Hello all this is my first post on the forums here at f150online. Im here because my fathers work truck (1993 ford f150xl 4.9 leiter Inline 6) has been acting up for several weeks now and after he has invested several hundred dollars in parts we are no closer to finding a solution then we where several weeks ago. Basicaly the RPMs will sudenly drop drasticly and the truck losses all its strenght. the other problem we encounter is that at red lights it stalls. lastly the rpms somotimes start to oscillate dramaticly at any speed the only way i can describe it is almost identical to some one learning to drive stick and messing up with the clutch so the truck begins to jump forward then stop then jump forward and stop (the truck is ofcourse automatic) and this dosent happen when we start to move the truck but can happen all of a sudden while going 30-40 mph. The check engine light is also on when we first start the car but turns of after 10-20 seconds.

So what we have done so far is changed the fule filter, air filter, spark plugs, distributor cap, distributor rotar, distributor cables, fule regulator valve, idel air control valve. We also cleaned the throtle body, treated the fule with dry-gas and lucas injector cleaner. Also tested fuel pump and it is working.

The last atempt that we made at fixing it was to change the oxygen sensor this is where we ran into a bit of a problem at the auto parts store they told us that the car has 2 O2 sensors one directly in front of the cadalitic converter and the other direfctly behind it. this is the problem we can only find one we looked over every single inch of the exauhst system and where unable to find this elusive second O2 sensor i will atatch links to pictures i took of the o2 sensor we changed which is on an x-pipe directly infornt of the cadalitic converter and i took pictures of the rear of the cad where the second sensor should be located. If any one could please confirm or deny once and for all the exsitance of this second sensor it would be a great help because we could finaly rule out the posiblity of a bad o2 sensor.


Link to pictures first is engine compartment second is the o2 sensor already changed and third is just behind the cad where i belive the second one should be. (it is a gallery all three pictures are at one link)

http://imageshack.us/g/3/dadstruck1.png/

 

93 only has one oxygen sensor. And for future reference its catalyic converter. Get the codes, doesnt matter if it goes off, they will be stored. Also throwing parts at a vehicle wont fix it, u need to get the codes, read each code and either determine a sensor is bad, wiring, etc...by testing.

 

Do the instruments zero out when it's cutting out? I'm thinking it may be electrical, like a loose or grounding out connection. If you are running the lights, do they dim when it's doing it's thing?

Also did you clean/check the EGR valve and the ports?

I'm sure Jbrew or Steve 83 will be along soon, they're real sharp and I'm just guessing. MGD fan is good too, just don't get offended about his rough edges!he is usually just funning.

 

Sounds like a throttle position sensor, but that can be checked with an ohm meter according to a Haynes manual, also pull the trouble codes even if there ain't a check engine light. Here's how,

http://oldfuelinjection.com/?p=13

Stop throwing parts at the truck as well until you know why. Aftermarket items are usually crappier than old Ford parts.

Adrianspeeder

 

Hey Im having the same problem with my 93 f150 and also performed the same tune up to it. I was wondering if you found out what the problem was and if you could tell me. Thanks!

 

First and only post, 3 years ago - I highly doubt you will get a reply from the original poster.

The oldfuelinjection site is no longer, here's a cached copy:

http://oldfuelinjection.packrad.net/