Help...Cranks but won't start

[DirtDonk]

Keep an eye on those pesky mechanical fuel pumps still. They get old, they get tired, they get tired of sitting unused for extended periods, and especially with modern fuels it seems.
But from your initial tests at least, it appears it's still working well. At least when it's not overly hot. Only time will tell on that.

If it happens to you again in the same way, you might even try the old "when all else fails, beat on the carburetor" trick. Sometimes something just causes the inlet needle to stick in the valve and stop the flow of gas temporarily to the bowl. Rapping on it a couple of times with something will often "fix" it. Also temporarily, but enough to get you where you're going.
The tool used can be as simple as the plastic handle of a medium sized screwdriver. For awhile I had one screwdriver that really took a beating on the handle because every couple of days my Bronco would either not start, or would start and die, or would start and then flood out (needle stuck open in this case) and a few judicious smacks with the screwdriver handle took care of it.
You could literally hear the inrush of gas from the pressure side into the float bowl when the valve opened.

A metal tool like a wrench was the old standby, but the carburetor would look a lot worse for the wear after a few beat downs with a wrench. The plastic handles were much easier on the aluminum.

Paul

 

[DirtDonk]

...I have dual exhaust. Will look at the lines closer to the tanks and see if the driver side line is inside or outside the frame and make adjustments/shielding as needed.

If it's factory then it's on the inside of the frame and usually with dual exhaust, at some point the exhaust gets pretty darn close to the wiring, brake and fuel lines.
Hopefully yours isn't, but if it does then now's the time to find out and do what you can to keep some of that heat away from the important stuff.

Paul

 

[brnco_luvr]

Thank you Paul. Great advice, and education.

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[Rustytruck]

if you have electronic ignition ohms check the mag pick-up inside the distributor.

 

[brnco_luvr]

if you have electronic ignition ohms check the mag pick-up inside the distributor.

Can you please explain why?

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[gr8scott]

Can you please explain why?

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Connect your ohmmeter between the orange and purple wires. Resistance should be 400-800 ohms.
Anything outside that range and the pickup is bad.

 

[DirtDonk]

Or failing, which could be a cause of it working one minute, but not the next after it warms up.

Paul

 

[Rustytruck]

Can you please explain why?

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the mag pickup is a resistance coil as it ages the insulation breaks down and shorts down when hot. the magnetic trigger fails and the module can no longer trigger the coil so you get a dropped ignition. typically these will still work when cooled off at least for a little while and then you get stranded where you don't want to be. mag pickups fail 3 times the rate of the ford modules.

 

[darrell82]

Happened to me 2 weeks ago while wheeling in the Talladega National Forest. Descended down a very steep trail about a 1/4 mile, then turned around to go up it. The first steep incline my 72 302 sputtered and died. Observed the clear fuel filter before the mechanical pump and it was bone dry, even while cranking it over. Started it with shots of Deep Woods Off bug spray and saw gas spurting into the fuel filter but not flowing strong till we got the Bronco level. But even then, I always thought the flow was weak. That cut our trip short since we couldn’t make it back to the main trail. From what I observed the mechanical pump doesn’t produce enough suction to pull the fuel from the main tank especially when it’s lower like when climbing a hill. I also was thinking there may be some leaks in the fuel line which you will not see since the system is always sucking and not blowing. So, the other day I installed a 4-7 psi Holley Mighty Mite electric fuel pump right at the main tank. I ran the engine without turning on the electric fuel pump and the mechanical fuel pump draws gas through it with no problems. The electric is on a switch so I can turn it on and off as I need. So with the engine off I tested the electric fuel pump and as I had assumed gas was leaking everywhere up and down the run of the fuel line. It turns out there were at least 3 junctions of where the poly fuel line to rubber hose didn’t have hose clamps and one spot it the poly line that had a hole. I added hose clamps and fixed the hole and now it runs like a champ. Even fixed issues with the engine starving for fuel while WOT C4 shifting from 1st to 2nd. So now cruising I leave electric pump off, gunning it and climbing steep hills it’s on. Also if the Bronco sits for a couple weeks I’ll turn the pump on and watch it fill the fuel filter and bowls.

Cheers,

Darrell


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[nvrstuk]

Darrell- Even before I finished reading your post I was going to suggest that you check your fuel lines for a cracks or other potential spots were it could draw a vacuum. When the fuel tank is that much farther below the fuel pump because you're going up a steep Hill it takes more vacuum on the fuel lines to be able to pull that fuel up therefore on the level your fuel pump worked great but once it needed to pull that fuel up literally probably 2feet it would suck air.
Old timer story- back in 1976 1 week after I bought my Bronco we were up in northern BC hundreds of miles from any town fishing with my Dad and his Bronco had the same symptoms because it would not pull fuel from the tank...it took us forever but we finally tracked it down to a small crack in a fuel line at the fuel selector valve under the driver's seat. When under heavy load the pump would draw air instead of fuel. That took an entire day for us to find it. Our first thought t was faulty fuel pump...glad ya figured it out.

 

[DirtDonk]

Another old-timer story. Electric pumps were often found on trucks, if they were owned by either "old car guys who were boy scouts and believed in being prepared" or just old rock-hounds and explorers that had found themselves miles from nowhere without a pump.
My pickup had one on it too. Nice clean little panel with rocker switch under the dash.

One thing to watch out for absolutely though, is if your mechanical pump fails in such a way as to leak fuel, you will still have that leak, or possibly worse, when you fire up the electric pump. Whether it's out through the vent, or inward into the crankcase, neither would be desirable results.
I had a failure once and would have loved to fire mine up, but the diaphragm failed and spewed gas out of the vent tube at each pump.
Unfortunately, the brainiac committees that designed the 335 series engine systems chose a vent location that threw the fuel right on to the exhaust manifold! If nicely vaporized fuel under the hood was their goal, they achieved it!%)

One and only time I had a bad pump on that truck, and I could not use the backup electric pump to get on down the road.
But I did learn to add a 90° elbow with hose extension to direct fuel to the ground instead. So there is that...

Paul