Low fuel pressure, new almost everything ! Help

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trevorcrum

trevorcrum

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Hello A body freaks,

Referring to a 74 Duster, 360, Holley DP 600 M1 intake, mechanical fuel pump.

Car started stalling out about a month ago, wouldn't idle consistently, and I noticed low fuel pressure if it did around 2psi. Typically It's running around 6-7. I have a gage on the fuel rail that I bought from Summit.

So I changed the fuel pump ( mechanical ), and filter ( Looks like a gold stone top hat inside the Summit fuel Rod before the gage ). And the car seemed to run fine for a about a week.

Then same thing happened, car needed constant pumping on the gas to keep it idling, and noticed low pressure again at the fuel rail.

So I decided to pull the tank, and change it, as well as the sending unit, and fuel line. I notice the sending unit had no sock on it, and the plastic tube on the end was cracking and disintegrating badly. Tank looked pretty crusty inside. So new tank, sending unit, fuel line all the way up to the pump.

Started it up tonight and pretty much the same thing, low pressure ~ 2psi at the fuel rail at idle.

Any thoughts or ideas ? Why did my idle pressure go from 6-7 psi, down to 2psi after changing almost every component ? The only thing that didn't change was the hose after the pump to the fuel rail, and the fuel rail itself.
 
Nothing special. Summit gage on a dual line feed.
IMG_1476.JPG
 
You replaced some of the components and then found some more problems at the tank, including no sock on the sending unit. Could some big chunk of something got into your fuel system before you replaced the tank, sending unit, and line to the pump?

How about cranking the engine (with no spark) and pumping some gas into a jar, both before and after the engine fuel filter. Even though you replaced all this stuff, things could have changed, or clogged.

I just recently got chewed out from a carb rebuilder because of rust and stuff he found in my old carb. He told me I needed to replace the pump, filter, clean out the lines, and clean or replace the tank. I was planning on doing this, but now I've had to make this a priority.
 
Remove the supply line from the tank. Apply air pressure to the line at the fuel pump going toward the tank. It will blow any restriction out.
 
Some of those dual feeds have little copper colored GM-type filters in the inlets. They are not filters per se, but a last-chance to stop very fine sediments from getting through. It doesn't take a lot of larger particles to plug them up.
I threw mine away years ago, after installing a decent, very large canister filter,way in the back close to the tank.
But having said that, when a problem like yours pops up, the very first thing I do is a fuel pump output test, followed closely by RRRs advice, if the pump fails the test.
 
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you ever thought about trying a better fuel pressure gauge....like something from Autometer....you are spending alot of money...but you got a 5 dollar fuel pressure gauge...
 
70aarcuda said:
you ever thought about trying a better fuel pressure gauge....like something from Autometer....you are spending alot of money...but you got a 5 dollar fuel pressure gauge...
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Well, if it was just a low gage reading and idled well I wouldn't care but I'm assuming the two go hand in hand. Ill.check out the gates though. Thanks
 
AJ/FormS said:
Some of those dual feeds have little copper colored GM-type filters in the inlets. They are not filters per se, but a last-chance to stop very fine sediments from getting through. It doesn't take a lot of larger particles to plug them up.
I threw mine away years ago, after installing a decent, very large canister filter,way in the back close to the tank.
But having said that, when a problem like yours pops up, the very first thing I do is a fuel pump output test, followed closely by RRRs advice, if the pump fails the test.
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Thanks. Good idea with the canister at the back. I haven't been running without one as I thought the GM style was good enough. So I'm assuming you need back pressure on the line to get the pressure at the gage without the carb on. Am I just looking for good flow ?
 
Yeah those little guys are very efficient, but they have no capacity, well they do but not for big stuff.Big stuff just piles up in front of it and then it's game over.You gotta run a prefilter somewhere. I went down to Napa and got a bigboy EFI filter with a 3/8 hose barb on one end and a fuel line fitting on the other. That beast is about 4 to 4.5 long and about 2.5 in diameter.I put it on the suction side, Which they say is a bad idea, but mine has been there since 99, and 125,000plus miles, so IDK. From the pump is a 1-piece steel line, with a short jumper at the dual-feed and I have no fuel-pressure gauge at all;never have had. I mounted that bigboy in front of the passenger side front spring mount, and ran a new steel 3/8 line up to the front stub frame, then into it and up to where the hi-output mechanical pump resides. I tucked everything up high and secured it as necessary.
If you don't have a pre-filter, follow RRRs advice; Remove the line from the pump, and the short jumper at the back and blow out the line. Then install your prefilter, blow out the line to the dualfeed,and finally those little GMers. Then see how it goes.I think you mighta just had some crud move through the system and it piled up at the carb. But if you don't blow out the lines, chances are that crud is still moving along, and it will strand you somewhere that you may not want to be stranded at.You can skip the line flush if you're willing to take that chance, but carry a spare filter.
I skipped the flush this one time and plowed through 4 filters in about a month. You know, those $2.99 ers. HYup, there was me on the side of the road again replacing a filter on the old Volare. Eventually that tank popped a leak on top,of all places. It was perforated in several spots where that sound deadener had trapped moisture, so I replaced it;and no more problems.
After the line is cleared you may want to check the pump's free-flow output.This is done by Tee'ing into the pressure side fuel line and running it into a container, about 2 quarts/liters. Then you start the car,let it idle for about 30 seconds, and shut it off. Then measure the quantity of fuel pumped, and convert it to liters per hour. But skipping the math, if it pumps a pint in 30 seconds at idle shes ready to hit the road.
The factory spec is 1 qt(32oz) per minute, or less, at 500rpm.
If you're going racing, you'll need about a half pound of fuel,per horsepower,per hour.This works out to about 68ounces in 1 minute for 400hp, according to my math.Yeah, so, that's a lot,eh;You might have to rev it up a bit to make that number,no biggie.
I skip the Tee'ing and just dump the line straight into the container; the engine will easily idle for the duration of the test, on the fuel that is in the bowl..
 
