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Fuel pump loses pressure when hot.
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wafflecopter
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PostPosted: Tue May 30, 2017 1:40 pm    Post subject: Fuel pump loses pressure when hot. Reply with quote

Ive got a 71 super and the pump puts out about 6psi at idle and after about a minute of driving it stalls out and then will fire up again after 30 seconds and idle but pressure drops to about 0.5 psi. Any ideas?
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PostPosted: Tue May 30, 2017 2:23 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

wafflecopter wrote:
Ive got a 71 super and the pump puts out about 6psi at idle and after about a minute of driving it stalls out and then will fire up again after 30 seconds and idle but pressure drops to about 0.5 psi. Any ideas?


Waffle - there are about 12,031 types of fuel pumps out there, you need to identify exactly what you have. I believe a 1971 originally had a dome-top Pierburg with a screw on top of the dome. and my guess is that yours does not have that.
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PostPosted: Tue May 30, 2017 2:45 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

Remove the pump and check the pump push rod. The rod needs to slide inside the plastic block with zero effort. If it is sticky at all it could get stuck when the engine heats up.
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wafflecopter
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PostPosted: Tue May 30, 2017 2:46 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

Its a generator style aftermarket with the dome crimped on.
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PostPosted: Tue May 30, 2017 2:48 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

Q-Dog wrote:
Remove the pump and check the pump push rod. The rod needs to slide inside the plastic block with zero effort. If it is sticky at all it could get stuck when the engine heats up.

Once it stalled i pulled off the pump not even 30 seconds after and the rod was perfectly fine and didnt have the slightest resistance. Once the motor cooled down the pressure went right back up.
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PostPosted: Tue May 30, 2017 2:50 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

Well, fuel pumps do fail. Wink And 6psi is too high.
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wafflecopter
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PostPosted: Tue May 30, 2017 2:52 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

Q-Dog wrote:
Well, fuel pumps do fail. Wink

Even with less than 500 miles? I mean i know it was a generic autozone pump but id still expect more than that.
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PostPosted: Tue May 30, 2017 2:59 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

Ten seconds is enough to cool down, especially if the engine isn't super hot. The fit of the phenolic block is important here.

I would also remove the fuel sender and check the gas tank for a rag or other debris. Any dirt in the fuel filter?

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PostPosted: Tue May 30, 2017 4:00 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

asiab3 wrote:
Ten seconds is enough to cool down, especially if the engine isn't super hot. The fit of the phenolic block is important here.

I would also remove the fuel sender and check the gas tank for a rag or other debris. Any dirt in the fuel filter?

Robbie


The gas tank was the first place i checked for a clog but im skeptical that its a clog of any kind because i take it down the road and it dies in literally the exact same spot every time. A clog would be a little more random than that. Somethings causing the pressure to go to zero once its driven with a load for about a minute or so.
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PostPosted: Tue May 30, 2017 4:26 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

Seems to me I've read threads where people were having trouble with aftermarket versions of the plastic block. When it gets warm it swells and restricts movement of the rod. You might check it and see if there signs of the rod rubbing against the inside
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wafflecopter
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PostPosted: Tue May 30, 2017 5:15 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

Mudpump wrote:
Seems to me I've read threads where people were having trouble with aftermarket versions of the plastic block. When it gets warm it swells and restricts movement of the rod. You might check it and see if there signs of the rod rubbing against the inside


Its an original plastic piece but to be safe I already sanded down the base where it can restrict it and like I said above not even 30 seconds after the engine was off I had the pump off and the rod had no resistance at all.
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PostPosted: Tue May 30, 2017 5:36 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

Is your fuel tank vented properly? If you are going approximately the same distance each time when it stops you could be developing a vacuum in the tank, easy test is to try driving with the fuel cap off.
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PostPosted: Tue May 30, 2017 5:48 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

I tried driving with the cap off too. I know its gonna be something really dumb and obvious but i just cant find what it is.
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PostPosted: Tue May 30, 2017 5:54 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

The line through the tunnel may be choked with rust, or the screen in the tank is covered in crap. Try draining the tank via the inlet line to the pump, jack the front of the car up and leave the gas cap on and see if it flows freely and makes a few gallons in 10 minutes.
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PostPosted: Tue May 30, 2017 6:22 pm    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

Im gonna pressurize the tunnel line and make sure its clear and holding in the morning.
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PostPosted: Wed May 31, 2017 12:12 am    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

David_nc_72std wrote:
Is your fuel tank vented properly? If you are going approximately the same distance each time when it stops you could be developing a vacuum in the tank, easy test is to try driving with the fuel cap off.


For the record, Mom's bug was a routine 60 miles with a vacuum-sealed fuel tank. Now with a vent, we'll be driving it until the world's oil supply runs out. Laughing

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PostPosted: Fri Jun 02, 2017 9:13 am    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

I put a 1.5 to 4 psi electric pump on it and still didnt work. Aside from the tank the entire fuel system is brand new now. There may be a listing for a 71 super beetle in the classifieds soon if i dont figure this out.
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PostPosted: Fri Jun 02, 2017 9:52 am    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

wafflecopter wrote:
I put a 1.5 to 4 psi electric pump on it and still didnt work. Aside from the tank the entire fuel system is brand new now. There may be a listing for a 71 super beetle in the classifieds soon if i dont figure this out.


I may have you confused with another thread with the exact issue, bear with me. Did you pull the tank and replace/service the pickup tub on the underside of the tank? Either you or the other poster said they had not- yet never followed up. If your brass pickup screen (the stock fuel filter) is collapsed you will experience exactly what you are.

Nobody ever listed their car for sale because of this simple an issue. We're not letting your off so easy. Cool
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PostPosted: Fri Jun 02, 2017 10:48 am    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

wafflecopter wrote:
I put a 1.5 to 4 psi electric pump on it and still didnt work. Aside from the tank the entire fuel system is brand new now. There may be a listing for a 71 super beetle in the classifieds soon if i dont figure this out.


Ouch. Where approximately are you located?
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PostPosted: Wed Nov 29, 2017 11:17 am    Post subject: Re: Fuel pump loses pressure when hot. Reply with quote

I'm having the exact same problem with a buggy I just finished. Held 3psi at idle, would drop below one when revved some. Did that a few times, last time I revved it it dropped to nothing and didn't bounce back. New tank behind the rear seat all brand new lines and filter. I'm guessing I just need a new pump. Sucks cause this ones new! I don't know how people that turn wrenches for a living deal with all these crap new parts these days!!
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