Dropping Fuel Pressure

[Freddie]

I'm finally getting my Dommy dialled in thanks in no small part to Steve Cowan.

Lots of changes made to it and I am very close.

That said, a week ago my fuel pressure started dropping as engine gets hot.

Nothing has changed in the fuel system, been same for 8 faultless years. Until 1 week ago.

Aeromotive A1000 pump installed below tank at rear of car
-10 braided stainless fuel line to Aeromotive regulator
-8 retun line back to tank

was set at 5 PSI.

Used to start and stay at 5 PSI

Now it starts at 6, and drops to 3 PSI within 20 mins of driving and has got as low as 1.5 PSI

causes surging at low speed acceleration and a massive hesitation if I absolutely hammer it

Liquid filled gauge but it's never been a problem, as said this all started happening a week ago.

Custom tank and pick up and tank is clean

I checked voltage at pump = perfect
lines were touching valve cover, so I moved them both = they are no longer touching any part of the engine
no kinks in lines, as said no changes to fule system in 8 years

I put a temp gun on it. These are all engine hot after 20 min drive temps:

Fuel inlet line near head = 150 F
Return line near head = 160 F
Reg = 154 F
Fuel in primary bowl (shot through the sight glass) = 145 F

They all seem pretty normal to me. Not too hot to boil 98 Octane gas (not an E10 blend)

I know tank vents properly, but I'm about to check that and run it with the fuel cap off in my shed see if that changes anything. I did check fuel vent about 5 weeks ago and it was fine.

What am I missing?

[turbo camino]

When it's showing low pressure, pop the rubber plug out of the gauge housing and watch the needle jump back up to normal. They do get worse over time, repeated temperature changes pump atmosphere in and it'll build up gradually until it starts affecting the reading.

[Freddie]

Thanks will try that. I don't think it's a gauge issue though as it does affect driving.

I also just went out to attempt to run it with the gas cap off.

Couldn't even start it.

As soon as I hit the key fuel started pissing out the top of the vent tube on the secondary side of the carb and out the air bleeds like a fountain.

I shut it down, gave it a quick clean up, removed gas cap, hit it again...same fountain.

I'm thinking that now there is too much pressure and it's blown the needle and seat in the secondary

[turbo camino]

Have you adjusted the regulator trying to compensate since the problem started?

[Freddie]

Yes i did yesterday, I was thinking now it's too much pressure. Going to back off the pressure when it cools down

[Geoff2]

Make sure the fuel tank vent is not blocked.

[Freddie]

Vent is not blocked. But Steve and I suspect vent might be too small. 2 weeks ago I adjusted the fuel cap as it used to leak when I hammered it. My cap is adjustable to it tightened it up. My current vent is tiny. We suspect this tightening of cap which was acting like a vent may be the issue. Going to test that tomorrow

[enigma57]

Don't forget to check your fuel filter(s). Especially if you are running ethanol pump gas. Ethanol collects water like a sponge (even the condensation that forms above liquid level in your fuel tank) and that water will cause a paper element fuel filter to swell and partially clog over time. What we did is to add a fuel filter with a spin-on filter element to my son's Camaro and on my '57 Chevy, I have one with a drain valve at the bottom as well, so I can check for (or drain off) any water that gets into the fuel delivery system. Mine is for a late model Land Rover diesel application. Picked it up from a friend in the UK a few years ago.

Just something to consider when checking out your fuel pressure drop. I had an '88 Buick daily driver with sequential fuel injection some years ago when ethanol was first forced on us here in Texas. Ran great...... Was getting 30 MPG cruising at highway speeds on straight gasoline. When we had to begin using 10% ethanol, mileage immediately dropped to 26 - 27 MPG. And 6 months later, my fuel filter clogged as I described above and I had to change it out twice a year from then on.

Best regards,

Harry

[Freddie]

All quick updated

RESOLVED

Steve nailed it.

Vent was NOT blocked but was too small.

Cap has been leaking for years. I decided to fix that. Apparently it had been operating as a vent along with the 4AN vent tube.

I replaced that with an 8AN vent tube. Cap still adjusted to stop leaks.

Just run it up to temp and let it idle for about 10 mins. Re-adjusted my fuel pressure to 5 PSI. I sat there and did not move the entire time.

[turbo camino]

Good.

Now, do yourself a favor and get rid of the liquid filled gauge. They simply cannot be trusted.

[Geoff2]

Vent too small? What am I not getting? Was ok for 8 years & SUDDENLY fuel pressure drops because the vent was/is too small????

[barnym17]

Leaking cap was acting as vent.

[turbo camino]

Even if the only tank vent were the single -4 line I don't see how it could have caused this. It's hard to maintain a significant vacuum in a container when there's a ~.150" hole in it.

[Freddie]

I agree, strange, I thought no matter how small the vent was, it was venting, and that was checked twice that it was not blocked. I'm no scientist but I thought a vent was a vent. But somehow someway, it was too small to allow it to vent properly (basically pressure would drop as vacuum built). Now it's back to the way it was. I can't explain it, but I'm happy all it cost me was a 8AN elbow connector ($12) and it's working now.

[Freddie]

Latest update after 30 min drive.

It was NOT a vent issue.

Holds pressure for about 20 mins.

Then slowly drops. By the 30 min mark I was at 2psi

so I am going to pull the filters and reg next