Chasing Fuel Pressure

Once again, the GS has me chasing my tail…

I have a newer Edelbrock 120 GPH pump pushing from the back, and an older Stage-1 pump on the engine. While I had the carb off a few weeks ago, I added a Earl's liquid filled gauge after a a new filter and before the carb.

Fired it all up to see the gauge fluctuating between 2 and 3 lbs. at idle.

I wasn't happy with how I originally mounted the rear pump, so I re-positioned it lower with smoother inlet and outlet paths. It has a 3/8" line from the tank, through a large-ish WIX metal canister filter and into the pump.

Now I barely saw 1lb. at idle – and with the engine off, but with the electric pump running.

The car was scheduled to be part of a special Muscle Car class at the Caffeine and Octane show this past Sunday, and it's just 3 miles from here, so I went. Ran fine, though I didn't push it.

The Stage-1 pump is old enough that I can't remember when we changed it last. I bypassed the stock pump this week and now I see 3lbs on the gauge by way of the electric pump.

My first thought is a bad gauge, but it has shown changes each time I make changes, just always under 3lbs.

I am ready to take out both pumps and put in a RobbMc unit and be done with it, but I’ll be pissed if it doesn’t solve it

I need to make a stop at Summit Racing sooner or later for a throttle cable, so I may go tomorrow and pick up a new gauge too and see if it’s that.

What are the chances the stock pump is causing a restriction? Without the mechanical pump, the pressure indicates low, but steady. With the mechanical pump inline, the gauge fluctuated if it showed any pressure at all.

Better yet, what are the chances the old stock pump and the new gauge are both whooped?

I think I know the answer, but always open to ideas and experiences!

 

I just went through nearly the same problem this past month and ended up putting the RobbMC 1/2" sender in the tank, all new rubber lines in the stock 3/8" fuel lines, RobbMC billet filter under the driver's door and a new Stage 1 fuel pump. My pressures are 7-8 at idle with some jitter and about 6 psi steady at cruise. Under full throttle pressure drops to 4-5 psi. (I did not use a pusher electric pump) My HP is about 470.

All this after spending a week watching my Summit liquid filled pressure gauge. Big difference between my gauge and yours is I added a braided line to get the gauge out of the engine bay and taped to the windshield so I could see FP under all conditions. If I were you, before you do anything else, I'd get that gauge outside the engine compartment. They are known to become unreliable with all that engine heat. Once you are sure your gauge is OK, I'd try a new Stage 1 pump if the issue persists. (I may have a spare pump soon).

By the way, what is your engine build? Approx. horsepower?

 

The numbers above were all with the engine cold, not after driving. I read your thread

9.1:1 462. 224/230-112 .480"/.483" cam, Gessler Street/Strip Stage-1 heads. Original B4B, shorty headers and a AED 850 Holley. 4405 lbs and has run a best 13.15 @ 101.88. Best guess via my engine dyno program (measured everything when apart with flow numbers) is 450 at the crank/380 at the rear wheels. Online calculators for weight/et/trap speed match closely.

We went to a pusher pump as it feels like it runs out the last bit of the 1/4 flat (and had NO2 on it, but never ran with it at the track). The NO2 is on the shelf. The tank has been cleaned, pick up sock is gone.

I don't mind over doing the pump a bit at this point, as it'll support the aluminum heads in it's future. I do have a AutoMeter remote fuel pressure gauge for my Mustang on the shelf too, but figured it would be easier to try another small gauge than plumb that sucker up. But, it might be the right thing to do...

 

You said your filter is prepump???? Electric pumps dont suck well....they push.......so having one suck through a filter can cause all kinds of issues....bypass the filter and recheck.

Liquid filled gauges in the engine are only actuate when cold. You would be better to have a nin liquid filled in there and deal with the vibration. The psi will drop reading on a liquid filled gauge as it warms up

 

Good advice. If you decide to relocate that filter, the rubber section below the driver's door is an excellent place to install a pre mechanical pump filter.

 

I know this sounds a little odd but I had an oil filled Autometer gauge on mine and it read low, particularly on hot days. A hot day in the sun and the darn thing showed little to no fuel pressure. I read that the oil filled gauges could do that in hot weather so I went to one that was not oil filled and bingo, no more problems.

 

I highly suspect that gauge unless your electric pump is on its' last legs, somehow. Have you tapped the glass while the pump is on to see if the needle goes up at all? Sounds to me like it is stuck at 3 lbs. and won't go higher.

