So I've been fighting a problem with my truck for the past couple mounths and am fresh out of ideas. I am having a issue with fuel pressure on my truck. The truck starts and idles great. I can let it idle for an hour and not have any issues i can even work the throttle through the rpm range and still not have any issues. The problem is when i start to drive the truck and gass it under load the fuel presure starts bounce around. It is set to about 5 psi normally but when i drive it for a bit it will start bounce around 3-6psi and if i continue to drive it it will drop to about 0-3 psi. After parking the truck and letting it idel for a while it returns to normal.

The truck is a 1977 3/4 ton with a 360 and a 3 speed transmition. The Fuel system is all new from the tank to the carb. The lines are all -6 push lock with AN fittings. I have an 80 micron filter right out of the tank and a 40 micron filter just past the holly fuel pressure reg. I am using a holley blue electric fuel pump and an edlebrock 600cfm carb with electric choke. The carb is spaced with a moroso spacer to keep heat out of the carb. Motor is stock. I also have a full msd igniton with a 6a box, pro billet distributer and high vib coil. All aftermarket pasrts are new

I have checked and re checked the lines and filters to make sure they are still clean. I have also replaced the electric system and tryed a new system using a carter stock pump but that made the problem worse so i went back to the electric system. At this point i have no clue what could doing this but i am at my wits end and could use any help i can get.

 

Is this regulator a bypass (return line) type?

About this hose---how much is in the suction side, and are you sure the hose is not sucking shut

Is this 80 micron for certain rated for the flow you might be getting?

 

You didn't say whether or not the truck runs poorly when the pressure goes funky. If it runs fine I would replace the gauge.

 

It isn't a gauge problem. I have used two different gauges just to make sure the reading is correct. Also the sound of the fuel pump will change slightly when the pressure drops. The truck will start to bog hard and then the engine will sputter and die. The fuel pressure is %100 dropping to a level that makes the truck hard to move more than a few blocks at a time. It's the why that is killing me.

The reg is not a bypass type. it is the standared holly reg with one port in and two ports out (i have run two different regs and had no change). As far as the fuel line I have about two feet on the feed side of the pump. I have not been able to find any sucking in of the hose. This stuff is NHRA compliant Aeroquip push lock hose and is very thick walled and strong stuff. It is considered hard line by the NHRA. The fillter is for sure rated for the flow. It is the most comon earls pre filter. I know of countless guys running it on higher flowing setups than mine.

 

I have been over this thing time and time again and it just dosn't make sence. I have also done checks to make sure the tank is vented and there is no pressure build up. All of the filters have been replaced in case of any hidden problems. I have tryed this setup with two different carbs with no change. I will also mention that the gauge is in line between the reg and the carb.

 

Have you tried
- a timed (GPH) fuel flow test into a 5 gallon bucket for 30 seconds to a minute from the carb side of the regulator?

- a direct(from battery) hot wire to the pump

- a direct (from battery)ground to the pump

- to check battery voltage

 

Maybe the pump itself is bad. Don't these come apart?

 

I will try a gph test into a bucket and check it against the pumps specs when i have a chance. The power that runs to the pump comes from the battery to a manual switch and then to the pump so that is fairly direct. I can try running the gounds right to the battery and see what happens. I have checked volts at the pump and they seem to be fine.

Keep in mind that I have tryed to run this settup with the stock style mechanical fuel pump and also had the same issues but the truck was wores at idle most of the time so i went back to the electric. This makes me think that either i am very unlucky (which is looking like that is true) or the fuel pump alone is not the issue.

I keep thinking this could somehow be carb related but I have tryed two different carbs so it is not likely. This sounds counter intuitive but is there anything motor retlated that could effect this?

 

May be more than one issue?


On a newly plumbed system I might suspect a miniscule particle of hose material stuck in the regulator. Heard of this many times from racers!(I run a 10 micron exit filter from pump)

Is the engine and the ignition system well grounded to the body/frame

Bypass fuel pump switch(test)!

If you have a master battery cutoff switch, bypass it (test)! The proffessor(WJ) got caught out by one of these!


