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Car Runs For Only 6 Miles

I have an '86 Mustang GT convertable. This car just turned 100,000 (California) miles and is in incredible condition, but it will only run for 6 miles, then die, won’t restart (cranks fine) and waits for the tow home. When the truck off-loads my Mustang, it starts right up and I drive it into the garage. This has now happened the last 3 times I’ve attempted to drive it. I have replaced the distributor, TFI module, coil, PCM power relay and fuel filter. Also, when I turn the key on after it dies, I hear the fuel pump prime so I eliminated that as the culprit. The fact that it will only run for 6 miles every time is more puzzling than, well…the weekly puzzler. Help would be greatly appreciated.

Hearing the fuel pump prime doesn’t eliminate it as a suspect. You need to put a pressure gauge on it and find out whether or not it is within specs. If it is within specs at idle, then you need to see what happens with it when it is put under load. It might turn on - but not provide sufficient pressure.

That is the next thing I would do.

You might want to check out the plug wires. Try the OEM wires. I have found the fancy colored ones then to be expensive but not work well.

The simplest check, and the cheapest too, is to check the fuel filler cap. If the breather on it is clogged, you can get just this kind of thing.

Thanks very much for that suggestion, I’ll give that a shot.

Thanks, I appreciate the suggestion, however, I do have a recent set of OEM plug wires installed.

On your suggestion I just looked at the filler cap. I don’t really see anything that would get clogged. There’s kind of a spoke-y looking thing inside at the bottom of the thread, but it looks clean. Is there some other way to tell if it’s clogged other than just looking down into the cap?

Next time it stops, go open the gas cap and try to start it again. If it starts, the canister vent system is plugged. The gas cap doesn’t have a vent.

And if this is the case, how do I go about unplugging the canister vent system?

I just had another thought. As I now remember, the vent canister is this plastic rectangular box filled with charcoal. There’s a hose from, I believe, the intake manifold going into that canister and then out of the canister into the gas tank. From the outside, it all looks intact. A couple of months ago I had to have an emissions test to renew my car’s registration. They have a new additional test here in Calif where they remove your gas cap, attach some kind of hose gismo, and test the emissions in the tank. Is it corrrect to assume that if the canister was plugged I would have failed the emissions test? One other thing, as long as I have owned this car, when ever I fill the gas tank (usually at 1/4 tank), as I unscrew the cap a lot of pressure is released. I’ve never owned a car that did that and have always wondered if that’s normal. It always did that, even when the car was a dependable runner.