What could cause a low fuel pressure situation

[prayers1 153]

I have a 351 Cleveland, new QF 735 carb, new SS tank, new sending unit and metal fuel from tank to pump. The mech fuel pump is new and I just installed a cowl mounted fuel pressure gauge.

 

Problem is when I lean on it and the rpms start to build up to 4000, the fuel pressure drops to 3 psi. During highway or normal driving, it stays at 6-7 psi.

 

I changed to a LKQ fuel pump and filter today and I have the same problem.

 

What do you think is causing the restriction.

[Hoover 12]

A possibility is that the diameter of your fuel line may be too small. When accelerating, the carb needs more fuel to provide the power. A small line cannot supply any extra fuel.

[sportsroof69 13]

What pump are you using? How much power does the engine make?

[LoneWolf2U 136]

Why do you think there is a restriction? your opening the back barrel and demanding more fuel. Is the engine falling flat or your worry of the guage reading?

Seems like its working correctly as long as it is supplying enough fuel for the demand.

[prayers1 153]

I have 3/8's from tank to carb.

sportsroof69-I have the RobbMc 550Hp six valve high volume fule pump, Max output Volume 98 gph @ 4.5 psi and Max output pressure is 7.0 psi. Est hp is 375-400, per builder not dyno.

 

Mach1Rider-Its falling flat on it's face and a couple of time the engine just died. Under normal accel it's ok, but once I nail I now can see when the rpms go up the pressure goes down.

 

An interesting note, not all of the time but it does happen often, when I get into it and it shifts into 3rd, I hear a sound like a turbo whine and thought that it was related to the trans. Could that sound possible be coming from the vent tube, thinking that the bowls are dry and air is being sucked in.

 

Another avenue of thought, while the car sat for a few months waiting for the engine rebuild, I noticed 3 large droplets of dark brown fluid on the fuel line 3 ft prior to where it separates under the drivers seat. I cleaned it off and never saw it again and I've never had any fuel deposits on floor.

 

The only restrictions I can think of, is a possible defective fuel line or the sending unit sock filter.

 

Keep in mine, that prior to all of this, I had a rusted fuel tank. I replaced it with a new SS one, a new Scott Drake sending unit and blew out the fuel line that had been previously replaced. I ran this system with the 289 . Since the 351C, I added a new carb, pump, filter and hoses from pump to carb.

Edited August 23, 2014 by prayers1

[LoneWolf2U 136]

Now the tech part, where is the fuel pressure guage connection in relation to the fuel filter? before or after it. If its before the filter, your reading may be pressure against the filter which means there is a problem of fuel getting past it. If after its pressure to the carb inlet which means there may be clogged fuel screens at the inlets to the bowls.

recheck all the hose clamps to insure they are tight.

[prayers1 153]

Installed after the pump and filter. Its a Spectra Dual feed line that came with a fuel pressure gauge. I took that gauge off and in its place, I installed a braided line going to the cowl mounted Autometer gauge.

 

I have to rule out the pump, because it happen with 2 separate pumps. It's weird that I never had this problem with the 289 or maybe it wasn't pulling enough at the time.

 

I will check the fuel inlet lines when time allows.

 

P.S. Glad to see your around!

[LoneWolf2U 136]

So its after the filter, means the pump is supplying fuel. then its between the filter and carb that has your flow slowing.

As for falling flat at high rpms, After you check and rule out the fuel screens look at your sec jet size. Too small and it will limit the high side of the rpm band.

 

Ps still around but mostly lurking.....:whistling:

[jmlay 80]

When removing your fuel cap do you hear air rush in, indicating a vacuum is building in the tank? When installing the new tank did you also install a new filler neck? There is typically what looks like a small imperfection in the filler neck that serves as a vent for the tank. If the one currently installed does not have this or the CAP is not vented this could be your issue. WAG

[prayers1 153]

I used the original fuel neck, I replaced the pop on gas cap with another aftermarket type. With the package, it gave 2 options, 1 was a very small twist on that was concealed by the pop on cap and the other was more like the original, in that it had the cap mounted on the pop on cap. I used method 1.

 

I did not hear any air escaping and I don't see much of a vent in the small cap. I wonder if I can drill 3 1/8th holes in it or would that cause future problem.

 

I did fill the tank last night and the fuel starvation still happened. IMO, by filling the tank ruled out a vapor lock.

[LoneWolf2U 136]

The replacement filp cap is NOT vented. Drill a 1/16th hole in the center of the cap for venting. This could be part of your pressure problem as it causes the pump to try and overcome the vac in the tank.

[prayers1 153]

Mach1Rider, I will do that today. You would think this would of been done by the Mfg.

[prayers1 153]

I took apart the fuel line from tank to pump and blew it out with compressed air. I also had the line a part at the 2 piece rubber hose section. I placed a longer hose on each end as I blew compressed air into it. the gas in the pan looked suspicious. When I went to reconnect the line at the fuel tank a lot of gas passed by via the line and into the pan. I was surprised to see how cloudy the gas looked.

 

I also took apart the dual fuel inlet line, the carb did not have a filter. The gas that came out looked cleaner and the clear site hole on the bowls looked like the gas was clean in there as well.

