Condensate under oil fill cap?

I added oil to a '98 Silverado, and noticed water condensed under the oil fill cap. Also, there were a few tiny bits of oil in the neck under the radiator cap (I hope this doesn’t indicate a head or manifold gasket starting to fail.) And, the PCV hose that goes back into the air intake was disconnected.

Do you think any of this stuff is related?

The moisture on the oil cap is not much by itself, but … Well I guess you already know. I would have a compression test done. (I am assuming the oil was on the inside of the radiator cap.)

Engine size and mileage? Lots of short trips?

Keep an eye on the coolant reservoir level. If it disappears steadily and you do not see an external leak, you have an internal problem.

If the oil turns a light brown milky color, keep checking the oil cap/run your finger inside the valve cover (looking for light brown sludge) and look for oil droplets floating in the coolant reservoir.

Check the intake manifold gasket for external leaks (most were at the bolt corners).

Coolant leaking internally could well mean a leaking intake manifold gasket and should be repaired immediately before permanent damage is done to the crankshaft journals and bearings.

Any white smoke from the tailpipe? Over-heating?

Like JEM said, compression and or a coolant pressure test.

Shake the PCV valve. Does it rattle?

With the engine idling, place your thumb over the end of the valve (with the hose connected). Any suction? If so, it’s ok and plug it back in.

There’s a lot more to it than this, but it’s a start.

144K, 6-cyl. engine. The owner doesn’t drive it often. She heard a “sloshing” sound and thought she had a bad heater core, and asked me to look at it. Heater core looks fine, no evidence of leaking. I think the noise she heard was just an air bubble moving through the core, which I heard mostly right after start-up, not much once the engine was warm.
I didn’t see any obvious coolant leaks, nor did I smell coolant. I’ll look at the reservoir for oil. No white smoke or overheating. I’ll also check PCV. I’ll tell her to monitor coolant level. Thanks, RR!

The reason I alluded to the intake manifold is because GM installed millions of faulty gasket kits (no lock-tite on the bolt threads for one thing) in most of their engines (V6s being the most in the fault line) between 1996 and 2003.

The manifold bolts worked loose, air got inside and married the coolant and the next thing was a breached intake manifold gasket.

The coolant disappears from the reservoir because it was going into the crankcase and stripping the journals and bearings.

As a result of this MANY engines had to be rebuilt at the owners expense.

The gasket kit replacement in these V6s cost in the neighborhood of $700 due to the labor involved trying to GET TO the gaskets.

No recall and not a lot of goodwill either.

They finally released a new gasket kit with new specs and for the most part seemed to correct the gasket issue. (My Silhouettes’ 3.4L was one of them, gasket changed, oil/filter changed, original Dexcool flushed out and new Dexcool replaced)

That was 4 years ago and have had no problem since.
I drain and refill the coolant every two years (NOT 5 like the manual says).

Now that I’ve all but twisted your ear off, FWIW, another ‘sloshing’ sound heard from the drivers seat can come from plugged fresh air vent water drains (below the windshield).

Hopefully, the problem is NOT with the manifold.

Retorque the intake manifold bolts. Pressure test the engine coolant system. There shouldn’t be any air bubbling in the engine. For air to go in, coolant went out. Use the air-bleeding procedure, and listen for bubbling in the engine, again.