2006 Dodge Caravan

What's the symptom?

If you mean it's moving forward and backward, that is controlled by one of the crankshaft bearings called the thrust bearing. On some engines those are selective fit inserts that are next to but not part of the main bearing. In most engines that is a main bearing with two sides folded down to form the thrust surfaces. Clearance between the thrust bearing and the thrust surfaces on the crank are measured in thousandths of an inch. Excessive wear will let the crank move enough to push out the rear main oil seal on some engines, and to allow the counterweights to hit the engine block webbing and make a knocking noise. That noise can occur more often when accelerating because the torque converter can balloon up from increased fluid pressure and push the crank forward. The same can happen on cars with manual transmissions when you push on the clutch. That puts forward pressure on the crank too.

The biggest cause of thrust bearing wear is using low grade engine oil or not changing the oil at recommended intervals. There were also two instances where the wear was caused by insufficient heat treating to harden the crankshaft. It was the thrust surface of the crankshaft that wore away, not the relatively soft bearing. The first was on Ford V-8 engines around 1972. The second case was the Chrysler 3.3L V-6 engines in 1989. The Chrysler engines were all taken care of under warranty. It took the mechanic less than six hours to pull the engine out replace the crankshaft, wash the engine compartment, reinstall the engine, and tie up any loose ends.

Caradiodoc

Hmmm. Does that mean the seal is backing up against the sprocket? I don't think the crank can move far enough to push the front seal out. The snout will just slide through the oil pump housing.

If the seal is indeed backed out, I'd look at washing all traces of oil off then reinstalling it with thread lock sealant or silicone gasket sealer. The Mopar gray stuff gets harder and seals better if there's still an oil film. The black stuff requires perfectly oil-free surfaces. You could also try peening over a few punch marks in the oil pump housing to retain the seal better.

If the seal isn't backed out, look for nicks on the crankshaft snout, possibly from removing the previous seal. Also be sure the spring is inside the seal that pulls the lip down tighter on the snout.

Caradiodoc

Yup. There's a rounded "radius" on the snout of the crank where it becomes larger in diameter. The sprocket will go all the way up to that spot. If you don't press it far enough, the vibration damper should still push it back when it is tightened down. If you can't slide the sprocket all the way back, check for gouges in the key way slot that are catching on the front end of the key.

Caradiodoc