Rattle During Acceleration

The stock 5.9 was meant to run on 87. The spark plugs should not be prematurely firing. You would have to advance the timing to do that. The timing is computer controlled and I assume you do not have a modified computer. What is likely occuring is premature ignition in the cylinder. The explosive mixture is ignighting too soon in the cylinder up stroke. The noise heard is the expanding gas trying to force the cylinder back down as it is coming up. The will mostly happen at WOT or when there is heavy stress on the engine (hills, towing etc) A dirty engine and carbon deposits can play a factor and cleaner may help. As indicated above the most common thing on the 5.9/5.2 is failure of the intake manifold pan gasket. A quick check is to pull off the air hat, open the throttles and with a light see if there is oil resudue in the bottom of the intake. There is a better check if I can locate the link on how to check the vacuum. Running plus or premimum may help the pinging but it is not correcting the problem. Higher octane fuel resists ingnition therefore it may not ignight too soon. Pinging is not good. Minor pinging likely will not do anything. Severe detonation will distroy the engine.

You cannot adjust the timing. The timing is controlled by the computer so rotating the cap is not going to do anything.

Search for "ping" in the magnum engines technical forum. There's a thread with a poll about whether 5.2's or 5.9's ping more. There are a lot of informative posts in that thread. The big issue with these engines is that the plenum pan gasket tends to develop a leak, allowing oil to get sucked into the intake causing the cylinder heads to get carboned up, causing preignition (pinging). As stated above, use a flashlight to look down through the throttle body-- if there's oil in there, you have a plenum gasket leak. Another way to check this is with a vacuum gauge-- your mechanic should know how to do this, or we can tell you if you're interested in doing it yourself.

I had some bad pinging, but the visual check and vacuum test indicated no plenum gasket leak. I gave the engine a Sea Foam treatment and I've followed up with Chevron Techron every other oil change and that seems to keep things under control. Others claim good results with a 180 thermostat and Autolite plugs, but I'd recommend decarbonizing in any case.