Try a voltmeter on the battery and start the truck and watch the voltage droop and recovery. Should recover to 14+ volts before settling to 12.8 - 13+ish.]\

Possibly the ignition sucking the battery down under load and limiting the voltage at the pump.

Try extending the voltmeter into the cab and driving.

Check the spark plugs and tail pipe for extreme soot.

Check the coil high tension lead at 1/2" air gap to ground. With well insulated pliers!!!. It should easily jump 1/2"

Just picking at straws, process of elimination. Keep the information flowing.

 

How close is the fuel pump to the fuel tank? Should be within 20" at MAX..
Also... Set your Regulator up to 7-8 PSI.... And you have #6 line from tank to carb Rite?
You did blow and clean all the lines before installing them?

I agree with the flow test, Unhook it at the carb and put the hose in a can and Flow it at 7-8 psi...
If it will free flow 5 Gallons, then I would say the reg in the pump is no good. I think that pump should push a pressure of around 15PSI or something close to that.

 

I've been over the electrical system front to back and back to front. I do have a volt meter in the cab and have checked the rest of the system with my hand held. Everything on that end looks to be good. I have pulled the plugs a few times and they look cleaner than i would have expected for the amount of stalling this thing has done. Ive gone over the ignition and not found any issue. The fact that i have had the same issue or at the very lest appered to have the same issue with the mechanical pumps makes me think that it isn't a power issue. I won't rule out the pumps though because something could be killing the pumps or there is a small chance that i somehow got stuck with three crap pumps.

Just a note when i ran the mechanicle pumps i ran new lines then when i run the electric and when i run the electric i remove the mechanicle pumps and use a block off plate. I am not trying to puch through the old pump with the electric one.

As far as the lines they were cleaned and blown out before i put them in the truck and i have removed them and cleaned them again just to be sure. The lines have also been compleetly replaced 2 times now. It looks the electric pump inlet is 18" from where it enters the tank (mind to the pickup in the tank is about 8" or so long. As far as the pressure i set it two 5.5 psi which is what is called for on an edlebrock carb. 7-8psi would be good for a holly but with an edlebrock it isn't a good idea. The holly blue pump should max out around 14-15psi. I am going to do a flow test the first chance i get and see what happens there.

 

How about a piece of turd in the tank that gets sucked onto the pickup under high demand periods and then floats away after?

 

With all the fluid diagnosis and parts elimination process, I've got to think it's a voltage drop in the vehicle while driving or an electric motor issue.

An amperage draw under load and free flowing may give a clue. The voltage at the pump when pressure drops will give some info but won't tell the whole story.

 

True Coronet: But just as a quick check the reg will hold it at least to 8 psi to check flow... And load the pump slightly....

My real thought in the beginning is just where Mr 4 speed is heading....
My thinking is if all is good from the pump forward, That part is golden.
And the problem would lie in the pick up with a crack or restriction in the line.
Then I think a pressure or vacuum test is in order from the pump to the pick up.
Might even be as simple as "in the case of a fuel cell" that there just isn't enough fuel to reach the pick up? Or it's hooked to the wrong pick up.

Once I was sure all was good from the pick up to the carb fitting. I go into the Carters because On most of them that I have worked on in the past. The float level is set to low out of the box.

Great Points Guys...

Just my 2 cents..

 

Yeah, I forgot about the mechanical and electric with the same issue. It must be behind the electric, Maybe a pick up sock collapsing.

 

It's good to see that my past thought prosses was on the right track. After i couldn't find any issues with the fuel system i took the tank out of the truck. The pickup assemble wasn't coming out of that thing any time soon so i cut the thing loose. The pickup sock was loose floating around in the tank and the pick up tube was full of some ****. So i thought hot dam here is my problem. So i made a new acces plate for the tank and a new pickup assembly. The new pickup was a little bigger than the previouse one and had a T fitting on the bottom so that way there would be two points of entry instead of just one. I did make sure that the pickup would be submerged at all times. Then i chucked the old gas and cleaned the hell out of the tank. Let it dryout and i even wiped all the little bits of dust out of it with a clean rag and then sealed it up. I then replaced all my fuel line in the truck and checked all the filters and the pump and then put the thing back together thinking i was home free. But no. Didn't make much of a difference. Ran a little better but not much.