Another thing, unless that electric is going bad, the fuel pressure should not fluctuate at all while the car is idling because, unless the output pressure of the electric is set lower than the mechanical, the electric pump's higher pressure should override what the mechanical pump is putting out and smooth the output flow. Now if the electric is somehow restricting flow because of some problem then I could see how the mechanical is just pumping out what is sent to it and therefore doing the small changes to the fuel pressure as it is pumping. (Note: I just looked up the pump and if it is the one you are talking about it is preset to 6.5 lbs. which leads me to believe that it is slightly lower than a Stage 1 FP output so there may be some slight pressure fluctuation while running but that shouldn't affect total output but it does point the finger at the gauge being bad and only showing 3 lbs but changing slightly when the car is running. My thinking there is that it really is 7lbs. but dropping to 6 lbs.)

However, before you get too carried away changing things take the line off up front from the electric fuel pump before the mechanical. Connect a length of line from it to at least a 1 galllon container so you can see how much liquid is in the container. Turn the pump on and time how long it takes to pump out 1 gallon of gas. Measure the time and fuel level carefully. Seconds count. First of all if there really is a problem the way your gauge shows you'll clearly see it in the way the fuel flows out of the hose. Fairly weak. With an electric pump of that size it should flow out in a fairly strong stream. If it takes longer than a minute to pump 1 gallon free flow then you may have a problem. It is probably rated at 120 gallon/hour free flow but that is unfettered by the long lines in/out and should take 30 seconds to pump 1 gallon. It won't do that with all the lines but the closer it gets to 30 seconds the better but I would say anything easily under 60 seconds you are probably good.

Even if your mechanical pump is not up to snuff the electric should push the fuel through it when the car is running without too much problem and it should not cause that much of a restriction short of some major blockage inside.

All fuel pump manufacturers recommend a pre-pump filter. But, a pre-pump filter is usually not a very fine filtering media(barely any restriction). It is only meant to catch "larger" items that could damage the pump if there is no sock. It sounds like you have a good setup there with the large Wix filter. I doubt it should cause a pressur eproblem unless it already picked up some major debris.

If you have good flow through the electric by itself I'd re-hook everything up and take the car out and get on it good. If it doesn't show signs of nosing over than I'd say you are good. A little bit of nosing over or even outright dying out will not hurt the engine as long as you don't do it a lot or by staying in it. If need be that electric should run that engine all by itself with no problem assuming it flows well in the static test I outlined. Adding a long line to watch fuel pressure is defintely recommended to really check what is going on when getting on it.

 

LOOK at your liquid-filled gauges.

There will be some sort of rubber plug on them.

You MUST remove the plug--at least enough to "burp" the gauge before taking readings. The temperature change inside the gauge causes the liquid to expand or contract, and as the gauge is sealed...the pressure difference INSIDE the guage interferes with the mechanism. The same sort of problem would occur with elevation changes, if they're big enough.

The usual result is that the gauge reads too low when hot.

IF (big IF) the gauge doesn't vibrate where it's mounted, you can poke a needle into the rubber plug to make a vent. Some plugs might be molded with a little nipple that could be cut off. Of course, if the gauge shakes or the rubber plug isn't at the top of the gauge when mounted, the liquid runs out of the vent.

And if all this is too much hassle, you don't need to buy a non-liquid-filled gauge. Remove the plug, dump out the liquid, poke a hole in the plug, and put it all back together.

 

Do you have a bypass regulator in the system? Maybe mis-adjusted?

 

So….

Small update.

Went to Summit Racing on Saturday, but screwed up as I thought my dual-feed Holley line had a -6 out the back, but it’s a -4. I knew that (bang-head). So, while I had everything here I needed to make the -6 line, except for the (hopefully) known good pressure gauge, which I did pick up, I came to a stand-still. I’ll make the 90-mile round-trip back to Summit for the -4 to -6 adapter tomorrow. I am then two-minutes from duplicating @rkammer and his setup.

@Buick - No regulator

@Bens99gtp and @Golden Oldie 65 - all done when "cold" so far. Engine off, and then at idle and just warm enough to run on it's own. Oh, and yes - more than a few gentle nudges on the gauge. Turns out, I didn't do my due diligence well enough - there are a few crappy reviews at Summit on this gauge. In my defense, I didn't get it from them, but from a buddies speed shop in NJ when I ordered a bunch of stuff at once.

@LARRY70GS - I totally agree in theory – that’s why I have a Auto Meter Fuel Pressure gauge with isolator for my Mustang – just not looking to turn this one into a gauge factory (yet) or spend more on the gauge than the next pump Just want to verify I actually have any fuel pressure, and so far, the jury is still out.

