1999 Toyota Tacoma pinging

You probably have a faulty knock sensor, this picks up the vibration from engine ping and sends a signal to the to alter the timing

You may have to do a pin out test to check for any high resistance from the knock sensor back to the ECU, and you can test the knock sensor is working by checking the timing with no check wire connected and tapping the block with a small ball peen hammer you should see the timing alter slightly if the sensor is working.

This is a different expert.
You may want to concentrate on the EGR system. An inoperative EGR will cause pinging all day. You need to make sure it is fully functioning and the passage isn't plugged.

How do you know that?

That tests the valve itself and the passage but it doesn't test the feedback sensor or vacuum solenoid.

CIRCUIT DESCRIPTION
The EGR system recirculates exhaust gas, which is controlled to the proper quantity to suit the driving conditions, into the intake air mixture to slow down combustion, reduce the combustion temperature and reduce NOx emissions. The amount of EGR is regulated by the EGR vacuum modulator according to the engine load.
If even one of the following conditions is fulfilled, the VSV is turned ON by a signal from the ECM.
This results in atmospheric air acting on the EGR valve, closing the EGR valve and shutting off the exhaust gas (EGR cut-off). Under the following conditions, EGR is cut to maintain driveability.

Before the engine is warmed up.
During deceleration (throttle valve closed).
Light engine load (amount of intake air very small).
Engine racing.

The testing procedures are a bit complicated.

1. INSPECT AND CLEAN FILTER IN EGR VACUUM MODULATOR

Remove the cap and filter.
Check the filter for contamination or damage.
Using compressed air, clean the filter.
Install the filter and cap.

HINT: Install the filter with the coarser surface facing the atmospheric side (outward).

2. INSTALL VACUUM GAUGE
Using a 3-way connector, connect a vacuum gauge to the hose between the EGR valve and EGR vacuum modulator.
3. INSPECT SEATING OF EGR VALVE
Start the engine and check that the engine starts and runs at idle.
4. INSPECT VSV OPERATION WITH COLD ENGINE

The engine coolant temperature should be below 50 °C (122 °F).
Check that the vacuum gauge indicates zero at 3,500 rpm.
Check that the EGR pipe is not hot.

5. INSPECT OPERATION OF VSV AND EGR VACUUM MODULATOR

If you have no TOYOTA hand-held tester, check these procedures:

Remove the 3-way connector with the vacuum hose.
Connect the vacuum hose (from port Q of EGR vacuum modulator) to the EGR valve.
Plug the vacuum hose (from VSV for EGR).

Check that the vacuum gauge indicates low vacuum at 3,500 rpm.

6. REMOVE VACUUM GAUGE
Remove the vacuum gauge, and reconnect the vacuum hoses to the proper locations.

7. INSPECT EGR VALVE

Apply vacuum directly to the EGR valve with the engine idling.
Check that the engine runs rough or dies.
Reconnect the vacuum hoses to the proper locations. If no problem is found with this inspection, system is normal; otherwise inspect each part.

8. REMOVE EGR PIPE
Remove the bolt, 4 nuts, EGR pipe and 2 gaskets.

HINT: At the time of installation, please refer to the following items.
Install 2 new gaskets.

Torque:
Bolt : 18 Nm (185 kg. Cm, 13 ft. Lb)
Nut A: 19 Nm (195 kg. Cm, 14 ft. Lb)
Nut B: 20 Nm (200 kg. Cm, 15 ft. Lb)

9. REMOVE EGR VALVE

Disconnect these hoses:

Vacuum hose
EGR hose
Water bypass hose (from IAC valve)
Water bypass hose (from water bypass pipe)

Remove the 2 nuts, EGR valve and gasket.

Torque: 19 Nm (195 kg. Cm, 14 ft. Lb)

10. INSPECT EGR VALVE
Check for sticking and heavy carbon deposits.
If a problem is found, replace the EGR valve.

11. DISCONNECT VACUUM HOSES FROM EGR VACUUM MODULATOR
12. INSPECT EGR VACUUM MODULATOR OPERATION

Block ports P and R with your finger.
Blow air into port Q, and check that the air passes through to the air filter side freely.

Start the engine, and maintain speed at 3,500 rpm.
Repeat the above test. Check that there is a strong resistance to air flow. If operation is not as specified, replace the EGR vacuum modulator.

13. RECONNECT VACUUM HOSES TO EGR VACUUM MODULATOR
14. REMOVE CHECK VALVE

15. INSPECT CHECK VALVE

Check that air flows from the orange pipe to the black pipe.
Check that air does not flow from the black pipe to the orange pipe. If operation is not as specified, replace the check valve.

16. INSTALL CHECK VALVE

HINT: Install the check valve with the orange pipe facing the EGR vacuum modulator side.

17. INSPECT VSV FOR EGR

What I might try to simplify things is to "Tee" in a vacuum gauge at the valve feed that you can monitor while driving and when you hear the pinging, see if there is vacuum going to the valve.

Excessive carbon on the piston tops can increase the compression ratio enough to cause pinging also. You can try de-carboning the engine.

Higher octane fuel does nothing at all for carbon. You need a professional carbon cutting chemical that is slowly fed through a vacuum line and then take the truck out and run it hard to blow it all out. Sea-Foam will work but you need some kind of tool to meter the flow. It will choke it out if fed too fast.

It's not worth buying equipment for one car. You're better off letting a shop do it. Most of them offer the service by a few different names like Induction service, upper engine cleaning or de-carbonization. Most will do it for under $100.