The title says it all. What could cause my 1991 f-150 XLT Lariat to only misfire and stumble between 1500 and 2000 rpm, and only sometimes. Have already replaced the ignition module, tps, idle air control, EEC, and much more. Already inspected wiring harnesses.

 

Sounds like a secondary coil winding short

 

Changed the coilpack. No difference.

 

How old are your plug wires?
have you run it at night with the hood open to check for miss fire?

 

Spark plugs, plug wires, CPS, fuel filter or even the torque converter - transmission? At this point you have probably exceeded the price of a shop diagnostic with parts replacement. I would probably have the shop diagnose at this point.

 

Sadly Jim, most shops now a days can't diagnose troubles. And when he shows up with a 91, they'll be less abt to be able to figure it out than him. Most shops now are expensive parts changers when it comes to vintage trucks.

They really can't scan anything under 96.

 

Silly me. and slap me on the back of the head here....

shut up KW

Have you checked it for codes yet?

 

I think I'm really going to miss Corey and his shop should he ever retire...he's helped me out with some problems I couldn't isolate on several occasions.

 

Yeah, same here with my buddy Ray

And everyone else has gotten E X P E N S I V E

 

Disconnect the Egr valve and see if it goes away.

 

You mean EGR?

 

Yes. .... I'm ...dyslexic .or auto correct. ..

 

Disconnected EGR. No change.

 

I'll rephrase my reply. I unplugged the EGR position sensor on top of the EGR valve, no change. So I reconnected it and disconnected the EGR vacuum line and plugged the line. This time right at 1700 rpm the engine just quit and died. No missing or stumbling, just boom, dead.

 

Ok. Totally disable the erg.
Then see what happens.
I have seen an o2 sensor cause
your type of issue.
Any codes?