I have a Ford F-150 98, V8 4.6L with 197,000 miles. I bought it used and had no problems. Then after a heavy raining season a year ago it got went. It began to have problems, I took to get read by the machine and changed everything is says, so I changed many things: Gas Pump, Gas Filter, Air Filter, Spark Plugs, Spark Plug Cables, Oxygen Sensors (before and after catalyst), EGR Valve (Pressure Sensor, Vacuum, everything I could find). So nothing worked, I took to truck to the machine to get the computer reseted, so after a few days I tried again and... the machine said that EVERYTHING I changed was the problem, again! Of course this happened during a year, I changed one thing then tried a different one. The main problems are when I start it, it shakes and like "drown" as if more air was needed or there was too much air. So that's one, the other one is when I'm speeding, I can't get it past 40 mph, it starts shaking (slightly but noticeable). This also happens when I stop at a dead end (light) hit the pedal again, won't work, have to take the foot off then try again and the will kind of work. I'm really out of ideas now, I don't know what else to change or what could be the problem. Any sugestions will be greatly appreciated.

 

Take it to Ford for a complete diagnostic analysis. It will cost $80-100 but they will find out what's wrong and give you estimates for repairs.

 

Sounds as if you have a vacum leak, try checkin at the back of the engine, pull and inspect all hoses for leaks. and have you cleaned the MAF sensor? a tiny bit of dirt can also cause some headaches.

 

Cats aren't clogging up are they? What brand of plugs did you use? These engines really only like either Motorcraft or Autolite double platinum plugs.

 

The spark plugs are Motorcraft platinum, and the MAF sensor is not just clean, it's new. About the leaks, it did have one but for the heater, it was leaking coolant, but it was solved, no leaks now. So, any other idea? or ideas? =/

 

I tried again, but now it's worse. When I manage to start it up, it begins to shake. As if there was no gas, or not enough power, or not air. The thing is that the RPMs go down when I hit the pedal. Also if I start it up and don't hit the pedal it will turn off. I'm wondering if it's the gas pump or if the gas tank it's dirty and need to be cleaned. What do you guys think?

 

Like I said in the first place, with all those symptoms and all those new parts you need to have that F150 looked at by a professional with good equipment. I recommend a Ford Dealer. You don't half to let them fix it but they will sort out the problems. Good Luck

 

I am just ignorantly throwing this out there, but did you look into the IAC? And also the PCV? Or Clean the Injectors? Replace the Coils? Hows the timing? This is in addition to the other things you have already done.

 

Double check that the plug wires are in the correct order if you haven't already.

The idle air control valve has a lot to do with a smooth idle. I would clean it and the port in the throttle body as well as the rest of the throttle body. Start here before throwing any more new parts at it. You did say you replaced the fuel filter. You can also test the fuel pressure at the rail. Some vehicles are very sensitive to even a few pounds low pressure.
Good luck.

 

I agree, check the fuel pressure, it should be at least 32psi.

 

like LXMAN said, Cats?? Those can just completely screw everything over.

 

Hi everyone, ok I check the coils, and they're working. The spark plugs are fine, also the order too. I checked the alternator and it works fine. The MAF, PCV and IAC are new. I checked the fuel pressure and it's correct. Now I was wondering, when I was checking it yesterday, I could hear some sort of air escaping from somewhere around the throtle body, my educated guess would be a gasket, but my question is could this cause the truck to have this problems? not starting well, shaking, rpms going down and even turning off when hitting pedal and also when no hitting and just leaving it like on for a little while? Also, I'm thinking about the fuel pump relay, do you think that could be a cause too?

 

I totally agree with the posts recommending that you find someone who knows what they're doing to get a thorough diagnosis. Throwing parts at it can be a seemingly never ending road.

That said, you haven't said anything about the secondary side of the ignition system which would be my first guess because you said that this started after it got wet.

Good luck,
Doc

 

Spray some carb cleaner or wd-40 around that area, if the RPM's change, then you have vacum leak, and yes this it can cause these problems.

 

Ok, let's say that if there is vacum leak, how can this be solved? Because the noise I could hear was under the throttle body. How's the we-40 gonna help here?