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'79 F-250 shut's off after warmed up

 
PlumberPat
New User
Posts: 1
Joined: 02/11
Posted: 02/02/11 05:23 AM

I have a '79 f250 with a 351m, it's my plow truck, It ran great half the winter, but it developed a symptom lately of after running for awhile it would shut off. I naturally checked a few things and decided it must be electrical but after replacing the ignition module, coil, and distrubitor still no progress....I don't believe its vapor locked, but is it typical for a mechanical fuel pump to get weak after the motor is warmed up? It's gotten to the point that i'm lost?  

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MustangMan
Moderator
Posts: 588
Joined: 07/06
Posted: 02/02/11 07:33 AM

My first guess would have been the Duraspark module, but you replaced that, along with the coil and pickup. So next thought would be fuel pressure. You can buy a cheap gauge and zip tie it to your cowl or wiper arm. Tee the gauge into the fuel line between the pump and carb and keep an eye on the pressure. You should have roughly 5-7psi. If it warms up and you see the pressure dropping you have a pump issue, but frankly, most mechanical pumps usually just have a failure of the diapghram and stop working.

BTW, what's a "plow truck"? I've never seen one of those here in Florida... LOL

Good luck with it!
Mark  
Mark Houlahan
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Mustang Monthly Magazine

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fair67driver
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Posts: 161
Joined: 03/08
Posted: 02/02/11 10:38 AM

I have a 79 250 4x4 supercab 351m with a plow as well. I had a similar problem with mine in the summer(without the plow). I swapped on another fuel pump and never looked back. It has not missed a lick since.  
1967 Fairlane 289-4v auto...for now
1967 Galaxie 500 302-soon to be 534
2007 Mustang
1979 F-250 EXT CAB
4X4
2006 Taurus

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waynep7122
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Posts: 146
Joined: 10/09
Posted: 02/02/11 07:56 PM

if its running on when you try to turn it off....  

its idling to fast...

its got a vacuum leak causing it to idle to fast...

the fast idle cam is binding and the fast idle screw is holding the throttle shaft open like its supposed to do.. except its supposed to release when it gets warmed up...


the fast idle cam is not dropping out of position as it should as the choke pulloff mounted to the passenger side rear of the carb is bad...  not pulling the fast idle down.. and causing a vacuum leak.. increasing your idle speed...

the carb mounting gasket is burned out... causing a leak...

the carb mounting gasket.. ( adaptor to manifold ) is burned away...

the EGR valve is leaking slightly...  causing a rough idle.. which somebody sped up the idle speed to compensate for .. now its running on when you try to shut it off...

the carb mounting / egr plate is corroded by the exhaust gasses and leaking exhaust gas into the intake...

the mounting bracket for the Dash pot that slows down the closing of the throttle lever when you take your foot off it is loose and jamming into the linkage..   or the dash pot is stuck and holding the throttle lever open slightly....

vacuum leak someplace.. like the power brake booster...  

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waynep7122
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Posts: 146
Joined: 10/09
Posted: 02/02/11 08:06 PM

fuel pump testing...

disable the ignition system...  

disconnect the fuel line at the line into the carb...  install a section of hose on the end of the fuel line.. so you can run it into a clean, clear plastic bottle..   2 liter. or 3 liter...   have somebody crank the motor for 10 to 15 secconds without stopping..   watch the individual pulses of fuel into the bottle...   each pulse should be even... the same full pulse.. if they get less or change volume..  you either have a fuel pump problem . or a clogged line...

i like to hook up a pressure gauge to the fuel pump.. crank the engine a few times till pressure builds..  stop...  watch the gauge..   it should hold pressure..     since the fuel pump pressure gauges are also vacuum gauges...  15 pounds of pressure.. 30 inches of vacuum..

test the fuel pump for vacuum...  hook it up to the large line in...  crank the engine a few times..

examine the front of the block for a clean areas where the fuel pump vent is..  

the diaphragms start to leak.. the fuel will squirt out of that hole for the first few turns...

if the check valves are bad or dirty in the pump..  it won't hold pressure .. or make vacuum...  

fuel pumps for these are cheep...

be sure to take the distributer cap loose... bump the engine around till the rotor is pointing toward the number one spark plug wire position..  this should be the proper cam position so the fuel pump eccentric is at the correct location for ease of fuel pump installation..    works the same on chevies also..  

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waynep7122
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Posts: 146
Joined: 10/09
Posted: 02/02/11 08:12 PM

last thing...  if you have the sending unit out of the tank..  toss in and make sure they fall to the bottom.. some super magnets.. sometimes you can use a piece of heater hose and stick it all the way into the tank.. using care not to bend the sending unit lever..    you can now.. install little magnets into the bottom of the tank...

or take some old big strong speaker magents.. and stick them to the bottom of the fuel tank..


why... the magnets will pull any Red dUST to them..  keep it out of the fuel system... at least until somebody pulls the external magnet off the tank...


i had problems with the magnets i dropped in vanishing..  they fell in.. but fell upwards and stuck to the top of the tank...  

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ERIK427
New User
Posts: 7
Joined: 11/09
Posted: 02/03/11 02:24 PM

Icon QuotePlumberPat:
I have a '79 f250 with a 351m, it's my plow truck, It ran great half the winter, but it developed a symptom lately of after running for awhile it would shut off. I naturally checked a few things and decided it must be electrical but after replacing the ignition module, coil, and distrubitor still no progress....I don't believe its vapor locked, but is it typical for a mechanical fuel pump to get weak after the motor is warmed up? It's gotten to the point that i'm lost?

A fuel pump can do this but not likely. I would check for a cracked ring, sounds like you have something binding up after it gets hot. How easy is it to crank over after it stalls?  

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