Coolant in the oil: all possible causes?

I have an '88 Dodge Dakota with 220K mi. It had been sitting for a while before I bought it (with 198K), and it had a bit of a problem with coolant contamination of oil. (This went away after 100 mi of use.)

Well, it’s back to leaking coolant in the oil–a lot. It has that “coffee-colored slime” on the top of the dipstick. I suspect a bad head gasket, but I want to rule out all possibilities before tearing it apart.

So, what could cause oil/coolant mixing? I’m thinking either a bad gasket, or an internal failure in the engine.

So…check for compression on all 6, then (with adequate reading) assume a failure of either the head or intake gaskets?

A compression test may or may not tell you anything. You’ve apparently got a coolant to oil leak. That doesn’t necessarily have anything to do with combustion chamber sealing.

You may have a failed intake manifold gasket, a failed head gasket, a cracked head or a cracked block. The gaskets are more likely, unless this thing has gone through some very cold weather with water in it instead of antifreeze. Check for new freeze plugs. If it’s had some replaced it may be because the old ones were pushed out when the coolant froze.