My 97 F-150 with the 4.6 Liter engine, over time developed a rough running condition that culminated in a P0301 code being generated. I replaced the spark plugs, Spark plug wires, and both coil packs. The wires were AC Delco and the plugs and coil packs were Motorcraft.
I have my own Launch fuel injector cleaner machine so I pulled the injectors and back flowed and did a uniformity, and leak test. Uniformity was good and none of the injectors leaked a drop. The truck no longer throws a code and it starts up and ideal perfectly. When I drive it
it runs fine until I get to 35 miles and hour and it starts vibrating. It continues to vibrate until I get below 35 and the engine will temporarily run rough again until it smooths out, but it doesn't throw a code.
Things that have recently been replaced is the transmission position indication module on the side of the transmission and the fuel vapor canister solenoid above the spare tire by the fuel tank.
I started the engine and let it idle and I removed the power supply for the #1 fuel injector and the engine lost power. I hooked it back up and the engine resumed normally.
I tried to perform a compression test but the well is so deep that I can't be sure if I am getting a good seal. The #1 cylinder was displaying 110 psi., but again I can't be sure of a good seal.
I was thinking maybe camshaft or crankshaft sensor, mass flow sensor, or worse case exhaust valve and or head gasket.
Has anyone else experienced the same issue with this engine?

Some additional Information: This Truck has 120,500 miles on it, but it is babied. I don't tow anything with it nor haul anything in the back. This truck never gets kicked into passing. It's a long bed, long wheelbase truck that is pampered.

 

Fuel pressure/volume was tested?
Fuel starvation is one thing that can cause a flat throttle-drivability issue and not always set code..

 

Excellent Suggestion. I just went online and ordered an OTC Fuel pressure inspection kit. It will be here in two days. In the meantime, I started the engine and pulled the vacuum line off the fuel regulator on the rail and noted that there was a good suction from the vacuum line and no fuel came out of the regulator. When the Test kit arrives I'll check the standing pressure with just the key on and with the engine running. I'll take the vacuum line off and run it and will check the values.
Thank you for that suggestion.

 

I will try that in the morning since it's pouring down rain here at the moment. However, while I was driving home from work today I did slip it into neutral and the vibration was still there. I was also talking to another guy at work today about this issue and another guy said he had a newer model Ford with the same 4.6-liter motor that was doing the exact same thing. He said he thought he had transmission or worn drive train u-joint issues because of the vibration. He said that his threw a code for an oxygen sensor failure. The same thing... the engine starts perfectly, idles with no issues, but at 35 mph the vibration begins.

 

Seemed you were describing a loss of power as well out beyond 35. (?)
Could be a joint. If you remove the shaft, index mark it to the pinion flange.
4x2 or 4x4?

 

Fuel pressure spec for your 97 is 30-45 psi at idle. You already determined your fuel regulator is good by seeing there was no raw fuel at the vacuum line. Too many buy these thinking it will solve a low pressure situation but the regulator actually keeps the pressure from going too high. If your pump is strong, your pressure should go up as you remove the vacuum line and hold steady as you goose the gas. If the pressure drops a lot briefly, your pump is weak.

Where do you feel the vibration. Driveline issues will be most notable at lower speeds and present in the seat and tend to dissapate at higher speeds. I believe your vibration is actually a misfire indicated by your fault code or the notorious converter shudder common with the fords.

 

Hank, Thank you for that suggestion. I went through the range of gears while driving to work and the vibration was still there, so I took the truck to 60 mph and put it in neutral. The RPM fell to idle and the vibration was still there?
This is a 2 wheel drive long wheel base truck, so now I'm thinking the U-Joints could be worn. I am going to order some and change them out this weekend.

 

Raski, Thank You for posting those Fuel pressure spec's. I ordered an OTC fuel pressure analyzer kit yesterday, so it should be here in a couple of days. The vibration manifests itself in the dash, doors, and sheering wheel. If the engine had not thrown the P0301 code I would have thought it was a U-Joint or transmission issue. On the other hand when dealing with a 22 year old truck I'm sure it could have multiple issues.

 

Hayapower, The old truck is a 4X2 long wheel base. It has 2 drive shafts. I am going to change the U-Joints out this coming weekend.

 

Kicking the trans in Neutral, and a smooth idle should elimate vibs caused up front. Being its a two piece shaft, take a look at the center/carrier bearing as well.