Help De-Smogging my '79 F-150 w/351M - Ford Muscle Forums : Ford Muscle Cars Tech Forum
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Old 03-14-2010, 11:58 PM   #1 (permalink)
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Help De-Smogging my '79 F-150 w/351M

I have a '79 F-150 4x4 with the 351M/C6/NP205 powertrain. . .we got this truck about a year and a half to two years ago for $800 and have been using it to haul our quads around. . .and it's been great. It starts up every time, runs smooth, and and stays cool. . .until just recently.

It's suddenly developed a nasty habit of stalling/dying on me. . .it will idle in Park just fine, but as soon as you shift into Drive or Reverse I'm lucky for it to keep running for more than a minute before it slowly idles down and stalls out. . .

. . .I've been told it sounds like it might be a vacuum leak and I hadn't done much to it since we bought it so I went ahead and replaced all the plugs, plug wires, fuel filter, distributor cap & rotor, ignition coil, ignition control module, PCV valve and I got new manifold/carb gasket. I also replaced the rubber fuel line from the pump to the filter/filter to the carb and the rubber vacuum line from the PCV valve to the carb and the line from the carb to the vacuum advance. . .and because the previous owner had obviously "tinkered" in the engine bay I went ahead and got rid of all the other vacuum connections (only vacuum connections being used now are the PCV valve, distributor advance, brake booster, and transmission modulator valve) and capped them with vacuum caps. . .so the EGR valve is capped and the extra two connections in the back of the carb are now plugged as well. . .the vacuum port at the front of the factory intake manifold was already capped.

One other thing is that the idle stop solenoid isn't connected to anything. . .nor has it been since we bought the truck and it hasn't caused any problems before. . .so I'm doubting that's the culprit.

Now, the truck starts great, but it idels faster than before and it takes FOREVER to idle down from choke. . .the first day it seemed to run fine and was idling just fine in Drive and Reverse. . .I started it up today and had to move the quad trailer around and it did the same thing as before. . .dying pretty quickly after putting it in Drive or Reverse. . .

Could anyone tell me if how I got rid of the smog vacuum connections would have caused this? and/or tell me the correct way to de-smog this engine? or I guess it could be the transmission modulator valve (I don't know exactly what this valve does, but since it has to do with the transmission and the problem happens when it's put into gear it makes sense that the transmission modulator valve might be the culprit. . .but I wanted to know what the valve did before I just arbitrarily replaced it).

I just picked up a 400M to replace the 351M so I'm not going to be keeping this engine in the truck, but I haven't even started the rebuild on the 400 yet, and I probably wont' be ready to do the engine swap until late this fall. . .if then. . .so I do need to get the 351 running reliably again.

Thanks for your help!
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Old 03-30-2010, 11:16 PM   #2 (permalink)
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Re: Help De-Smogging my '79 F-150 w/351M

It sounds like a bad brake booster to me. The next time you start it and get the high idle down to normal, try putting it in gear and let it sit there for a minute with the park brake or a wheel chock and no pressure on your regular brakes. if it doesn't happen still you know what to replace. Just a guess? good luck.
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