Hello, I'm new to the site. Joined because I need some help with my truck.
I have a 1997 F-150 with the 2v 5.4L. I bought the truck with a blown head gasket and replaced the motor with one from a junkyard which allegedly has around 110,xxx miles on it. The "new" motor is identical to the one that came out with the matching VIN code "L". After I did the swap and hooked everything back up, it hasnt idled right. I replaced a coil, put new spark plugs in it and recently installed a new IAC valve.

When I crank the motor, it idles low until it builds oil pressure then the RPMs jump up to around 1000 and then fall down to around 600 (and sometimes even lower) then jumps back up to 1000 and falls back down. It does this the whole time it is idling and when it is idling in drive with my foot on the brakes. I thought it may have been the TPS so I swapped it with the one off the old motor and unhooked the battery to potentially reset the computer but the problem still occured.

Sometimes when i pull up to a red light, I put it in neutral to take the strain off of the transmission and the RPMs shoot up to around 1500 for 5 or 10 seconds then fall back down and continue to fluctuate. Also, after installing the IAC valve, it started making a resonating sound from the air intake that kind of sounds like a horn. Im thinking it may be the gasket on the IAC valve isn't lined up properly but I'm not sure. I'd like to get this thing running smoothly because it is a beautiful truck and the one I've always wanted.

Any help would be appreciated. Thanks!

 

I had the same problem with the droning noise. Its the IAC, Im waiting for a new one to come in. You may also have a bad MaF or a small vac leak. Take the good stuff off you old engine. Why did you rplace the whole motor with the gasket. You could have just replaced the gasket?

 

I am having the exact same issue on my 1999 F150 5.4L. My engine failed (threw a rod at 143k), I found an exact replacement with 57k miles and had a shop install the replacement motor. I also had them replace the oil pump, water pump, hoses, belts, front & rear seals, valve cover & pan gaskets, fuel pump & fuel filter.
After about 20 minutes of driving, is when mine acts up. Runs great for the first 20 minutes then the idle surges and the engine runs horrible under load. The shop has tried swapping the EGR & IAC from the original motor but the problem persists. Never gets a Check Engine light or any codes. Shop drove it with a fuel pressure gauge and when the idle issue starts, the fuel pressure is fluctuating also, still no idea what is wrong. Any ideas on what to check next?

 

Fluctuating fuel pressure could be the fuel pump, fuel filter, or fuel pressure regulator.

 

replaced the Fuel Pump & Filter. Replaced the fuel pressure regulator, fuel pump relay. Still have the problem.

 

I found out what my idle problem was. On the EGR valve there is a tiny green hose coming out of the top that opens and closes the EGR and there is a tiny red hose that regulates the fuel pressure. When I hooked the new motor up, I had them switched. This was sending constant pressure to the EGR instead of the fuel rail and vise versa. I switched those tubes and she purred like a kitten. The intake howl is another story. And I know it must have something to do with the IAC but even when I put a new one on it, it made the same noise if not louder. I have no idea how to fix it myself so I am going to let a mechanic take a look at it.

And the reason I swapped the motor is I dont know for sure what all was wrong with the original motor because the oil had water AND gas in it and replacing it was more time effective. Plus, I have a spare motor that I can rebuild now when this motor takes a dive.