I have a '90 F150 4x4 w/351w E4OD 3.08 rearend - 33" tires
Only mods I have are removed 2nd cat w/18" glasspack (2.5" SISO) and homemade FIPK. Ever since I've been keeping track of my mileage I've seemed to average around 7-8mpg 100% city driving. Even after a fresh tune-up (plugs, wires, etc.) I've known its been running rich for some time now but it hasn't really bothered me... well the other day I was bored and decided to make a video/sound clip of the exhaust. When I was revving up the truck a single flame shot out of the tailpipe!! This surprised the heck out of me and I'm now wondering what to do (new sensors, etc) to get this thing running right. (Although I must admit it was kinda cool seeing it in the video...)

Also it starts/idles fine, has good power... just crappy mileage.

 

I'd check your fuel pressure. I've got a feeling your regulator might be going south.

 

Handyman43358,
What makes you think it could be the regulator? Just curious.

 

controls how much fuel ....more pressure when injector opens more fuel going into motor

 

Well then, how much pressure should there be and can it be checked with a tire guage?
My fuel rail has a valve on it....

 

Don't check it with a tire gauge. Go to Auto Zone or Advance and normally they have a fuel pressure gauge you can use. If not, they aren't that expensive. Fuel pressure at idle should be around 40. You can pull the vaccuum line off the FPR and if it has any fuel in it at all, that means it's going bad.

 

Well, theres two main sensors, initially it is the Engine Coolant temp Sensor, that controls the amount of fuel. Once it gets up to temp, the computer gives control to the O2 sensor, so if you have no vacuum leaks, and fuel pressure is ok, I would suspect most likely the O2 sensor.

 

After farting around on the forum, I found something that I had been doing wrong. In setting the timing I was neglecting to remove the spout connector.....DUH...
Engine runs a lot better, so I checked the voltage on the TPS and it was .90 vdc fully closed. Being a perfectionist, I adjusted it to .98 vdc. (I read that's what it is supposed to be....) Runs even better. A little more digging and I found my evap. canister purge solenoid was bad....not that it was causing any problems, but......
Now that everything is set as it should be, I have one final problem to tackle.....That pesky rapping sound from the engine.
I think it's "piston slap" and here is why:
1. start truck and let it idle.....noise is there.
2. increase RPMs and rapping rate increase accordingly.
3. shift truck into gear (any gear)...noise gets really faint.
4. accelerate down the road and still faint....
5. decelerate and the noise is loud again
6. oil pressure A+....just replaced crank and all bearings as well as rings
7. removed belt...noise still there....
8. new exhaust manifolds and gaskets.....no leaks.....
9. engine could have as many as 213000 miles on it.....not sure though.
Summary:
If the engine has no load (like forward motion) the noise is there....put a little resistance to it and noise is almost gone.

Can anybody tell me with 99% certainty that piston slap is what I got?
I'm dreading pulling the engine to have it bored out........
Opinions welcome!

 

I didnt know fords had a problem with piston slap. My wifes suburban on the other hand, which is a 2003, DOES indeed have piston slap. It is something I have to live with as GM says it is "normal". Anyway, it only does it during the first few seconds after it starts, then goes away. It sounds like diesel during those few seconds.