I would highly recommend getting the tank flushed out while you are doing this. It is not hard to remove the tank. Disconnect the 2 fuel lines, remove the 3 brackets holding it down, and just lift it out. Just changing the filter and leaving in the crud that clogged the original will most likely cause the same problem again in a shot time. The gasoline cools and lubricates the fuel pump, running it with a restricted flow will burn out an expensive pump.

[DRACO A5OG]

+1 what E Sully wrote:

Damn, I thought my filter was bad:




I just had some debris at the bottom and a piece of metal shaving on the side. I also replaced the fuel pump for safe measure plus filter.

[Cattler]

I recently had a similar problem (in an 87!) with a grey "foam" on the fuel in the tank. I replaced the pump, the short fuel line between the tank and fuel pump inlet (a good, while you're in the there considering the rubber's age), and the engine compartment fuel filter.

I highly recommend you do the latter if you suspect any fuel contamination from the tank itself. We cut the filter apart and found that it had done a good job of catching the crap from the tank and certainly needed to be replaced to ensure good fuel flow.

Now, no issues.

best,
Dave

[Walter_Middie]

I drained the tank using the backward spark plug socket trick I learned from this forum:



My filter was cleaner than I thought possible after almost 40 years:



Not much fuel drained out of the tank, and I was thinking that I had the rear of car jacked up to change the fuel pump and maybe the angle of the car combined with the low fuel level was keeping the new pump from starting. I put 5 gallons of gas in the tank and had the front jacked up now to check the filter. The fuel pump primed - connected all the hoses and the car started right up. Problem solved.

[dshepp806]

When I dropped my screen on my 89 , she looked quite clean.......changed it out, nonetheless,...sending my old screen ass'y to a friend in Australia (free),...Good feeling to see that original screen looking SO clean!!!!!!!!!!!!! A keeper!!!! but a good friend needed it more than I.

BEST!

Doyle

[wildcat077]

Originally Posted by hernadorico

I highly recommend you do the latter if you suspect any fuel contamination from the tank itself.

Do the latter what,
send the filter to Australia ???

[RWebb]

Yes - the Coriolis Force reversal will push all the sediment out of the filter.

[DRACO A5OG]

Originally Posted by Walter_Middie

I drained the tank using the backward spark plug socket trick I learned from this forum:

Damn, better than my Ghetto SST:



Still worked though

[bugstrider]

Nothing like a 5 year bump on an old fuel tank thread...

Does anyone have the torque value for the tank strainer? Been looking but obviously in all of the wrong places.

Thanks

[drtyler]

Bentley manual shows 18 ft-lb.

Originally Posted by bugstrider

Nothing like a 5 year bump on an old fuel tank thread...

Does anyone have the torque value for the tank strainer? Been looking but obviously in all of the wrong places.

Thanks

[bugstrider]

Originally Posted by drtyler

Bentley manual shows 18 ft-lb.

Thanks!

I actually just found it in the mentioned manual.

A case of looking in all the wrong places, lol. Why is it when you post a question, you magically find the answer.....


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