As far as a pressure check i have run the thing into a pale before and held it to 9psi and it held steady. I didn't use the reg though i used a set of vice grips on the line.

I think I might put a presure gage between the pickup and the pump just to see what its at. Not sure what it should be though. I have also always run this thing with at least 1/2 a tank in it so it shouldn't be an issue of the fuel not getting to the pickup. I'm going to start pulling my old mechanicle pumps apart aswell as some of the other old fuel system components and look for some clues. I can't help shake the feeling that there is something wrong in the tank but i went over that thing with a fine tooth comb.

 

Got buddy you can borrow a known good carb from? I dunno, just one less variable...

 

Good pull out what you made from the pump back.... Cap thing off at the t in the bottom of the tank and test it from the pump back to the t by putting vacuum to it and see if it holds......
Also at that point... Block off the carb inlet fitting, and then pull vacuum from the pump... If everything holds. the system is tight.... If it leaks down, your drawing air within the system and you got to find it....
If you cant vacuum test it, then pressure test it with you shop air with 20psi, use a soap and water to find it....
Once thats done we can move on...... Bench testing the pump.. just an FYI, I have had 2 of those pump new in the box to be bad... And boy did Holley get an ear full from me over it... Oh and Summit also....

 

I will do a pressure test tomorow and check it all for leaks. It could be possible that there is air getting in the line some place between the pump and the tank. Not vary likley but possible.

I'll also start checking over the pump again. What was wrong with the two that you bought? Was the bypass in them sticking because i have heard of that. I did pull mine and check it and it looked pretty good.

Mr4spd i don't have any freinds that have an extra carb on hand but i have run two different carbs on this and had the same problem. This one is brand new and the other one i used is like 15 years old or so but has always worked great untill i have had this issue. I think the only thing carb related that could drop the pressure is if the secondairys are dumping to much fuel or if i had some kind of leak in the float bowl and im not seeing any real sing of over fueling. I think that when they are opening i am simpley just not getting enough fuel into the bowls to replace what is being used.

I think i will try to do a gph test right off the line that feeds the carb under 8psi or so. I'll check this before i drop the tank and do a pressure test. Droping the tank in and out of this things isn't fun. I would just take the box off but it is a long box and this truck is lifted and on 35" tires so it would be a massive undertaking.

 

One had the by pass defective..... The other the bushes where junk....

 

I really couldn't see a carb causing this anyway, just thought the evil spirit causing all this might live in one of the float bowls or something. I think Bob's on the right track, and the reason the pump swap you did before didn't work was because of crap in the tank, you know two problems at the same time.

 

I did a pressure test on all my lines and the pickup and could not see any air leaks. I checked over everything with great care just to make sure all was well. I didn't get to check the pump or do a gph test before i got slammed with more rain. So i will do that as soon as weather permits and post what i find

 

please do, can't wait to hear what the outcome is. best of luck man

 

So i checked the pump today and i got about 15gph past the reg. Then i tested the flow with no reg and the fuel was coming out in spurts. I think that the reason it came out steady with the reg was because the reg was holding enough back pressure to smooth out the flow. To make sure that it was an issue with the pump and not something else like an air leak or pick up issue I tryed the pump on its own with the feed line in a gas can and the out line into anouther can. When i turned the pump on the flow then came out hard and fast. While i was cursing the gods trying to figure out what the hell was happening the flow started to puls again and then just went to a steady low flow. I timeed the flow and it was around 20gph. So i now know that the pump is being not working right. I will double check the volts at the pump just to make sure that it isn't some kind of intemitent electrical issue but i have checked before and its been good so far. I will get a pump at the end of the week and swap it out. I will also re run the lines so that eveything is in the best possible location and try it all again.

 

Return that fuel pump....It's junk.... Get the 140GPH pump...
Jegs Brand.... Holley Black pump..... or Berry Grant.....