I've been a long time viewer and seldom poster on this forum for a while, but have finally found a problem that I haven't seen covered yet. I have an 89 F150 4.9L that has been misfire and stumbling hard at wide open throttle, and also has a rough idle. The problem is intermitent when cold, sometimes it does it and sometimes it doesn't, but when warmed up it does it 100% of the time. I've put a new o2 sensor, EGR valve, EVP sensor, TPS sensor, FPR, cap, rotor, plugs, and wires into in the last few months and still have this problem. I've checked the exhaust for a clogged cat and it showed no back pressure, cleaned the plenum, checked all vacuum lines for leaks, I tested the EGR vacuum solenoid and it opens when grounded, and right now I've pulled the injectors and have them soaking in cleaner over night.

The truck was driven up until around 2005 when it was parked in a field. The owner needed some work done around the farm and didn't have the money to pay me so I took the truck as a trade in 2009. Im certain the truck hadn't been run in the 4 years it was there because the battery was completely shot when I picked it up, but after replacing it it fired right up.

I've been hunting this problem for a while, it started out as a hardly noticeable misfire that I put off, but has escalated to where the truck is no longer safe to drive. I've taken it to a shop and they diagnosed it as the o2 sensor and possibly the injectors. I don't have much hope in the problem being the injectors as they looked in great condition when I pulled them.

Can anyone help me out with this? I've not a clue what else could be causing these problems.

 

The injectors could be clogged/faulty and not firing properly and thus giving you a miss.

Sounds like you covered pretty much all the other bases.

 

intake gaskets possibly