I'm here asking for help as I'm lost on what to look for now. I have had my truck in the shop 3 times trying to stop this pinging that has been happening at highway speeds in 4th and 5th gear under load (manual). It was pretty bad until they finally changed my mask airflow sensor which was giving false readings. After this it was quiet on premium fuel. But when I put mid grade fuel in, there it was again. Climbing hills on freeways or backroads is when this happens, and it really grinds my gears as I thought to have had it fixed 3 different times. They checked plug wires, spark plugs, fuel filter, mass airflow, EGR, I replaced EGR solenoid. What else could be causing this? I even ran a bottle of intake seafoam which I may do again. The motor only has 130k plus miles on it, and I would think it wouldn't be having this issue. As I had a 94 4.0 before that had 170k miles and didn't. Please give some advice for what it may be as I'm pretty desperate. I don't want to keep taking it to shops if it's only going to cost me money, as I'm a 19 year old college student with not mounds of money. Thankyou everyone who shoots some advice my way.

 

This is what mine does occasionally too. Most likely carbon knock. Seafoam helped a bit at first but the PM3 lasted much longer. I usually do this with only one bottle of PM3 about every 8 months or so. You can just run premium but it can burn incompletely and add to the problem. Here's a link with the TSB procedure in it:

Carbon Knock TSB link

Recently I tried the Hot Shot gasoline extreme and there was a noticeable improvement.

 

Welcome to the forum

I assume the CEL(check engine light) is not on and you have never had any codes?

You had a 1994 Ranger, what year is this one?

The 1998-2000 ran hotter cylinder temps so were more prone to pinging, but still ran 9.0:1 compression ratio so should run fine on Regular(87 octane) fuel

After engine is fully warmed up, let it idle
Unplug the 2 wire connector on the IAC Valve
Idle should drop to 500rpm or engine may even stall, either is GOOD, no vacuum leaks
If idle does not drop then you have vacuum leak causing the pinging


There are a few reasons for pinging/knocking, pre-ignition, air:fuel mix is self igniting before spark plug fires

One is high compression, compression is HEAT, this heat is needed to change liquid gasoline into a vapor, only gasoline vapor can be ignited by a spark, yes the movie guys take liberties with this one, lol
Compression ratios under 9.3:1 can usually run fine on 87 octane(octane is a heat rating)
When compression ratio gets above 9.5:1 then you need to go to 89 octane, and even higher compression means even high octane
Carbon build up is a double whammy, layers of carbon in head chamber and piston tops increase compression ratio, and carbon holds in heat, cooling system can't work as well to pull heat out of the cylinders
As said octane is a heat rating 87 octane will self ignite at a lower temp than 89 octane, which ignites at a lower temp then 91 octane
Heat is heat, so if your temp gauge on the dash is above 1/2 then your engine is over heating
Normal range is just below 1/2
And you can try a lower temp thermostat, although this can lower MPG a bit, stock thermostat is 192deg, you could try a 180deg t-stat is see if HEAT is the issue

Another reason is the air:fuel mix, if is Lean, then it will heat up too fast and self ignite
A richer fuel mix helps cool cylinders as it take more heat to vaporize it, so it uses up heat
O2 sensors help computer to adjust the air:fuel mix, O2 sensors should be changed every 100k miles, they use chemicals to "see" oxygen in the exhaust and they simply run out of those chemicals over time
An easy test to see if air:fuel mix is the issue for pinging is to FLOOR the gas pedal when engine is pinging, when computer "sees" WOT(wide open throttle) it ignores the O2 sensors, and dumps extra fuel into the engine for MAX power, fuel economy is out the window, lol, its all about POWER with pedal to the metal

EGR system is used to COOL the cylinders, yes hot exhaust gases do cool cylinders, cooler cylinders prevent the spike of NOx gases, a toxic gas, hotter cylinder temps cause more NOx to be generated
When exhaust gases are mixed with air:fuel mix they slow down the burn rate of the mix when cylinder fires, and this prevents the spike in temp, so prevents the spike in NOx production
As a side effect the cooler cylinder temps also helps prevent pinging, so a working EGR system helps

Spark timing, not much you can do about this, but you can ASK
Get your VIN number and call a Ford dealer, see if there is a Computer update for your VIN, it is possible that your software from the factory had an issue over time, and Ford changed the software
A dealer would charge $100-$150 to load the new software, but it beats chasing your tail trying to fix a software issue with hard parts that won't fix it, lol
The 4.0l spark plugs need to be gapped at 0.054, a big gap, and you should use Motorcraft or Autolite brands, 4.0l tends to "eat" other brands, lol