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Ok, it's been a while but I rebuilt the carb, installed a new fuel pump ( my second new one now ) and with and started her up pretty quickly. Pressure was nice at 6-7psi, and took her out for a quick spin. She drove great so I brought her back into the bay to tune the carb. I started, made it past the idle setting and then noticed that the pressure dropped to about 2 psi....shortly after she stalled out. Yes I have gas in the tank, and I can pull it to the mechanical pump with a hand vacuum pump. But I'm wondering why my pressure is so low.

Has anyone had a failed eccentric ? Or does anyone have an idea how I can check this without pulling the water pump ? I'm running out of things to check, and unless I bought two failed pumps in a row from summit, this seems to be a possible cause.
 
Try removing the gas cap when this happens. If this straightens it out, the vent maybe stopped up as mentioned if it has one, or the gas cap may be defective if it is a venting cap and has failed. It could be pulling a vacuum on the tank and slowing the flow of fuel.

Also, I've never seen whether you have a regulator and or return. If you are running a dead head style regulator on a return system, the fuel pressure will be very low, because the regulator is not restricting the return flow.
 
Pulled the pump and saw with a mirror that the eccentric seems to be operating normally. I don't have a regulator or returning line as its mechanical. I do have a vent line which runs to the engine bay but no charcoal canister. It basically just dangles up near the alternator. Ill check it tomorrow and see if it's plugged / run with no cap on. Thanks for the ideas. Ill probably get Carter pump soon but would like to confirm its a pump issue first. Thanks all.
 
Posts 4,6, and 10, going back to Nov 13. That's 29 days and still no output test results..... Trevor, Trevor......This is the number one go-to when having fuel issues. Think about; you can totally plug the output line at the pump and make 7 plus psi. But with an empty fuel bowl, can the engine possibly run?
Or this; you can dump you pump output into a pail at Zero psi, and have enough fuel in the pail after 15 seconds, to have run a pass or maybe two.
Or this; you can remove your pump from the engine, clamp it into a vice, put a gauge on the output side, and stroke the handle manually. The guage can read anywhere from 3 to 7 psi, but did any fuel at all move anywhere?
It's simple, if the bowl runs dry; either the inlet is dry or restricted, or the outlet is restricted, or the pump handle ain't working right, or the pump is no good.
You, yourself replaced everything on the inlet side.And you pulled fuel through that new supply side.
You proved the eccentric is operating.And with the pump off I gotta assume you checked the pump arm return spring.
You have tried 2 new pumps, with same results.
You have removed the little sintered bronze filters.
Rusty said blow the lines out.
There are just two things left; the "hose from the pump to the rail" that you didn't replace, and that shouldn't be there in the first place, the rail itself,and the float-valves. Oh and the pump output test,lol.
So one more time; remove the rail stuff it in a pail, start her up and run it for 15seconds on full float bowls. Stop. Measure the fuel discharged. It should at least equal the amount of fuel your car needs to make a qtr mile pass, which is 1/2 pound per horsepower, per hour. Do the math for your engine.
Ok I'll help you with that. A 300 hp engine needs about 13.3oz @2200.And here is how I got that;
(300hp x .5 #/hp)/(60minutes x 1/4 minute) =.625 pounds of fuel in 15 seconds. I USg weighs about 6.25 pounds IIRC. So you need .625 x 6.25 =3.91pounds. And since all of a gallon is 128 oz, then you need 3.91/6.25 x 128=80 oz. And 80oz /32 oz per qt equals 2.5 qts to hit the traps. So rev your engine up to peak torque, say 3800 to 4000rpm and pump away!

