2005 Dodge Ram 1500 SLT, 4.7L with auto transmission and about 87K miles on it.
I had engine idle problems, when I would turn the truck on and watch the tach, and it would go from 400 RPM to 1200 RPM, bouncing back and forth every few seconds, almost like my foot was gunning it.
Then, when I would put it into gear and drive off, I could hear engine pinging.
I had previously changed out the original copper plugs for platinums at about 60K miles, and I've noticed a drop in performance and power since then so I just changed the platinums out for new coppers this last week. Pinging sound is gone, so I don't think I was getting good combustion or I was getting a late spark or something.
But, the revving problem is still there.
It tends to go away after driving for awhile. But, if I come to an abrupt stop at a rapidly changing stoplight, it comes back. The RPM's dip to near 400 from the rapid change in engine demand, then bounce back up. Once the truck is warmed up, the lunge/fall is less dramatic than when it is cold, but I'd say 400-900 is the swing when it happens when the truck is hot.
I'm thinking of two things right now as tests...
-Replace the fuel filter. Haven't done that to the truck yet. Probably time. Perhaps there's a restriction in it, and the fuel pump is getting wonky "hungry" signals from the engine ECM?
-Run some fuel injector cleaner through a tank.
But, I understand this can also be caused by a failed head cover gasket leak, a faulty PCV valve, or a vacuum leak elsewhere. I don't see any oil leaking on either of the head covers, the PCV valve rattles when I pull it so it isn't stuck. Vehicle vacuum systems have always plagued me. Had a car with bad brakes for several years due to a leak in the master brake cylinder that I could never trace down.
I've also heard that the distributor cap can contribute to this. A weak spark along the spark lines, or on the contact itself. Given the fact that my pinging problem is gone, I think that unlikely, after the change in plugs. Am I wrong?
Anyone got any ideas?