New member here. Looking for some help from those more knowledgeable than I.
Up front, I am very mechanically inclined and work on all of my own stuff. Trucks, tractors, farm equipment etc. But I am more of a diesel guy, and me and gas engines never seem to get along.

I've picked up a 1989 F-150. It has the 302 engine with a 4-Speed.
This truck came from a family member who has used it as a beater to get back and forth to work in for the last 15 years, and was truly treated as a beater. It was parked 5ish years ago and has sat until a year ago. Fired up ok and I got it home. Learned that the frame was shot from the cab back, so I replaced everything. Chopped the frame just behind the cab and put in 7" C channel, replaced the leaf springs, hangers, shackles, shocks and mounts. Feel much better about it structurally now.
Pulled the bed off and will be finishing out a wooden flatbed.
New in-tank fuel pumps while everything was off, and a new rear tank. Front tank was in decent shape, so I cleaned it and am reusing.

Since I've had the truck, I've had a weird issue once it warms up. It starts great if the engine is dead cold. Once it gets warm it really struggles to idle and wants to die. If it dies or if you shut it off, the truck will not start again until everything is dead cold. It has to sit for at least 24 hours.
Now, it seems that issue is creeping to become more present. Truck will only start 20% of the time. Cranks great, and sounds like one cylinder wants to fire. But will never light off.
Fuel pressure seems to hold steady at 35-40 psi.
Everything on the ignition side has been replaced other than the ignition control module. Not sure how much that has to play here, and I also have done some reading about capacitors in the ECU.
My test light on the engine side of the ignition coil shows a weak light, but maybe a test light isn't the right tool to test after the voltage is bumped up. If I put my multimeter on it, what should voltage be?
It also doesn't have a single solid vacuum line. That's on the list, but I feel like the truck should at least start without the lines?

Looking for some guidance and if this issue is more common than I think.

I intend this truck to be a farm truck. Run from field to field, to town and back, get fuel etc. so I'm not worried about everything being prim and proper. I just want something that's reliable, will start every time I hit the key, and cheaper to maintain than my 3/4 ton 24V Cummins.

 

Checking computer is a good idea. Run through all grounds and make sure they are good. Vacuum lines are important!! Especially to MAP sensor, but it should start.

 

Next time it starts, get it up to operating temperature and pull the engine codes. You can count the flashes of the check engine light instead of using a volt meter.
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How's the fuel filter? It's worth changing if you haven't already.
When it's cranking and not starting, do you have spark and good fuel pressure?
When you cycle the key to the run position, you should hear the ECM relay and the fuel pump relay click on. After about 2 seconds, the fuel pump relay should click off. They're on the driver's side inner fender, either laying on the fender, or in a black plastic cover. If you open the hood and stand beside the truck while you cycle the key, you'll be able to hear the relays and the frame rail mounted high pressure pump.
The test light should blink while the truck is cranking.(On the primary side of the coil). The tach should move while the truck is cranking.
Make sure these two grounds are connected to the battery negative terminal. One is the power grounds for the ECM, the other is the ground for the ECM relay.

 

Ignition coil testing




If the coil checks out and you're losing spark and tach movement, here's how to test the ignition control module that's mounted on the distributor
How To Test The Ford Ignition Control Module (Distributor Mounted) https://easyautodiagnostics.com/ford...l-module-tests

 

Fuel filter is new.
From what I can tell, while its cranking I have spark. Fuel pressure is 35-40 psi.
Fuel pump kicks on with key. I think I can hear relays clicking too.
Tach moves while cranking.
Both grounds that you pictured are connected and test good.

First video of the frame replacement is up, if anyone is curious.

 

OK. I think I found something last night.
I have repeatedly failed to get the truck running over the last 2 weeks. It will start briefly and stumble hard for a few seconds, then will die. Foot off of the gas or 1/4 way down, doesn't matter.

Last night, I put my foot 3/4 way down and it coughed to life, then roared to 3,000 - 5,000 RPM's and ran great. Let it run at 3,000 RPM for about a minute, then eased off the gas peddle.
As soon as it got down to 2,500 RPM, it stumbles hard and the tach goes down to 0. Give it some more gas and it comes back. Ease back down and same thing. Tries to die and the tach flatlines.
Will run fine over 2,500 RPM.

I think I have a crankshaft position sensor issue. Or is it camshaft?

Thoughts?

 

One possibility is a large vacuum leak.
At the vacuum tree on the driver's side of the upper intake, remove all the vacuum lines except the ones to the MAP sensor (on the firewall) and the fuel pressure regulator. Temporarily block off the vacuum tree ports.
Also look at the vacuum connection for the PCV valve. It's either at the rear of the upper intake, or at the center, underside of the upper intake. Block it off temporarily also.

 

Thanks for the reply Soup Bean.
I definitely have a large vacuum leak. All of the lines are rotted and cracked. It's on the list.
However, my main problem is failure to start, and even after it does, failure to restart when warm.

I know vacuum leaks will play a big role in running correctly, but I assume the engine should still start and idle, and restart even with leaks?

 

This engine doesn't know how much air is entering the intake, the computer is running fuel delivery off of preset tables, and input from various sensors.
While you're troubleshooting, pull the vacuum line off the fuel pressure regulator and check it for fuel. There shouldn't be fuel on the vacuum side of the regulator. Also, if that vacuum line has failed, it's the same as having the accelerator pressed. The regulator relies on vacuum assist to meter fuel back to the return line and lower fuel pressure for idling the engine.

 

Are the leaks big enough that there isn’t enough fuel to keep running? If it does start do you want the engine “idling” at 3,000 rpm? Why wouldn’t you fix or temporarily cap off the known vacuum leaks?

 

I agree the leaks need fixed, and will be. Maybe I'm downplaying how much effect they have.
The main issue I'm chasing and wanting fixed first I guess is the hot no-start.
I don't see how vacuum leaks would cause a hot no-start.

 

Copied and pasted the wrong video

 

 

When you turn the key to the on position, the computer takes a reading from your Manifold Absolute Pressure sensor, throttle position sensor, and engine coolant temperature sensor to select a preset table, that determines how long the injectors will stay open on each intake stroke of the engine.
With multiple vacuum leaks, too much air is entering the engine, for the amount of fuel the computer is allowing in via the fuel injectors.
A hot engine requires even less fuel, so the computer decreases the length of time the injectors stay open, making the mix so lean the engine won't start.
Your throttle position sensor tells the computer that you're trying to accelerate, so when your foot is on the accelerator, the computer keeps the injectors open longer, which is why the engine runs at higher rpm.