No,no don't do that! At 2200rpm, your engine might be needing to make 50hp to cruise so it would only need 50/300= or 1/6 as much fuel. 1/6 of 80 oz is 80/6=13.3 oz
So if your pump exceeds 13.3 oz at 2200, then she will likely push enough fuel for a 300 hp engine to reach the finish line.
Finally, the FSM says the factory SBM pumps ran on 3.5 to 5psi, and the output should be 1quart in 1 minute or less, at 500rpm.How many oz in a qt? That would be 32 in the USA, and that includes even Texas.
If you can't get that much out of the rail, move down to the next junction, and repeat the test, and so on until you find the problem.
There is a wild card again. If you put a soft squishy hose on the rear junction,between the sender and the hard line, the pump can collapse that and shut off the fuel supply. Or if the pump sucks air there, well you can't run the engine on air alone. If you use gearclamps back there you have to double them up on each side, and stagger the screws 180*. The joint is high enough that it will not usually leak so do not think that because you do not find fuel puddles under the car, that the jumper is not sucking air.
And I'll tell you a little secret;
This air infiltration at the jumper is a hard thing to diagnose.
See every time you start it up, the gauge shows pressure and the bowl fills up. After a bit, the pump starts mixing the sucked in air with the fuel and the floats drop to find the fuel, so the pressure drops. But the fuel is not forthcoming, so the engine stalls.Now the air and the fuel in the pipe between the pump and the valves separate and the fuel drops back to the pump, and fills the output chamber. The fuel and the air in the supply line also separate, with the air moving to the high spots and the fuel separates to the low spot. So now there is a big slug of liquid gas sitting in the line with air probably at both ends.
So now after 5 minutes you have maybe manually refilled the bowls, and you hit the key, and she fires right up. The pump pushes the air through the inlet valves where it escapes through the bowl vent, and the gauge again reads full pressure; sound familiar? After a few minutes the pump starts pulling air in again at the back,the pressure drops, and soon the engine stalls, again.
So whats a guy to do?
OUTPUT test! With the discharge hose submerged in the jar, the bubbling tells the story. There should not be any bubbles!
Or you could just replace the hose and double clamp it, right from the get-go...........................................
Yeah so, one or the other; don't make me come over there and do it myself, cuz I can drink a lot of beer, and you better hide you wife!
Kidding; I don't drink beer anymore........lol. And I'm 63 so your wife should be safe. Honest.If I get outta line you can slap me. I'm only 62inches and 135 lbs,so I mean, there's no fight left in that, if there ever was. Ok,Ok, I'm 69inches and 200, so step aside we got work to do;put out that cigarette and hand me the pail I brung?
 
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Assuming the low pressure and the idle are related might bite you, but they probably are.
But then it could be a packed with junk gauge and a plugged up idle circuit just as easy.
 
Wiped a lobe on the cam fuel pump once. It move the pump enough for the car to idle and run at low rpm but while I drove it the carb used more than the pump put out. Discovered it by pulling the pump and cranking the engine with a post in the hole and noting how little the post moved. Ended up putting on an electric pump. Problem solved. Hope this helps.
 
trevorcrum said:
Pulled the pump and saw with a mirror that the eccentric seems to be operating normally. I don't have a regulator or returning line as its mechanical. I do have a vent line which runs to the engine bay but no charcoal canister. It basically just dangles up near the alternator. Ill check it tomorrow and see if it's plugged / run with no cap on. Thanks for the ideas. Ill probably get Carter pump soon but would like to confirm its a pump issue first. Thanks all.
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The charcoal canister line is not a vent line....although you can use it for one. Just because you run a mechanical pump does not mean you cannot or should not have a return. A good return setup will benefit you greatly.
 
I didn't read the whole book but replace every piece of rubber line you have. Sometimes they will swell up inside like blockage in an artery and restrict flow.
 
So..problem solved for now.....I tried to run a short line in to the pump from a gas can to output tank while priming the bowls. When setting it up, I filled the input line with fuel and noticed it leaking out the pump on the back vent hole suggesting a broken diaphram. Swapped it out with a new one and started right up. Statistically improbable but possible I received two bad pumps in a row from summit. Perhaps lesson learnt is not to buy the crap China ones for $15. The first one lasted about 5 hours and the second lasted about 1. Am I really this unlucky or is something causing me to destroy fuel pumps ?
I guess ill know in a few hours if this one rips too. In that case ill welcome any 63 year old beer swilling, under 6' person who wants to lend a hand to come over and help before they hit on my wife. But would really like to know if that line from the top of the tank to the engine bay canister ain't a vent line then what the hell is it ?
Thanks for the help everyone. Ended up replacing all the hoses, running without the gas cap, and replacing the fuel rail so it all suggestions helped in the end.
 
To be honest: I would either order new 3/8"s restoration fuel line,from inline tube..or replace the O.G.40 year old line with your own choice..( hand bent aluminum line,aftermarket fuel line) It's forty years old,and you have added more horsepower to an already not stock ,quality system... I would do it ,just for peace of mind..(ask me,how I know...)
 
trevorcrum said:
Liquid filled.
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In case you were unaware, liquid filled gauges read low as they heat up- 1lb for every 30* increase if I remember correctly. That's why most of them have a vent that you're able to crack open and normalize the pressure inside with the ambient pressure. Does your have a vent?

0-100_psi_liquid_filled_fuel_pressure_gauge_with_internal_pressure_releif.jpg
 
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