Weber Fuel Pressure Unstable

[rhk109]

First a bit of history...

So I had a shop replace my old fuel lines and clamp style PMO regulator with stainless lines, PMO fuel bars and a proper Holley fuel pressure regulator and guage. The new parts were installed in a dead head (no recirc.) manner. The shop left the high pressure Pierburg fuel pump in place.

Now the issue...

When I got home, the guage was reading 0 psi. Although, the car was running fine. I cycled the ignition and watched the guage go to full scale (15 psi) and the jump back to 0 psi. After that the guage would not register any pressure. Again, the car seemed to be running just fine. So the shop sent out a new guage. A month later (several shipping snafus) the new guage arrived.

The new guage is installed and is hunting anywhere between 1 psi and 6 psi. At one point I saw it hit 15 psi. It will generally idle around 3.5 psi. However if I give it a little throttle (2500 rpm) the pressure drops to 1.5 psi and stays there until I return to idle when it usually settles out at 3.5 psi. Sometimes it settles out around 6 psi. In general, it's pretty inconsistent. Car still seems to be running just fine.

Unfortunately, the shop is 1.5 hours away and I sure don't want to make that drive with this going on. So I'm trying to sort it out over the phone with them which is a PITA and takes forever.

Anyone ever seen these symptoms. Any ideas? Faulty regulator? Is the high pressure pump incompatible with this arrangement?

[berettafan]

Hmmm....let's think this through before the experts show up;

-car is running fine so float bowls have enough fuel and are set right
-if pressure dips when giving throttle my guess is it's because you are creating a bit of a vacuum (bad terminology i think) hence the line pressure drops (ie float drops, needle valve opens and fuel flows in).
-i wonder if extended running at 2500 + rpm would eventually empty your float bowls?


i can only surmise that you have a bad regulator. i don't know what happens to the needle valves at 15psi but if the answer is anything other than 'nothing' you have a fire hazard.

Holly pumps (red?) have been recommended here IIRC so perhaps that would be a better choice. I have a Carter pump going to AN plumbing and a dial type regulator and other than a fuel starvation issue (tank i think) it keeps a rock steady 3.5psi.

[JP911]

I've gone through some issues getting my fuel pressure situation sorted out and the bottom line is this: get a fuel pump that provides as close to target pressure as possible. Ideally, the OEM Bosch carb pump. Next, consider the Carter (best aftermarket, IMO) or the Holley Red.

[berettafan]

i THINK the Holly Red is a higher pressure pump isn't it? i seem to recall that being the case but figured holley pump w/ holley regulator must work well.

[JP911]

Holley Red:
Free Flow Rate: 97 gph
Maximum Pressure (psi): 7 psi

Carter Low Pressure:
Free Flow Rate: 72 gph
Maximum Pressure (psi): 6 psi

Carter Competition:
Free Flow Rate: 100 gph
Maximum Pressure (psi): 8 psi

[berettafan]

Is there a way to calculate what flow at 3.5psi would be?

(sorry to hijack!)

[pope]

I don't see how you can reduce the pressure of a 7psi pump without a return line circuit...

[pope]

Flow and pressure are not related. I mean that pump A may have 5psi and 100gph and pump B may have the same pressure with 80gph.

[avendlerdp]

If you have a dead head setup and a high pressure pump your fuel pressure will be unstable. The regulator will open up when the carb line pressure drops a bit but then snap shut under the tremendous rush of fuel that was being crammed into the fuel line by the pump. The regulator will even emit some "farting" sounds as this cycle increases in frequency.

It's no problem using the pump you have though. Just run a return line BEFORE the regulator to allow the pressure to escape. It must be smaller than the feed line but does not need to be from the engine compartment back. It can just be a "tee" fitting arrangement near the tank that gives the fuel a place to go when the regulator is shut.

Dead end setups only work if the pump delivers relatively low pressure and doesn't pack a bunch of fuel into the line when the regulator closes. All other solutions are work-arounds for the lack of a return. The stock Bosch carb rated pump that costs $$ has an internal return.

[avendlerdp]

If you have a dead head setup and a high pressure pump your fuel pressure will be unstable. The regulator will open up when the carb line pressure drops a bit but then snap shut under the tremendous rush of fuel that was being crammed into the fuel line by the pump. The regulator will even emit some "farting" sounds as this cycle increases in frequency.

It's no problem using the pump you have though. Just run a return line BEFORE the regulator to allow the pressure to escape. It must be smaller than the feed line but does not need to be from the engine compartment back. It can just be a "tee" fitting arrangement near the tank that gives the fuel a place to go when the regulator is shut.

Dead end setups only work if the pump delivers relatively low pressure and doesn't pack a bunch of fuel into the line when the regulator closes. All other solutions are work-arounds for the lack of a return. The stock Bosch carb rated pump that costs $$ has an internal return.

[berettafan]

The return line would, i THINK, come from the fuel pump and not be visible in the engine compartment. So dead-head setup on the webers would still be viable.

And THANK YOU for that comment as it just cleared up a mystery for me

[berettafan]

Correction; the return COULD come from the pump itself.

I have a deadhead setup but do have a return line (this was the mystery i mentioned; only 1 line going into the carbs yet two hooked up at the tank) and pressure works perfect.

Or do i not really have a dead-head setup? My fuel line splits and terminates at each carb (vs. forming a loop). I might be speaking above my level here....

[avendlerdp]

Originally Posted by berettafan

The return line would, i THINK, come from the fuel pump and not be visible in the engine compartment. So dead-head setup on the webers would still be viable.

And THANK YOU for that comment as it just cleared up a mystery for me

What you are describing would be called a dead-head "look" setup. It will work fine although it does not benefit from the fuel being constantly circulated through the filter(s). Unless you have a cleverly placed filter under the car.

[pope]

berettafan, maybe this thread will help you:
Fuel hose size and other fuel Qs

[berettafan]

Right; that makes sense. I believe the other issue for cirulating non-deadhead systems is cooler fuel per Grady.

[avendlerdp]

Who ever thought not being a Dead-head would be cooler? Kids these days...

[rhk109]

Fuel "dip" stays low at least for 5-10 seconds. Is this possible if the regulator is OK and the pump psi is just to high? Car drives just fine, but I know it's not right.

So what's the consensus? High pressure pump with recirc or low pressure dead head?

Thanks for the input.

[avendlerdp]

+1 for High press recirc.. Cool fuel and constant filtering are good things. Oh, and not having to buy and install a new pump...

[pope]

If your car has the plumbing (ie return pipe under the cabin) then it's very easy to set up a return line circuit.

[rhk109]

I found this on another site. Does this work? If so, is it a cluge?

"If you want to use it as a bypass reg.you need to run the line directly from the pump to the carb,then tee it somewhere in that run and put the reg. in with the line to the tank as a return.

This is the way we do all the alcohol setups, the best part is when you put the reg. in there hook it up backwards. Thats right ,pressure in the out port and the return out the in port. Adjust with the screw as normall."