We've had pinging on our 1999 Maountaineer 5.0 since new. Although the owner's manual say 87 octane is fine, we've run 92 premium since new and it still pings but not violent like 87 octane. After 77K miles (current), I decided to start troubleshooting the ping. I tee'd a vacuum gauge on the EGR's vacuum line and going down the highway, I get 5-7 inches of vacuum. Is this normal or too low? I then put a vacuum line to the EGR valve with my gauge still installed and when I apply 5-6 inches of vacuum to the EGR valve, it doesn't open (idling in neutral) and me sucking on the vacuum line. The diaphram is good on the EGR. How do I increase the amount of vacuum to the EGR valve? Or is my vacuum normal and the EGR is requiring too much vacuum to open? I had an Oldsmobile years ago that had a bunch of pinholes in the EGR valve housing to induce a vacuum leak. I plugged them one at a time using BB's and super-glue until I got the desired amount of EGR. Not only did that stop ping but boosted our mileage substantially. I guess this Mountaineer isn't that easy? BTW, we get 9-10 in town and 14-16 on the highway!
You should be able to test EGR function with a good scan tool (Snap-On MT2500, Modis,etc..), not the "code reader" piece of crap they sell for $100 or so. If the EGR is functioning, as you command more EGR flow% your idle will slow down, and eventually stall with enough EGR. You could also try this a bit less prescisely, by using a hand vacum pump connected to the egr valve. As you apply vacum, it should open the egr valve, and cause the same idle issues. I think this will work, but I'm not 100%, because of the DPFE sensor.
By the way, you kinda hit on one of the secrets to better highway mileage: Increased EGR flow rate. Now, it won't work all by itself, so don't try to make the egr valve flow more, with your BB thing or anything. EGR is controlled by the ecm on modern cars, and increased flow requires changes to the spark and fuel table as well.
As for the pinging, it's quite common for vehicles to need higher octane gas as they age. This is due to the buildup of carbon deposits on the combustion chamber. It essentially makes the chambers smaller, raising compression ratios, and requiring higher octane to prevent pinging. If it's not excessive, and you use the higher octane, it can actually give you a little bit more power (higher compression ratio), but eventually it gets out of hand. Your vehicle has fairly low mileage for this to be a big problem, but it's possible. I would recommend that you do the "seafoam" treatment. There are plenty of threads on here describing it. Just buy a bottle of Seafoam at you local autozone, and slowly feed it into the running (and warmed up) engine thru a vacum line (make sure it feeds all cylinders, like a vac line that enters just after the throttle body, or into the main plenum area of the intake) When the bottle is empty, shut off the engine. Let it sit for about 30min or so. Start up the truck, keeping your foot on the gas enough to keep it runnning (1500-2000rpm should do). OH YEAH, ISSUE THE NEIGHBORS GAS MASK!!!!
Um, yeah, as they say on the bottle "fumes will be extreme for a short time". This procedure will clean all thise carbon deposits out of the combustion chambers, as well as off your valves, and injectors. You may find that you no longer need premium, and that the truck runs better/smoother.
