Hi guys
The other day my truck conks out at a stoplight, then as the day goes by the "miss" and stalling gets worse.
A day later I discover that my O2 sensor connection wire was severed and hanging by the carb!

I hooked the wire back up but the problem is steadily getting worse and now it's bucking and missing even when driving.
I took out the plugs to check them and they have a whitish ash on them meaning it's leaning out?
The "sucking" sound in the new carb is intermittent and I think it's not getting enough gas, especially at idle.
At idle it just misses till it cuts off

What could cause this?
Did the O2 wire ground out and hurt the O2 sensor or the ECM?
This truck has a "dumb" computer or no?

Please help as I cannot figure out what happened
thanks!

 

Welcome to the forum, ovalhead72!

Your 1986 originally was equipped with a computer which controls the carburetor [rich/lean] and the engine timing.

The O2 sensor does need to be connected or the computer will go into its failsafe or "limp home" mode.

First, check for vacuum leaks. You engine has many hoses, most are plastic and become brittle with age. Also, the carburetor is very prone to loosening, creating a vacuum leak at the base. It is held down by two bolts. Using a 1/2" wrench, snug the nuts, if loose. Do not overtighten. Look for missing hoses or caps which cover vacuum ports. There is a vacuum tree on the intake manifold with many ports. Also, there is a black plastic vacuum manifold located on the firewall near the engine on the passenger side.

Look it over and get back with us. By the way, the 4.9L engine is great, but hates vacuum leaks!!!

 

OK
As I said the problem started as a single event where the truck stalled at a light.
The problem has steadily gotten worse and the truck went from sometimes stalling to now it stalls and bucks at every stop
Additionally, at first the problem only affected idle...it has steadily gotten worse to where it's now bucking throughout the driving range also.

From past issues with the Ford TFI ignition I am wondering about the "new" ignition module I installed as the truck got a little "warm" the other day when it was very hot here (107 degrees), not overheating but these ignition modules once damaged by heat seem to degrade rapidly.

I have the old O2 sensor and the old ignition sensor- I am going to put them back on and see what happens.

As for a vac leak it would have to be one that is getting worse somehow...
I have checked all vac lines and they seem OK and I don't hear the "hiss" from a leak either.

The white ash on the plugs indicates a lean condition, and the exhaust stinks like nitrogen or something, incomplete burn?

In "limp" mode shouldn't the truck run "rich" to protect from detonation??
It is certainly running lean

New parts that have been installed include-
O2 sensor
ignition module
wires, plugs, cap and rotor
new coil
brand new carb
new TPS

It has the old fuel pump unknown condition but I have a see thru fuel filter and it puts out a good amount of fuel
It has the old MAP sensor also, unknown condition.

Thanks for any help

 

Well I went to go to Autozone to get a new wire for the O2 sensor and lo and behold it keeps cutting out, etc so then it won't restart at the gas station no matter what I do...
?!
So, suspecting the Ignition Module, Ford's incredibly brilliant concept of a ignition component that cannot stand HEAT and fails every time it gets hot, I replaced the "new" ignition module with the old one and BAM she starts right up!
Still a miss at idle but this truck has always had a miss even after replacing so many things...at least it runs now

I was going to take out the TFI ignition system and replace with a aftermarket one...any help with any of you guys replacing the TFI with an aftermarket system??
What are your thoughts?

Thanks

 

Use the "Search" button at the top of the page and type in: Duraspark 2 conversion. This topic is frequently discussed.

 

You said your truck has a brand new carb. If this new carb is not the correct feedback variant, then your current ignition control system (computer) will lock your ignition timing and performance will suffer. Stock on your truck was the EEC-IV system, and it needs all of its components to work properly. If components (like the carb) have been changed to non-stock, then folks often retrofit to the earlier Duraspark II system.

 

yep Ignition module was the culprit!
Stopped at a gas station and truck wouldn't start....arrrgghh
Grabbed the old ignition module put it in and truck starts right up, though still missing some.

My temp gauge mysteriously stopped working awhile back and now I know why!
The thermostat was broken inside the housing and was stuck closed for who knows how long I found the top hose was red hot near the housing and the radiator cap was cold
Put in new 160 degree thermostat and all seems ok but truck is still trying to run hot and now I think the water pump might be weak not sure but gonna replace it anyway
No head gasket issues
I love this engine it's like a tractor engine

And I gotta get change over this ignition

Unknown about the carb, previous owner put it on