Hey there, new to this forum and ford trucks. This is my first ford and I’m having issues. 1986 F150 302 EFI starts fine, fires right up actaully, idles fine. However when I give it partial throttle, like I’m talking about just barley cracking open the throttle blades, the truck stumbles and dies. If I let off the throttle it catches itself and idles fine again. If I give it more throttle it revs fine.
I’ve replaced vacuum lines, plugs, wires, not running an air filter because I’m not driving it right now, cleaned throttle body, cleaned IAC and new gaskets for both.
I was beginning to think ignition timing but I wasn’t 100% sure. If you guys could help me out that’d be great.
This is also my first distributed vehicle. I’m coming from the Buick Grand National community so some of this stuff is new to me😂

 

Welcome to FTE, lots of helpful lads out here.

The first thing I would suggest is that you have checked or check the codes in your computer to see what it sees as a problem.

Engines are easier to diagnose over the internet when all the codes are listed, and in order of test.
KOEO - (O) Self-Test Codes =
KOEO - (C) Continuous Codes =
KOER - (R) Self-Test Codes =
Make sure your A/C, Heat, Radio, etc. are turned off when testing, but worth checking if you have a light.

Cleaning the AIC is good, but if it is 30 years old, is it working properly….?

Off the top of my head, sounds like your TPS has a dead spot near closed throttle. You can test it, or just replace it. If you have the time, testing is a good learning curve, for you may need to verify the TPS voltage output on a new one.

The timing rarely changes unless someone has been messing with it, but worth checking if you have access to a light.

Would be a good Idea to replace the Dizzy Cap & Rotor if not already done.

 

I am having the same battle with my 86 ranger with efi. It's the same as your truck, only two less cylinders.

I would check the throttle position sensor. Only way to check it is to shave a little bit of the insulation off the wires, and check it with a meter with the key on, engine off. Make sure that the voltage rises smooth when you move the throttle.

Mine was ok, but I found that my throttle blade stop was too far in. I could unplug the idle air control solenoid while the truck was idling and it would keep idling. It should slowly die when you do this. If the idle stop screw is too far in, it gives the engine too much air. If you find that you can back the throttle stop all the way off and it still is running, you have a vacuum leak somewhere letting air in.

Once you dial back the throttle stop screw so it stalls when you unplug the IAC, then go back to your meter and wires on the TPS and if you have any wiggle to the TPS when you loosen the screws, turn it till it's about .9 volts or in that neighborhood.

Backing the throttle closed stopped by hesitation, but I still have a part throttle shudder sometimes. And when you mess with this thing, it takes a little while for the computer to relearn some of the adjustments.

 

Thanks for your input guys! I’m currently re wiring the electric fan the previous owner put it. It was super sketchy lol. After that’s fixed I’ll check TPS voltage. How would I go about checking for codes in these trucks? For my grand national we have a box called the Scanmaster that’s wired into the ECU that tells all engine vitals including codes. I doubt ford trucks have these?? Finding a balance between TPS and IAC is nothing new to me. That’s a common “tunning” issue with GNd��

 

This is how you check the codes.

EEC IV Self Test hookup

Fords can be aggravating, there is no way to get streaming data from any of the sensors without just using a meter and what they used to call a "break out" box, which was really a thing that split the computer harness so you could probe the wiring with a meter.

These older trucks are a little odd also in their engine wiring. They were batch fired speed density systems, no mass air flow sensor. So things cannot be changed around too much before bad things start to happen.

 

I backed the throttle screw out enough to where it was not touching the blade actuator with the IAC unplugged and the truck still ran. So I have a vacuum leak somewhere. Are there any other vacuum sources other than the 5 hoses on top of the intake plenum? I’ve replaced the throttle body gasket and IAC gasket but I’ll double check those areas. Haven’t had time to check for codes yet, I’ll do that when I get home today.

 

PCV and power brake booster leaks?


There are 2 ways I know of to check for vacuum leaks.
1 - carb cleaner and with the motor idling spray along all the mounting areas of the intake to heads and intake to ?? as I don't know the EFI trucks but any place you think there could be a leak.
2 - Smoke test, look it up on Youtube. You fill the intake area with smoke and look to see where smoke is leaking out of. It is pretty cool test from what I have seen.
Dave ----

 

I found my vacuum leak!! I knew about the smoke test but I didnt know how to do it until on my way home. I vape (yes I know stereotypical huh) so I blew smoke into a vacuum line and it came out of the EGR. I’m considering just deleting it since my state doesn’t run emmisions on vehicle older then ‘95. Do I have to do anything else besides take it off and block it?

 

I think you pull it off and make a plate to fit over the opening and bolt the valve back in place to hold the plate.


This way if someone does look the valve is in place and you have fixed the leak.
Now I got to ask, what if you just pulled the vac line off and plug it is the valve still closed and blocking the port?
I say this because you said you blew smoke in another line and it came out the EGR this would point to a leak on the hose side and blocking the port on the manifold would not stop the leak.
Dave ----


btw good job on finding the leak.

 

The vacuum line has been off since I got the truck. When I replaced all vacuum lines I capped it off. I only have three vacuum lines, fuel pres reg., MAP Sensor and brake booster. The rest are capped off. The smoke came out the underside of the valve through the holes that are in it.

 

Oh OK it was passing thru the intake to the EGR and the valve does not seal.....got it!
I am sure you can pull it off and make a plate (old tin can?) to block it off and put the valve back in place to hold it.
Dave ----

 

That’s exaclty what I did lol. Took some scrap that I had that was thin enough to “make a gasket” and honked down the bolts. No more leak! About to fire it up and see what happens

 

Update: My problem still persists. I pulled codes and got code 31. Checking in my book it says it’s for the EGR. Exact words are “EGR Valve Postion Postion Sensor our of range”

Checked TPS voltage too. 1 volt closed throttle and 4.6 volts at wide open. Smooth increase throughout its travel. Any ideas on what else I can try?

 

I think you will get that code because the EGR is not moving.
You still have the vacuum line to it disconnected also right?
If it will never move.
Try the smoke test again as you found a big leak there might be others?
Dave - - - -

 

I think you will get that code because the EGR is not moving.
You still have the vacuum line to it disconnected also right?
If it will never move.
Try the smoke test again as you found a big leak there might be others?
Dave - - - -