I have a 97' F-150 4x4, with a 4.6l V8 and about 135000miles on it.

The truck drives normally and accelerates fine, until you reach approx. 33-3500 RPM. Then it stutters and bucks. This stops when you go blow 3300RPM.

If you keep the throttle down, and let the engine continue to ramp up, the transmission shifts to the next gear and the RPM's go below 3300, everything smooths out and if you keep accelerating, when the engine gets to 3300 RPM it starts its dance.

My 'AutoEnginuity' Diagnostic software does not show any misfires on any cylinder.

NO codes are thrown.

I have replaced the plugs with Motorcrafts, cleaned the injector electrical connections, cleaned the plug wires (plug boot and coil boots). Cleaned the MAF. Replaced the fuel filter.

To make sure this was not a O2 sensor problem, I let the truck cool down overnight and then tried this while the O2 sensor system was in 'open loop' and it still does it. Plus, I replaced the B1S1 and B2S1 (upstream) O2 sensors about two months ago. The B1S2 and B2S2 (downstream) sensors are original equipment.

It was running just a tad rough, but only I or an engine head could detect it. After I did all this, it runs just like it is supposed to, except for the acceleration issue.

I had the transmission checked by a very good shop here in town and they say the tranny is fine. Its the engine.


My plug wires are about six months old. The coils are Accell's and were replaced at the same time.

1. Could a coil be breaking down under load?
2. Could a Crank Position Sensor be breaking down and cause this?
3. Fuel Pump or Fuel pressure regulator?
4. Could the Intake Manifold Tuning System (IMT) cause this?


I am now lost....

 

1. Yes, and so could the plug wires. If the Accel coil packs are anything like the Accel COP's, they are very suspect.

 

New wires should do the trick! Sounds like your arcing out to the valve covers.

 

Not heard of this with the Accels.

I like the extra power from the Accells. Do you recommend going back to stock or go to MSI's?

 

How about bigger wires?

I have not tried revving the engine at night to see if I have arcing...

 

Sounds like either a misfire or a fuel delivery issue. Have you done a fuel pressure test on it??

 

How old is your fuel filter? Might be your running out of fuel pressure..

 

Just replaced it yesterday. It was only 9months old.

 

Might do a fuel pressure check. See if its holding up. Sounds like it might be a fuel pressure issue to me.

 

Mine did the same thing at 128k miles, and, new wires and plugs cured it. As a general rule, when the plugs need replacing the wires do too.

 

what kind of wires should i get(any specific brand) i have a 97 with a 4.6

 

I use DURALAST from autozone. I have not had any trouble out of them. Just be sure to use the dielectric grease in the boots and on the plugs.

 

OEM Motorcraft.

 

My accel coil packs are awesome and have not let me down at all.

 

Did a fuel pressure check.

The spec is 30-45 psi with vacuum line connected to regulator and 40-50 without the vacuum line.

With the vacuum line connected I have 32psi. Without vacuum line I have 40psi.

When I gun the engine, the pressure jumps to 39psi then back to 32psi. I replaced the regulator anyways... no change.