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Mon May 04, 2015 7:33 pm

The car started acting up badly again yesterday, sputtering and backfiring. It has always been guzzling gas and I am trying to work that out too. I'm starting to go through the tests I hadn't done before and researching different things to try. I threw my pressure gauge on the fuel again to see if anything had changed in the pressure and to see if there was any change to it while driving. Still at 30 psi, but I noticed something and I wanted to ask if it was normal. When the car is running, it holds a steady 30 psi. Starts up fine, no issues there. If I turn on the ignition without starting the car, the pump comes on to prime, but there is no change in pressure in the gauge and it still reads 0. When I turn the car off, the pressure immediately drops to 20psi. It will hold there for awhile and slowly back off.

Is it normal for the pressure to just drop off that way? The injectors don't leak (that I know of, but I am going to pull them to change this seals this weekend and I will test them), there are no leaks from the fuel lines and no raw gas smell from anywhere . Also, shouldn't it register some pressure with the pump on even if the car is not running?

I don't really think this is the cause of any of my problems (I think they are vacuum, timing or sensor related), but in my researching I saw a post somewhere about the diaphragm in the pump starting to fail when the pump wouldn't hold pressure. I'm curious if this is what that means. Thanks for your help!

I forgot to mention...1972 FI Squareback

Mon May 04, 2015 7:48 pm

Yes....in reality the pressure wheN everything is in tip top shape......should drop from the stock 28 psi to the low 20s immediately on pump shut off. It should drop slowly over about an hour or so to the high teens. Overnight you should be at about the 10-13 psi range.
over a couple days it may, drop to read 0 but the lines should be full enough that you see at least 10-13 psi register on the priming cycle.

This is because the regulator is a check valve and the pump has a check valve. Withcage...mthe regulator will get leaking and so will the pump check valve.

Bump the key for two priming, cycles....read the pressure and tell us how fast it bleeds down without starting. Ray

Mon May 04, 2015 8:20 pm

So I loosened the clamp at the gauge to drop the pressure to 0, bumped the key twice letting the pump run the prime each time and it went up to 20 PSI. Turned it off and it dropped to 19PSI in about a minute and held there for the few minutes I watched it. I will check it every 20 minutes or so to see where it goes and report back.

I forgot to mention that when I took the screw off the "T" no fuel came out. The other times I had done this there was a dribble of fuel from the hole, but this time it was dry. I had driven the car last night and it had sat for about 15 hours before I put the gauge on. Also, the pitch of the pump changed about a week ago. Might mean nothing, but I thought I would point it out just in case it does. Thanks Ray.

Tue May 05, 2015 4:03 am

Seattle72 wrote: So I loosened the clamp at the gauge to drop the pressure to 0, bumped the key twice letting the pump run the prime each time and it went up to 20 PSI. Turned it off and it dropped to 19PSI in about a minute and held there for the few minutes I watched it. I will check it every 20 minutes or so to see where it goes and report back.

I forgot to mention that when I took the screw off the "T" no fuel came out. The other times I had done this there was a dribble of fuel from the hole, but this time it was dry. I had driven the car last night and it had sat for about 15 hours before I put the gauge on. Also, the pitch of the pump changed about a week ago. Might mean nothing, but I thought I would point it out just in case it does. Thanks Ray.

Unless you have reason otherwise yet....the first thing that can change pump pitch is restricted pump intake. The usual suspect is the in tank sock filter if you have one. I get rid of them and install an external strainer...after you clean the tank.
Another cause of chang in pump pitch can be the mounts getting loose. Ray

Tue May 05, 2015 7:43 am

I'm not sure of the relative heights of the tank and the tee. IIRC, with a full tank it should bubble out like a drinking fountain from gravity, even without a pressure head. Maybe not with an empty tank, but this may also indicate a plugged inlet somehow (kinked lines, clogged sock, water in the filter).

Tue May 05, 2015 8:40 am

KTPhil wrote: I'm not sure of the relative heights of the tank and the tee. IIRC, with a full tank it should bubble out like a drinking fountain from gravity, even without a pressure head. Maybe not with an empty tank, but this may also indicate a plugged inlet somehow (kinked lines, clogged sock, water in the filter).

The previous times the tank was half full or less, this time there was almost a full tank of gas.

So here are the results...

20PSI after 2 bumps of key
1 min 19PSI
30 min 15PSI
60 min 11PSI
2 hours 9PSI
12hours 0PSI (I checked before I left for work this morning)

What do you think?

I tried to change the sock once when there was a small enough amount of fuel to drain the tank. I struggled with it for a minute and decided to wait until I had time to remove the tank to take it out (looked to be much easier to do with the tank out and I didn't want to break the elbow). Once I get this tank down enough, I will pull the tank and remove the sock. I'll double check the lines around the pump as well as the mounting screws. The filter was replaced about 2 months ago, but I have another on hand because I planned on changing it again after I cleaned the tank. Thanks!