1977 F150, 4x4, 351M, 4 speed manual

I have just changed out the ignition coil pickup. I had done the control module last winter and didn't do the pickup. I had just recently had some strange hesitations and stalling so I went and did the coil pickup thinking it would help.

Turns out it has the same symptoms. It starts and runs during warm-up o.k. but as soon as I pull out and give it a little gas the engine rpms drop to near 0. If I'm quick on the clutch it will come back without stalling. Out of gear it revs just fine no stumble, no hesitation. Sometimes I'm able to go a little down the road with no symptoms and then out of nowhere it cuts out.

This is a recently rebuilt carb that has hundreds of miles of good performance to it's credit. It's got a new fuel filter and all vacuum lines are good. The ICM checked out OK on a meter.

So, my thoughts have always leaned toward the fuel pump but I would think I'd pick up some stumbling even when at a standstill. The in gear/out of gear thing has me stumped. If I hadn't checked already, I'd say it behaves like a choke problem. But I checked mine and it's working fine.

I'm hoping someone has had similar symptoms and can help me out. I desperately need this truck running and just can't figure this one out.

 

have you checked your timing? the chain may have jumped a tooth

 

Never thought of that. Going to be a while to dig out the timing light after our recent move.

 

While it's idling do a wire wiggle test. My 76 had similar issues and it ended being a short in the wires between the alt and solenoid.

 

Based on some reading here on the forum, I'm going to investigate the fuel strainer on the dip tube of the fuel sending unit. It only dies under load but idles and revs fine. From what I understand, Fords were sensitive this way.

I will also investigate the wires too...but it's most likely fuel. It only happens when I press the gas pedal and even just a little.

I'll follow up as to what happens next.

 

Double check the accelerator pump.

 

So, here's where my logic lies. I figured since it only dropped idle to almost zero and I could save it with a quick stomp on the gas that it was a fuel problem.

I replaced the fuel strainer in the gas tank and added new rubber lines since the old ones were shot. No change; same problem.

I got to thinking that it could be ignition related...My ICM died last winter but I never changed the pickup. So I replace the pickup; No change. Also threw on a new voltage regulator since mine is original to the truck.

Then I figured it could be a loose wire as it does happen only while moving. I cleaned all my connections, lubed them with dielectric grease and reconnected. Everything looks fine. Also did a scan of vacuum lines; everything checked out ok. No change.

I read elsewhere to put the radio on and the lights and see if they go out when the truck dies to determine where the electrical problem may lie. My lights were on and the radio continued to play but the truck wanted to shut off.

What's left? I just cannot come up with a reasonable cause that would explain what's happening. I was thinking fuel pump but I don't think it would start and idle or rev from 650 to 3000 without a hickup..but maybe.

 

Its pretty cheap to see if it is a fuel pump. But throwing parts at a problem usually don't solve it. I would say maybe try some seafoam. Get a can and use a vacuum hose from the carb to suck about half the can down the carb and pour the rest in your tank. It helped my old 78 with some fuel problems doing that. Different symptoms but still did the trick.

 

To follow up:

I've gone through the entire fuel system from tank to carb with new:

1. rubber fuel lines
2. inline fuel filter
3. fuel strainer at tank
4. fuel pump

Got in started right up, idled ok, warmed up, put it in gear and just cut out dead. Starts right back up but dies as soon as I let out the clutch.

WTF? Checked the coil and it appears ok with 1.5 ohms across the terminals and ...if I remember right about 13k ohms to the distributor wire. ICM checked out ok at Autozone and is less than a year old. There is no hesitation or stumbling, nothing. Just dies when I try to engage the clutch.

 

Reading all you've done and just thinking out loud... Is it possible that the clutch pedal is hitting some wiring under the dash that is related to the ignition switch and causing a short or something? Does it start right up regardless of if you've got the clutch pedal pressed or not?

I had a similar problem like that in a prior truck. The clutch pedal had gotten tangled with a low hanging wire for the brake lights, and the brake lights quit working any time the clutch was pushed.

Also, do you have one of the small fuel filters that screw into the base of the carb where the fuel line attaches? I've seen those get plugged, but they usually cause a stall and either a no start or a lot of cranking to get it restarted.

 

It's not that directly related to the clutch. I can get it into gear and move a short distance then it just cuts out. Occasionally, I can snap it out of gear, stomp the clutch in and tap the gas and it will come back up in rpms.

I checked all the electricals on the firewall and main harness; nothing out of place or worn.

It has to be related to the load on the engine...either the fuel cuts out or the electrical dies. I was thinking alternator but the gauge reads ok and my choke, which rides off the stator of the alternator is still getting current.

The last thing I did was replace the fuel filter at the carb. The old one had teflon tape on it which turned to gum. I'm going to blow through the carb inlet and mixture ports with mild compressed air to see if it clears anything.

Last ditch is to take the carb cover off and inspect the float, needle valve and jets.

What a freakin' nightmare.

 

My guess is that some of that "gum" made it's way into the carb and has plugged up something.... Possibly in the accelerator pump area.

 

Does fuel squirt down the carb when u turn the throttle?? I'm still leaning towards accelerator pump.

 

As soon as it warms up a little outside, I'm going to look at the carb. I'll report back what I find. Boy, if it's not a carb related problem I'm gonna be totally at a loss as to where to try next.

I have done a rudimentary root cause analysis and every time it comes up as a fuel issue.

 

I agree it's an accelerator pump issue.