1989 F150 misfire (never before experienced misfire condition)

[cleandan]

Hey guys, this is a new one for me with this issue. I hope someone can point me in a good direction.
First the truck.
1989 F150 XLT Lariat, 2wd, 4.9L (300ci) 6 cylinder with a 5 spd manual.
I have not driven this truck a lot because I have been chasing a hard engine cut out issue.
The last part I showed as faulty was the ECU.
I installed the replacement last Monday and drove it around a bit and things worked well...It was a nice day too so no heater needed.

Just a few hours ago I was driving the truck and wanted some more heat.
I noticed the blower was not really blowing on any of the four speeds other than high....Great, the blower motor resistor is shot.
Here is where the weird miss comes from.

When I place the blower on high it does come on high, and the blower makes a bit of noise, but nothing outside expectations for an older truck.
However, as soon as I put the switch to high the engine begins to buck and misfire...or at least abruptly cut in and out.

I can induce the miss by flipping the blower motor switch to high, off, high, off.
Off, or any of the other non-functional lower speed positions = no misfire.
High = misfire and bucking.

What in the heck is going on here?

Thanks for any input on this weird situation...At least I learned something new today.

[jmlay]

What does the voltage read across the battery when the issue is induced?

[cleandan]

I have not taken these reading due to me being in the drivers seat when I found the problem.
The blower resistor is bad, and the blower motor is VERY hard to turn.
Both are factory parts so they certainly lasted long enough.

I will replace the parts and see what happens.
I am just bungled about how the blower motor can induce a misfire in the engine.

This truck did have some auxiliary lights on it (running board lights) but that has long before I owned it been compromised.
It also has a flat four trailer connection, but no trailer brake controller.

I am wonding if some buck knife wielding electrician got into the wire harness and piggy backed off the wrong power lead, or made a ground fault situation that only shows when the blower fan is turned on.

What am I looking for with the battery power check when the blower fan is running?

[Stangman]

Sounds like blower motor is pulling down voltage and causing the misfire. You need a blower motor and resistor change them and start troubleshooting when your done and don’t be surprised it’s fixed.

[chris401]

Those years of ignition switches are known to split open with age. Seems there is only 3 anchors holding it together. When that happens the blower motor, wipers and something else I can't remember will cut out. If you create that scenario again try bumping the key area of the steering column and see what happens. Pull that lower cover, if the ignition switch is splitting you'll be able to tell.

[cleandan]

I was able to get a blower motor and resistor today...Both in stock at my local O'Reilly's.
Color me surprised.
Out with the old, in with the new and no more missing issues when running the blower on any speed.

I looked everything over carefully.
All the connectors were in pretty good shape, with only a slight amount of green on one blade that was easily cleaned.

The resistor looked pretty good but it did have some rust on the inside connectors, nothing burned or broken though.

The blower motor, on the other hand, had failed pretty good.
I could move the rotor shaft out about 5/8", and it was really hard to turn the shaft by hand.

I am wondering if the combo of the failed resistor, and the badly worn motor bearings, allowed the motor to back feed some current and cause issues.

Oh, the resistor plug was SUPER hard to unplug. Dang that plastic gets stiff with age and cold.
I am completely surprised I did not break anything in the process.

To be honest, everything went quite well all things considered.

Now the truck is in good order and it is also for sale.

I purchased this truck for the sole reason of giving my kid something with a clutch to learn how to drive a clutch vehicle.
A 300 six and five speed make for a pretty durable, easy combo to learn on.

She is done with the clutch vehicle, so I have no more use for this truck.

At least I fixed all the stuff with good parts and it is running and driving nice now.

Thanks again for any and all helpful suggestions.
This was a head scratcher with the misfire issue.