Recently, my 92 Spirit w/2.5 (147k miles) started pinging under load, I never heard this at all during the eight years I have had this car,. I have tried different fuel, verified the timing at 12 BTC and replaced vacuum lines. I do not have an EGR valve. I have a newer radiator and the engine temp is 1/3 up from cold. What other items should I check? I have no codes stored except the usual 55.
Could be carbon buildup. My PT (273,000 miles) started pinging under load a while ago. Since then, it developed a miss at idle, which, with the help of people here, has been pretty much narrowed down to a leaky/burnt valve. The heads have never been off the engine so I had no clue how much carbon could be there. Since I figured I had nothing to lose, I bought the spray Seafoam and gave it a shot. Didn't help the missing issue, but as a side benefit, the pinging is totally gone. No matter how much load I put on the engine, it will not ping. Also, aside from the idle, the car is running great. When I get closer to when I need to change the timing belt (about another 30,000 miles), I'm going to change the head to rectify the miss.
Not sure if you want to try Seafoam. It's the first time I ever used it, but figured I had nothing to lose. The spray was very easy to use, and it did make a difference.
An old mechanics trick to remove carbon build up is to simply pour some water down the carb or tbi a little at a time with the engine running and revving it to keep it from stalling. I used to watch my father do this when I was a kid and I've done it on some older cars I've owned over the years. Usually one or two 8oz. cups was all that was needed.
I've heard the water thing too, chuzz. I was exploring some options as to how to help my folks out with our '05 Caravan's developed taste for premium-- sounded like one of those fairground BB machine guns, albeit a lot quieter, under load, unless it had the pricey stuff in the tank. I decided to steer clear of the water for fear of accidentally hydrolocking the engine, and ended up going with the Seafoam spray in lieu of Mopar Combustion Chamber Cleaner. If you can get the MCCC, that's pretty widely held to be an extremely potent cleaner and better than most off-the-shelf products. However, the Seafoam spray did give a bit of an improvement. I didn't use the entire can, and based on my mom's insistence on using the absolute cheapest gas possible, I imagine that the combustion chambers in that motor are not pretty at all. Point is, the Seafoam is worth a shot. It might take a can or two. If you want to try water, I would use something like a spraybottle so that I could regulate the how much water was going into the engine. Might also be worth pulling the plugs and gapping them, that should give you some idea as to what's been going on in the combustion chambers as well.
When I've used the water trick, I just pour a few drops at a time, not a steady stream. But, the spray bottle is a good idea too. It just seems that whenever I needed to do this, I didn't have a spray bottle handy.
When my 3.0 car was still under the new car warranty I noticed it pinging under load. The dealer told me to use a higher octane fuel! 89 didn't do it so I have run 93 ever since. I have never been satisfied with that cop out but it never pinged again. Good luck solving your issue.