@No Lift - I replaced the 20-year-old braided hose from the mechanical pump to the carb, and checked the fuel pump flow from the electric to the carb with a new line (bypassing the stock pump): about 3/4 of a gallon in 30 seconds. Yes, 120 gph Edelbrock. Thought it would flow a little stronger, as the 140 Mallory in my Mustang was better, but it has a 1/2" line from the pump to the front of the car. Before I put it back together, I'll check it when the stock pump back in-line just out of curiosity.

@Schurkey - The Earl's gauge doesn't have a bleeder. The Auto Gauge one I just picked up does. Excellent tip!

Thank you all for your thoughts!

Dave

 

Welp. Electric pump alone - both gauges match.




Now to bypass the Wix filter in the back and then the electric pump...

 

I don't know if your just pushing through the mechanical pump without the engine running what kind of pressure you will see..........the mechanical pump not pumping might be a restriction

That electric pump alone should be enough for your motor ........not sure how easy it is to bypass the mechanical pump and see reading.....

Air and liquid gaskets do read the same when cold.....not so when warm

 

So the reading you are getting there is direct from the electric pump feed line at the carb with no mechanical pump involved. Interesting problem. Somehow pressure is being limited yet volume is not. According to your .75 gal./30 sec measurement your pump is doing 90 GPH(rated at 112 GPH free flow) which is good considering it pumping through at least 12' of 3/8" line with only a 1/2" pickup going for it.

I checked my record how a Comp 110 pump self regulated to 7 psi. worked. With 3/8" pick-up and line to the front it took 90 sec. to pump 2 gal. which is 80 gph. I changed the pickup to a RobbMc 1/2", 40 micron inlet to pump filter, and -8 hose pump to carb. It made a very noticable difference in flow down to 75 sec. for the same 2 gals. which equals 96 GPH. I pumped two gallons to make it more accurate.

When checking the free flow time of the 110 you can see the gas coming out of the open line smoothly and with a nice stream. Sort of like you are pouring gas from a vented 2 gal. container with a 3/8" hose outlet. Smooth and strong but not shooting out. I have also used the Comp 140 pump which is set at 12 or so psi. and the free flow output is noticably stronger in the way the stream of gas flows out of the hose which makes sense since it is coming out at higher pressure and flow. I want to say that was pumping about 32-35 seconds for a gallon with 1/2" pickup and lines but it has been a long time since I did that check on another car.

What all this is leaving me to believe is that the internal regulator in the electric pump is somehow jammed open slightly regulating the pressure to 3 or 4 psi.(assuming a pressure drop rear to front). Unless you are saying that the flow into the container was not smooth but spurting instead. If so this could indicate a restriction to the inlet of the pump. Possibly the Wix filter is installed backwards, if that is even possible or if it would even make a difference. Output from a normal electric pump should be smooth unlike a mechanical pump.

I would also agree with Ben that pump is easily able to fuel your engine by itself.

UPDATE: I just found this. Page 1 and 2 indicate how to adjust the internal regulator. Also note the first 2 star compaint from the Summit page about how it came from the factory at 3.5 psi. I'd set it at 7 psi. without the mechanical and the engine running and then see how it works with the mechanical. I'm suprised there is no set screw to keep the adjustment from moving although I'm pretty sure it will not move easily. Get adjusting!

https://static.summitracing.com/glo...17911792-Quiet-FloElectricFuelPumps130320.pdf

https://www.summitracing.com/parts/edl-1791

Ramin is probably right just that it is the internal pump bypass regulator.

 

Yes - same conclusion I came to as well. I dug out the instruction page and found the adjustment details.

I double checked the filter - it has "out" labeled, and I added a arrow with a sharpie last time I had it in my hands just to make sure... (old age).

The flow wasn't garden hose like, but it wasn't sputtering either. I'd describe it as "ok" lol

I also took a good look at the wiring from the relay at the front... "meh." I need to run speaker wire to the back anyway, so I am going to replace the hot lead for the pump as well. Tomorrow's project.

Thanks for following along with me! Work on the old car they said. It'll be fun they said.

 

Better than these new cars lol

 

Heh - messing with them too these days...

Got under the back and dialed up the pressure in half-turn increments. Basically nada. 2.5 turns = one more psi. Bypassed the electric pump and filter tonight and reconfigured the mechanical pump. Will start it tomorrow with just the mechanical and see what I get.

 

And... the Stage-1 mechanical pump pressure:

This is with no filters - in the back or the front. Ordered the RobbMc pump this afternoon, hope to have it middle of next week. They told me they build them to order.

I know it'll be overkill at the moment, but hopefully it'll be the last pump the car sees no matter what I do with it.