I have a 95 F150 with the 5.8L motor. It has over 200,000 miles on it, but it recently (past few weeks) passed smog with no issues. However, shortly after the smog test I started noticing a low rpm shake. It will shake a bit more than normal at idle, but seems to accelerate fine. If I am cruising with very low load on the motor, it will start to shake, almost as if the front tire was severly out of balance, however upon downshifting the shake will go away. It drives just fine at 70 mph on the freeway, but once I am down to 55-60 with little load, it will develop the shake.

Any advice on where to look first to solve this problem?

 

Is it road speed dependent, or engine RPM dependent? For example, does the shaking frequency increase/decrease with road speed, or engine speed?

 

It is engine speed dependant. It will happen going down the street in the neighborhood or on the freeway, but always when cruising at low rpm with little load on the motor.

 

Sounds like torque converter shudder, you didn't say if it was a auto (E4OD) I just assumed.

 

If it is an automatic, when it starts shaking try gently pressing the brake pedal (while still remaining on the gas). Pushing the brake pedal just enough to turn the brake lights on will disengage the torque converter clutch. If the shaking stops, that could be your issue.

 

It could also be your ball joints

 

I will take a look at those things and post up what I find. Thanks for your feedback.