I have a 1991 F-150, 4WD 5.0 5 sp with 119K and have tried everything I can think of to stop the annoying pinging. The timing is correct, fresh tune up, de-carboned, I have reg gas in one tank, hi octane in the other and no change between the two. The shop I normally go to says timing chain may need replacing but they have done no real troubleshooting as far as I am concerned. I have notice that when I first start it in the morning for about 10-15 minutes it runs like a new truck, plenty of power pulls along great, no pinging. Once it reaches operating temp at highway speeds it pings like an old taxi cab and it is driving me nuts. I just seperated all of the plug wires so there is no contact and still no change. I have a manual that says there is a knock sensor and was leaning toward that. If anyone can help me I would sure appreciate it. I really like this old truck and want to put it back into shape.

 

try premium gas

 

I just posted about my 2001 making a sound similar to what your describing, maybe we are having the same problem, look at my post "Old engine sound on 2001?"

 

The knock sensor is supposed to retard timing up to about 8 degrees.
If no work is done to check these things then you have the problem.
The engines run in a highly compressed state of timing in order to get more low end torque by keep the timing as advances as possible under such controls as the knock sensor.
I once tested engine operation with the knock sensor unplugged and got the same results you are speaking of.

 

Besides the knock sensor, check the cooling system. Excess heat can really contribute to detonation.

 

My next step is to replace the thermo / gasket and all of the hoses and a good cooling systems flush. The guage goes up about mid way when fully warmed up and may drop slightly at highway speed. Any idea exactly where the knock sensor is on a 91 5.0? Thanks

 

Just taking a stab here but could it be related to the computer going from closed loop mode to open loop mode? Maybe the O2 sensors are bad causing it to lean out the cylinders too much and make it run hot? It certainly sounds like a "hot spot" problem. My 88 5.0 pinged like crazy sometimes. It was extremely sensitive to the gas I put in it.

Just some thoughts