I just baught a 1994 F150 with the I6 4.9L engine. I noticed that it make a wierd under load coming from the engine. I baught it in AZ and drove it to ND and could only do 50 up the mountains and it would make a sound like pith...pith....pith. I took it to a mechanic and they said that was normal. When I first picked it up it had old gas and did it worse until I burned through the gas.

 

Make sure your fuel pumps are working and your fuel pressure is good when you rev up the engin. Next check you cats and muffler for fouling or pluged passages.

 

Pull the plugs and see what they look like--if theyre fouled out and not firing right. Sounds like to me you have a gas/air mixture problem which causes the engine to miss. Could be bad fuel pump or a bad sensor that controls air flow.
Next check fuel filter and fill up with fresh gas. Also could be plugged up cats. Just too many possabilities to say exactly what--good luck! Check back in here and youll get more answers

 

plugged cats??

 

If you are burning a lean F/A mixture your exaust will be so hot that the metal catalist in the catalitic converters will melt and cause a restriction in the exaust.

 

I had a shop replace the plugs and O2 sensor. Would they have been able to tell if the Cat is pluged? Also the mileage in town sucks about 8 Mile Per Gallon.

 

check pcv valve or a vaccum leak. Check these--they could be back
im thinkin vaccum leak...

 

How are your emissions? If the O2 sensor was bad that could have caused problems that will linger after the sensor has been changed. Change your plug wires, distributer cap and rotor. Think about changing your coil to. Take a look under the hood if any of theas things look old or like orignal equipment change them out I agree with Green that you should look for Vaccum leaks also. If you go through all this and still have problems I would think about the cats agan, an emissions test will give you an indacation not only on the condition of the cats but how efficent your burning fuel.