'79 Ford F-100 Pickup low miles ran great now sputters. Help this student!



The patient- It is a 1979 F-100 pickup with a straight six. It has 75K miles and I use 87 octane.

The problem- I need help diagnosing this condition. It runs very rough. It reminds me of my motorcycle when I once left the choke on for 20 miles. The truck engine sputters and shakes. It used to run perfectly! The sputtering got progressively worse after I first noticed it on my way to a car wash. I wont drive it now.

The history- I replaced the distributor cap and rotor and spark plugs and wires. It also has a new air filter. The oil is good. Eight months ago I had it looked over at the ford dealer and they adjusted it for DEQ (emissions).

My suspicions- stuck choke? (it does get worse a few minutes after its first start then stays terrible all day) Something with the carburator? Bad timing? Gremlins?

THE COOL PART- I made a video so YOU CAN HEAR IT! copy and paste this link into your navigation bar!! http://www…eX5S_xpaHg

you can also try searching Youtube for the exact title of the video: '79 Ford sputtering sound

Help me diagnose the problem and point me in the direction of a information resource. I am familiar with the ARRC. I have limited money for repairs so I would like to do this myself and I think I am perfectly capable. I once disassembled an entire motorcycle.


The choke stuck closed would explain your blackened spark plugs too. Easy enough to check- just warm the truck up, take the air filter assembly off, and take a look at the choke plate(s). Still closed? Open it up manually or with a pen or something (just don’t drop it down through the carb!) and see if things improve.

I might also look for vacuum leaks as well. Even if a vac leak isn’t the cause of your prob, on a truck as old as yours, it’s probably time to replace the vacuum hoses anyway.

Nice vid by the way. AND nice truck. I’ve got a pretty cherry '78 myself. I know they’re a hit with old people and farmers.

P.S. ANYBODY can disassemble a motorcycle engine! REassembling it would be much more impressive. : )


Check the screws that hold the carburetor together, they used to loosen up.

Get a new huge ignition module. It sits up near the left hood hinge. The parts place will ask if it is blue grommet or black grommet. The grommet will look like it is black from the top, but when you remove the bolts and turn it over, it should be blue. Clean it to check. It’s where the wires go into it.

Check the wire to the electric choke, it could be burned off or broken.


++++++++++ UPDATE ++++++++++++ UPDATE ++++++++++++

A friend and I worked out the suggestions so far.

  • The choke is NOT stuck.
  • The mixture is fine.
  • The vacuum hoses seem fine, the dealer checked them 8 months ago.
  • The wires to the choke seem fine.
  • It has a new fuel pump and filter

~ I forgot to mention that concurrent with sputtering it began to blow a little blue smoke.
~ The oil and coolent are clear and fine.
~ There is a good-sized leak around the muffler my friend suggested this affects backpressure, perhaps causing the sputter but not the blue smoke.


Uh-oh. Blue smoke is not good. My next step would be a compression test.


Will do! My friend may be able to get his hands on one or I may have to pay someone. I can rent one for 7.50. I can’t find a tool library for my neighborhood, bummer.


Remove the distributor cap and take a look inside it and under it. It should be clean and dry. Next, try to move the shaft from side to side. There should be NO slop. But the shaft should turn against a spring return, the centrifugal advance…How are the wires to the pick-up coil? Check closely where they go into the coil…

Put everything back together and remove the coil high-tension wire from the distributor. Hold the end of it about 1/2 inch away from any metal surface and have someone crank the truck. You should have a nice, fat, steady, blue spark…If this checks out, it’s time for a carb rebuild.

Also, check the torque on the bolts and studs that hold the intake manifold on the head…


I’d also add one more possibility. Have you checked the PCV valve? If it’s clogged, or if the lines to or from it are clogged and/or leaking that can cause stumbling and blue smoke. Fingers crossed-- you’ll be very lucky if that’s all it turns out to be!


Check the EGR valve and make sure it’s not sticking open. If stuck open it will run like garbage, can cause smoke, and soot spark plugs up.
It’s an easy fix if that’s the problem.