Looking for help on mysterious fuel interruption

[Lloyd1969]

I'm looking for some advice on an interruption in my fuel that's been happening sporadically the last couple of years. I've taken it to several mechanics who tried several things, but it still happens. I recently took it to the local classic Mustang expert and it stumped even him.

I have a plain Jane '69 fastback with a 351 Cleveland, so it's not the original drivetrain.

There doesn't seem to be any rhyme or reason to it. It won't happen for months, and then will happen every time I drive it. Happens when the car has been driven just a couple miles or only after 20 or 30 miles. During cold months and summer. When the (aftermarket) AC is running or not.

I've always been able to get it going again by disconnecting the fuel line under the hood and sucking on it. So it's a loss of vacuum somewhere.

The first thought was vapor lock, though that seems unlikely to me since I had the car in Florida the first seven years I owned it, and Indiana summers are wimpy by comparison. First mechanic added an electric fuel pump at the tank, in addition to the one under the hood. Both are working properly, according to tests. Ditto for filters, which have been replaced (several times) despite no detected clogs. No detected crimps in the fuel line.

Don't think it's crud in the tank, either. We dropped a new tank in the car when I first bought it, and put in a new sending unit last fall.

I'm at the end of my rope. I used to drive this car all over the state of Florida, put 5,000-6,000 miles a year on it. Now I'm petrified to drive further than the corner.

Anybody out there got any ideas?

 

[jikelly]

Bad fuel pump?

Perhaps the diaphragm in the pump is wearing out.

 

[rbohm]

it sounds to me like you have a clogged or partially clogged fuel line.

 

[truck90278]

When a fuel tank is not vented properly, you will get similiar effect. It will run for a while and then stop. As vacuum is not normally applied to the fuel system, it is odd that when you suck on the fuel line, it works. Might also try disconnecting the fuel line before it goes to the mech. pump and disconnect the line on the engine side of the electric pump and blowing it out with air.

Also try by-passing the mechanical pump just to eliminate that as a potential intermitten problem

Does the new sending unit have a sock over it to stop big items? Might want to re-check it.

Not sure about '69s, how is the fuel tank vented? On the earlier mustangs, the cap is vented. At some later point the gas caps are not vented. Whichever manner the tank is vented make sure the venting system is working. For earlier systems just replacing the cap solves the problem. On the later systems, the venting is part of the emissions system. If the tank is not real full, might try driving with a loose cap or remove the cap and see what happens.

 

[Pakrat]

Are you sure it's a fuel problem and not more like the fuel not getting ignited problem? The basic fundamentals to most car problems are fuel, air or spark, when one is down it stops. I did not see where you checked the carb? Also, are you running points still on the distributor? Have you watched the engine running in the dark and looked for arcing of the wires? I would also consider running a 1-2 gal dedicated fuel source directly to the carb (just hang the jug from the open hood) and run the car in place for a while and see if it still happens. If it does then you know from what point the fuel problem starts. If not then you know it's from their to the tank at least or maybe not fuel at all.

As for the vapor lock that usually happens from the heat under the hood or a misplaced exhaust pipe not so much the weather outside. Lots of sudden changes can bring it about like adding exhaust headers for example. A good way to check or eliminate this is to locate any spots in the fuel line that run close to any heat source and put a clothes pin on it. Wood disipates heat much faster than metal so if the fuel was boiling this would take care of it.

 

[bnickel]

you also need to check all the rubber fuel lines. there should be 4 connections if it's all still stock. there's a rubber line at the gas tank, one right next to the driver side front torque box, almost right under the front seat, there;s the connection from the metal line to the fuel pump and another at the carb. if any of these lines are cracked they will cause the problem you're having. if they haven't been changed in the last year at least it wouldn't be a bad idea to go ahead and change them all anyway.

 

[davetrue]

a hole in the fuel line or a kink, also a vaccumm problem w/ the tank ex. the tank vent. its either in the lines or vent.