I've been hunting a problem for weeks now and making no progress so I've decided to consult the experts here. I'll do my best to be concise. I have a 2002 F150 4.6L Supercrew w/ 147,000 miles that has intermittent miss / loss of power. I cannot get the CEL to come on even though the problem sure seems bad enough that it should be coming on. The truck will start without any issues and idle well for a little while. However, once put into gear or faces load (hills) the miss will show up. As I said previously, the miss is intermittent. The truck may run fine for 5 or 10 mins and then start missing and acting like its going to die at a light. Additionally, I might be cruising at 35-40mph, hit the accelerator to pass someone, the truck stumbles for a bit, and then speeds up as it should.

Things I've tried...
Searched for and fixed any vacuum leaks I could find (not much)
Changed fuel filter
Cleaned MAF
Cleaned throttle body (without removing)
Live data shows that the long and short fuel trims are within acceptable ranges (I think). short = -1 to -4 or so, long shows -5 to -7 or so. I think I remember reading that anything from -10 to 10 is considered acceptable.
Cylinder 5 & 8 have a newer plug and cop (CEL came one months ago for these)
Fuel Pressure - 35psi turning power on /off, 28psi running. When I hit the accelerator the pressure goes up as it should (I think) When I remove the vacuum line from the fuel pressure regulator while running the pressure goes from 28 to about 35 - 37. I also turned the key on/off a few times and the pressure was 35psi, I left it off for 5 mins and checked again, pressure was about 30-31psi. I changed the fuel pump last summer for a different issue (hard start). I'm wondering if the in tank fuel sock could be interfering with the fuel pickup? I did not change the in tank sock when I changed the pump. Also, with my fuel pressure gauge hooked up, when the truck starts to miss the pressure does not seem to fluctuate at all.

As I said before, without the CEL pointing me in some direction I'm at a loss. I can't explain why the CEL doesn't come on, the truck has almost died multiple times over the past few weeks while sitting at a light. The CEL came on when the cops were bad and the idle wasn't as rough then. When the miss happens it def feels like a cylinder is down... Should I consider replacing all other cops? Any simple way to test the cops I have to find a faulty one?

Thanks in advance for any help!

Chris

 

Sounds like its either a spark plug or ignition lead starting to fail but not enough to give a fault code to the engine control unit. Do these symptoms still appear if you run without the maf sensor connected.

 

Currently you have a low grade misfire.

Yes, - replace ALL coils once they start to go out. That way you won't have to deal with this again for a very long time. They can go out like dominoes shortly after the first fails.

Replace with Denso or Motorcraft coils. Otherwise your wasting your time with knock-offs that don't meet Ford spec.

 

I thought that the 4.6 has 2 coil packs which have 4 ignition leads each going to plugs.

 

They do. But that one doesn't.

 

Ah.if that's the case then changing all coil packs is the way to go. As stated previous if you only change one it will be stronger than the rest and will eventually start taking the rest down.

 

Denso are 24.99, Motorcraft are 37.99... either are acceptable? I'd def prefer to save money if possible. Should I replace all plugs while I'm at it?

I'll be able to test without the MAF later... just curious, what will the results of running without a maf tell me? If disconnected and the problem remains, what does this mean? If disconnected and it runs better, what does this indicate? Always trying to expand my knowledge base... Thanks.

 

Yea, Denso and Motorcraft are one in the same. So which ever you can get a better price on.

If it's time to replace the plugs or close to it. Use Motorcraft plugs. Don't use Autolite or Bosch.

Have no idea on the MAF. I would never run or diagnose without it connected (that doesn't mean it's wrong to do so).
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Monkey Man, - The 4sixes began using COP ignition systems half way thru the 2000 model year.

 

mafs can give erratic running without giving any fault codes so running without it connected will eliminate if its the maf or not. If you still experience the problem then it could possibly be another coil pack slowly degrading.

 

It won't run worth a damn with the MAF unplugged.