Try some combustion chamber cleaner. Most likely the culprit is carbon buildup in the combustion chamber. I started experiencing some pinging during the same conditions as you with my Jeep Grand Cherokee. Switching from 87 to 89 may reduce your pinging but doesn't eliminate it from my experience. My pinging finally totally went away with even using 87 by using a can of Mopar Combustion Chamber Cleaner....great stuff. 7 bucks for a can at the Chrysler dealer. Going on 30K plus miles after using it and have not had any pinging using 89 whatsoever. I should mention that I have been using Redline Complete Fuel System Cleaner every so often to continue keeping it clean...probably once every 7K miles or so.
You'll need:
1 can of Mopar Combustion Chamber Cleaner (available at local Chrysler dealer...7 bucks a can)
Toothbrush
Clean rag
Don't follow the directions on the can, doesn't work well.
(Instead, Follow These Directions):
Start the engine and let it idle or take your ride out for a drive until it reaches normal operating temperature (engine should be hot). Turn off engine. Remove the air hose from the throttle body. With engine OFF, spray some Mopar CCC onto throttle plate(not too much) and linkage and clean off carbon using a small tooth brush (make sure you clean the bottom of the throttle plate as well). Wipe off dissolved carbon with clean rag (should shine like new). Start the engine (make sure it's hot) and spray the entire can of Mopar CCC into the throttle body (the intake manifold vacuum will suck it in). You may have smoke coming out of the tailpipe during this, and your idle may drop or even try to stall (this is normal). It’s important NOT to rev the engine during this but to keep a steady idle around 1000-1200 RPM by controlling the idle with your hand on the throttle body spring pack (a glove may come in handy since the throttle spring pack may be hot). When can is empty, shut off the engine IMMEDIATELY. Wait 20 minutes for the cleaner to soak. While waiting, reconnect the air hose to the throttle body. Start the engine (may need to fire it up a few times) and DO NOT let it just sit and idle once it's started or else the carbon will just re-cook in your engine. Rather, rev the engine to 3000-3500 RPM's 10-15 times. Again, don't be surprised if you see A LOT of smoke, especially white smoke...it's normal. Take it out for a ride and drive the heck out of it for a few minutes. This will do a good job of cleaning out any excess carbon from the engine. I like to use a can of Redline Complete Fuel System Cleaner shortly after, like the next day, to further clean and condition the combustion chamber, pistons, valves, fuel system...all that jazz. Try that, post your results. Good Luck.