 

Even though I put the line altogether, I had resevations to start the car. I'm thinking the tank must be full of something and maybe the sending unit filter might be clogged.

 

After I had a previous problem, when the car sat for a long time with fuel in it and the entire tank rust within, since then I replaced the tank and sending unit and have only used gas from a station that advertises NO Ethanol or Alcohol. So what could be in the gas????

 

Here's a picture of how I ran the fuel pressure gauge and the gas that came out of the tank.

 

[LoneWolf2U 136]

Your carb looks like my Holley 750, are you sure there is no screen in the inlets? Mine has brass screens behind the large nut the fuel line attaches to.

as for the fuel in the pan, did the tank have some sort of protective film inside. Looks like it may be coming off.

 

As for the flip cap, They say it is for show only in their ad. I put a 1/16th hole in the center for venting and even with the tank filled up to the neck none leaked out while driving.

[prayers1 153]

Interesting about the protective film, never heard of that. I had changed the entire fuel system when I had the 289 and drove it like that, no problems with starvation. I'll call CJ Pony Monday and see what they say.

 

I'm draining the tank tomorrow.

 

Yes the Quick Fuel carbs are like Holley's. I did take the 13/16s nut off and there was no brass or paper filter. Usually there is a stone filter with a spring.

 

You think 1/16 hole is enough for ventilation.

Heres a picture of it

 

 

[jmlay 80]

Rather than drill a hole in the cap I would put a dimple or crease in the mating surface of the filler neck, where the cap seats. Something like this.:

 

[LoneWolf2U 136]

The dimple may work if it is deep enough that the rubber gasket does not seal it.

Before calling CJ, What color primer did your paint shop use?

Mmmm..... Lack of prep covering the open hole maybe.

[prayers1 153]

I don't know if you recall, but somewhere on this site explains the prior problem I had with a stock tank. The car sat at the Bodyshop for over a year with 1/2 tank of gas. The gas rusted the entire inside. I found this out after getting the car back from the shop.

 

So, to answer your question, the new tank was replaced after getting the car back.

The entire fuel system was replaced.

 

I'm getting a slow start today and I wanted to see if there were any other comments prior to tackling that job. Boy, I hate the thought of pulling the tank out. I did such a fine job of sealing and bolting it in.

 

I'm puzzled why the gas turned out that way. Like I mentioned earlier, I used only one station, high test with no additives. My thinking was that other stations have additives that will rust the inside of the tank if allowed to sit for some time.

[LoneWolf2U 136]

Tank did not have drain plug on right side?

hanged after paint leaves only coating inside of tank or someone playing games wih you.

 

Did you keep the gas in the pan? check to see if it settled overnight.

Color is strange looking primer like.

Edited August 24, 2014 by Mach1Rider

[prayers1 153]

Yes it does have a drain. I laid down a seal before setting in the tank, now I have to break the seal w/o bending the tank lip.

 

Car is always in locked garage, got rid of gas in pan.

Yes this does seem very strange.

 

Only thing I can think of is that the gas station has something seeping into their

tanks.

 

I will put some gas in a clear container and see what settles.

[det0326 179]

Its hard to tell from the picture what the mixture is that you collected in the pan by blowing through the line but it may be oil and water mixture from your compressor tank that maybe had settled in your air line and mixed in with the gas in your line. Just a thought. I would some how get another sample from the tank and take a look at it.

Dave

Edited August 24, 2014 by det0326

[prayers1 153]

deto32- When I reattached the rubber hose from the tank to the line, I kept the hose hanging into the catch pan at the separation point under the drivers seat. That is actually the gas that came out of the tank. No compressed air introduced.

 

I drained the tank and pulled the sending unit. The sending unit looked as new as it was the day I installed it. The filter sock was in very good shape and no signs of any contamination.

 

I was able to see inside the tank and it too was very clean.

 

Below are pictures of the sending unit, gas and a coke bottle filled with the last amount of gas.

 

That last bit of gas, I caught in the catch pan, when I tilted the car. I noticed a very small white swirl of something. That white swirl I believe made the remaining bit milky. The majority of gas looked clean.

 

I'm thinking that this may not be the cause of fuel starvation, but I'm not sure at this point. I did not take the carb a part since I'm waiting for more opinions.

 

[jmlay 80]

If those buckets are the gas you have drained. Please get a lid on it & move it to a safe location outside your garage & where children & pests can not get into it.

[det0326 179]

If the inside of your tank is as clean as this fuel I don't see a problem with the tank at all, your problem is somewhere else. You mentioned in a post above that you saw maybe a spot where it might have leaked a bit, if it came from a joint in the line it could be maybe sucking some air there at higher RPM's. When you get on the throttle hard does it stumble immediately are after it approaches higher RPM's?

Dave

[prayers1 153]

Mike,

Don't worry Ive already took care of it. The open Buckets were for pictures. In fact I've already used the fuel in my truck and daughters Jeep.

 

In regards to those droplets, it has always puzzled me. It happened so long ago, but it's something I always remember.

 

Tomorrow I will test the line by capping one end and blowing air through the other.

Visibly I checked the entire line and could not find a problem.