1990 f150 5.0 l 4wd seems like its starving for fuel back fires only have about 30% throttle then starts bucking and popping....read codes..replaced almost everything and still no change...idles perfect ..when starts it will progressively get worse till stall..but turning key on and off seems to fix for few minutes then must be repeated...list of all i have done...replace fule filter...fuel pump...map sensor..tps sensor...idle air control valve...other valve next to throttle body...replaced computer....still same thing...any ideas??

 

Since you gave a vague description, I will give a vague answer to check your fuel pressure.

 

And the fuel pressure regulator.

 

Look at the spark plug wires for correct routing and condition. Cracks in the boots or wires has this same affect. Also check the distributor cap.

 

Likely will cause the engine to run rich because the diaphram ruptured. Pulling off the vacuum line from it will confirm.

 

I just bought truck..it had set two years...its tank was empty filled up drove home ran ok ...80 miles..next day it started cutting out.i replaced pump filter drained tank..read codes showed tps codes low voltage and another tps code..?since then i have replaced all sensors and module and computer no change it has about one third throttle if given more it stumbles pops backfires...it is nota plug wire!!i have changed rotor cap tfi isc tps map sensor.relays ...even replaced filter teice!!!took to a mechanic..he is stumped!!!said these years are very problamatic and hard to diagnose with the simple computer thats isnt able to give anything but basic codes...oh and yes have gutted cat also....i iave no fn idea only thing i can possible guess is a huge leak / crack on intake itis a two piece intake

 

Went and put fuel pressure test on and had 15 lbs ign on not running ..30 lbs running..when i hit purge valve on gauge dropped immiedetely to 10 lbs then slowly built to 30...replaced regulator..seems to be stalling.....idk defective fuel pump its brand new??

 

sounds like you bought a spectra pump, and likely wasted a ton of money on other parts.

 

It wasnt a spectra spe it was the new pump just bought from oriellys! Returned got refund went to napa and bought a full assemply cost almost $200 but its its running great...and hey i have two of every switch sensor and computer!!so for future i will have parts!

 

The one thing I see mentioned the least in these forums is the one thing that doesn't throw a code when it goes wonky, and, in my view, it's the one thing that needs to be considered first when the codes don't make any sense, or are non-existent....the injectors. I had stumbling, surging, overall bad response usually after third gear kicked in, but sometimes on startup also. I checked for codes (mostly EGR codes that persisted even after swapping such parts, and including doing a cylinder firing test which passed on all cylinders), tested some parts, replaced some (including the fuel filter and a pump/tank because it had quit working long ago), cleaned and tuned this and that to no avail. I even suspected the transmission and/or torque converter for a while, but couldn't prove any of them were the cause of the problems. I finally picked up some injectors from the local u-pull-it, rebuilt them (I had already rebuilt mine which is why I never suspected them as being bad), put them in aaaaaaaand, bam!, problem(s!) solved. No more hesitation, no more stumbling, no more power loss. My mileage improved also and the truck runs like brand new again. Mr. Injector has the rebuild kits (no affiliation), or he can rebuild them for you. Anyway, if you try everything else and the problem remains, consider the injectors.

PS. If your mechanic spends all his time looking to the computer to tell him what the problem is, no offense, but you need to find another mechanic.