I have a 1998, 4.0 with 169K. My truck has been pinging for sometime and 91 octane has not helped. It only pings occasionally when driving up a grade. Its is definitely a ping and not a knock. Spark plugs and wires are good, coolant is up to date. L I change the oil religiously and take pride in maintaining my Ranger. I have no check engine light or codes. I have added two cans of Seafoam to the gas and oil and still pinged. Recently, I added Lucas octane booster to chevron 91 octane and the pinging went away. However, I do not want to pay $11 for a bottle of Lucas every time I fill up. How can I fix this?? Its driving me nuts. Please help.

 

Welcome to the forum

Pinging can be caused by three things, spark timing, fuel mix and heat

4.0l OHV engine runs 9.0:1 compression ratio so can used 87 octane fuel without pre-ignition(pinging) caused by compression heat.
This is IF compression stays at 9.0, if engine runs rich for long periods it will build up carbon layers on the piston tops and head chamber, this lowers volume so increases compression ratio, and it is a double whammy in that the carbon can get hot enough to pre-ignite the fuel mix on compression.
Spark plugs should show signs of carbon build up as they were changed over the years.
Did you ever see this?
Blackish residue

You can do a compression test on all 6 cylinders, expected would be 155psi, if a few were higher, in the 165-170psi area then they could be pinging

Air fuel mix gets too Lean
This will cause pinging as the Lean mix is easier to ignite and doesn't cool the cylinder as well as normal mix, as as cylinder get hotter, like when going up hill, Lean mix will start to ping, try pushing gas pedal down to the floor, WOT(wide open throttle) takes computer out of "economy mode", it ignores O2 sensors and adds more fuel for more power based on internal tables for RPM
If WOT reduces pinging then check fuel pressure, 1998 and up Rangers use Returnless system so 55-65psi is what computer expects and uses in its mix calculations, if pressure is lower then mix will be Lean

WOT is a voltage setting, so to test if your TPS(throttle position sensor) is working correctly:
turn key on
Press gas pedal down to the floor and hold it down all the way, this should turn OFF fuel injectors
Now turn key to START
Engine should just turn over and over and not fire or startup, fuel injectors are off
This is called "Clear Flooded Engine" mode

If engine fires and/or starts then most likely your throttle cable has stretched so TPS never shows WOT voltage to computer
Google: Ranger throttle cable mod
Easy fix

Then try going up the hill again at WOT to see if pinging is reduced


Spark timing is done by the Crank sensor and modified slightly by Cam sensor
And bad spark timing won't be strictly heat based as above is
So Cold engine ping is likely if it is spark timing
Spark plugs matter as well, the 4.0l likes Motorcraft the best then Autolite, and a BIG gap, .054
If you run other brands make sure they are set for big gap and are correct heat range
Hotter plugs lose less heat so can act as ignition source after they warm up and especially under load, i.e. going uphill
Colder plugs lose heat faster so less likely to cause pinging


And I assume engine coolant is running in normal range, just below 1/2 on the temp gauge after full warm up?

And just as a heads up, the 2 upstream O2 sensors do wear out every 100-150k miles, these aid computer in setting correct fuel mix, so if one upstream O2 was worn out it could cause pinging on that one bank of the engine under load.
If you don't know if these have been changed then I would replace both just because you are at 160k miles
Downstream O2 lasts twice as long since they see "cleaner exhaust" after Cat converters