Fuel issue

[GTSpecial39]

On my 71 Mach 1, ran into a fuel issue trying to figure it out. Was on my way home which would of been a 40 minute drive and the car started sputtering, popping and did eventually die. Pulled over to side of road to see what was going on. I have a fuel pressure gauge on it and showed only 3 lbs of pressure where it should be around 5.5 on my 351C. I did not see anything wrong, but took the insert out of the fuel filter and started it up sounded good and took off only for the same thing to happen numerous times the rest of the way home. After stopping, would wait 10-20 seconds, start it up and continue on only having it happen again after 2-4 miles. I started popping it in nuetral and revving the motor a little and finally made it home. The next morning went out to see if I could find the issue, started it up and the fuel pressure was up to 6 but as I watched it, it started to slowly go down to under 5. To me, I do not think its vapor lock or anything blocking the fuel lines, its all pretty new from the tank up and from the fuel pump to the carburetor, but the issue seems to be before the fuel pressure gauge. I did have a regulator and had taken it off a month or so ago since the drop in fuel pressure was happening then too. The fuel pump is only 2 years old, its a mechanical stock one, it is a Carter and is listed as 6 lbs pressure, so not sure if it is getting weak or what else is going on. Any thoughts would be appreciated......Thanks

 

[c9zx]

Well we could be Captain Obvious and say replace the pump, it may very well fix the problem. Is your gas tank venting/charcoal canister system unobstructed and working? If it became plugged and a non-vented gas cap is in use, suction in the tank would be created. I know it sounds like a long shot, and probably is. I hope you get it sorted without too much drama. Chuck

 

[clevelandcoupe]

Is there a sock on the fuel inlet on the sending unit in the gas tank ? I remember back in the day they would collapse and limit the fuel flow greatly.
Thanks, Jay

 

[Spike Morelli]

Another restriction point that happened to me, was that sometimes the car gets jacked up for repairs using the side rails under the body. The main steel fuel line coming from the tank runs right next to the driver's side body rail, front-to-back. A mis-positioned lift or jack can crush down this line here causing a restriction to the fuel pump/engine. If I remember correctly, this line can be replaced in either steel, stainless steel, and either 5/16" or 3/8". When my Mach was used for a movie, the scene required the actress kicking a jack out from under the car to pin a bad guy under it. The movie crew weren't aware of the fuel line there and crushed the line on my car partially, although it would still run. They gratiously covered the cost of replacing it when I discovered it while doing routine maintenance. This may not be your issue, but i thought I'd post it as a longshot.

 

[Blayne9067]

On my 71 Mach 1, ran into a fuel issue trying to figure it out. Was on my way home which would of been a 40 minute drive and the car started sputtering, popping and did eventually die. Pulled over to side of road to see what was going on. I have a fuel pressure gauge on it and showed only 3 lbs of pressure where it should be around 5.5 on my 351C. I did not see anything wrong, but took the insert out of the fuel filter and started it up sounded good and took off only for the same thing to happen numerous times the rest of the way home. After stopping, would wait 10-20 seconds, start it up and continue on only having it happen again after 2-4 miles. I started popping it in nuetral and revving the motor a little and finally made it home. The next morning went out to see if I could find the issue, started it up and the fuel pressure was up to 6 but as I watched it, it started to slowly go down to under 5. To me, I do not think its vapor lock or anything blocking the fuel lines, its all pretty new from the tank up and from the fuel pump to the carburetor, but the issue seems to be before the fuel pressure gauge. I did have a regulator and had taken it off a month or so ago since the drop in fuel pressure was happening then too. The fuel pump is only 2 years old, its a mechanical stock one, it is a Carter and is listed as 6 lbs pressure, so not sure if it is getting weak or what else is going on. Any thoughts would be appreciated......Thanks

a liquid filled gauge gauge can give false reading with temperature increase.

 

[TommyK]

Sounds like the fuel is boiling.

 

[71ProjectJunk]

I am assuming that the fuel tank, sender, fuel lines, and pump are all new. One thing that could be happening to you is one of the rubber lines has started to collapse internally, this is quite common now because of the alcohol in the fuel. Check ALL the rubber lines, they may look good from the outside but be collapsed on the inside. Also make sure that none of the fuel lines are sitting on top of the engine or touching the engine or exhaust as this can create vapor lock. Also check your fuel filter, may be getting clogged. Fuel pump may be going south, but seems like a remote possibility if it is new, but with the influx of overseas parts anything is possible. To isolate al the fuel system from the fuel pump back, you could run a rubber fuel line from the fuel pump inlet to a small portable gas tank and see if that fixes the issue. If it does you know the issue is in the fuel lines, the fuel tank or the fuel sender, if it does not then you only have the fuel pump and the filter to blame. Another possibility is that you do not have the gas tank properly vented.

 

[ROCKET70]

If you have a fuel filter before the fuel pump that can cause this problem. I've seen this happen a lot of times remove it and the car runs fine.

 

[mrgmhale]

I know "c9zx" already mentioned tank venting as being a potential problem area. I felt I ought to push that out there with some added emphasis. I have come across vehicles in the past that had a non-vented fuel cap, where their fuel cap supposed to be vented. The use of a non-vented cap caused the fuel pump to create a vacuum in the fuel tank after some fuel had been drawn out, resulting in no more fuel flow. Although use of a vent fuel cap may correct the vacuum retention problem, it may not be the "correct" fix.

He also mentioned the Fuel Evaporative System that may be causing the problem. The 71-73 Mustangs have a Fuel Evaporative Control system that works well when it is correctly connected. I have attached a PDF that shows how it should be connected (see pages 19-21). I see situations where folks connect the metal vent tube's rubber "vacuum" line to Intake Manifold or Ported Vacuum, thinking that is what is supposed to be used. Or, the metal vapor vent tube (or its attached "vacuum line) is blocked off. Either say, the fuel tank can will up with a vacuum, again causing liquid fuel to not be able to flow to the fuel pump. It is worth checking to make certain your Fuel Evaporative Control system is connected correctly, which will ensure the fuel tank is not subjected to a vacuum condition.

I also have some YouTube videos showing how the Fuel Evaporative System should be connected. It is at:






Here are some annotated photos that may help also.