I have experienced what nearly everyone who has ever had a GM vehicle with a 3400 engine may have experienced at least once, a head gasket failure, but, not the common type, and it was certainly slow developing. I hope in telling a little bit about the signs, that most can catch it early if they are experiencing the same issues as I was.
I've learned that there are actually 4 types of head gasket failures:
1. External leakage where you actually have coolant leaking out of your engine (This is the one that everybody expects).
2. Internal where coolant leaks into the oil (usually easy to figure out).
3. Internal where coolant leaks directly into cylinders and directly out the exhaust. (The most silent and slow occurring one that no one expects).
4. Any combination of the above.
The type I had was the 3rd one and it apparently had been present since I bought the car. Since I bought my second Nox, I had noticed that even after everything else was corrected, the engine still had varying RPMs and it kept setting off the 02 sensor at what seemed at random.
When I would connect Tech2Win, there was always an extremely slight misfire on cylinders 1 and 4, however, if you pulled the spark plugs, they looked clean. Also, just before the 02 sensor would go off, there would be a slight bump in the engine. I now believe this was coolant being passed through one of the cylinders directly into the first 02 Sensor. If you pull the sensor and clean it, their might be white water deposits left on it. This is a sign of gasket failure.
This went on for 7 months, until recently, the car had no heat, and, eventually the engine would try to overheat.
Now, take note, there was no mixing of oil that was occurring, and, any coolant passing through was passed directly to the exhaust manifold.
When the leak got bigger, because pistons can act like an air compressor, they pushed air into the coolant system and were displacing the coolant causing the engine to overheat...... Just before this started to occur, there would be some puddling of coolant (it will look like condensation) at both before and after the muffler.
Also, note that if this kind of failure is happening, there will be almost no loss of power or any other signs until the vehicle goes into "limp home" mode.
I advise that if you are experiencing any of the prior tell-tale signs such as extremely slight rough idle/misfire and/or if the 02 sensor with "Catalyst Efficiency Bank 1 Low" code, even after replacing all the parts, have a compression check done immediately.
There will be no mixing of coolant with oil or any other signs usually with this type of failure and their might be coolant dried to the first 02 sensor or the spark plug(s) of the cylinders involved.
Now, just to let you know, you can save yourself on the road if a failure happens a couple of ways. One is to, every few miles, release air from the coolant system by very slowly unscrewing the bleed screw and relieving it while the engine is running. This may take several stops for your return home, but, it will get you home... just use your heater as an indicator and watch the temp gauge. Just make sure to pick up plenty of water along your way.
Another method is to find a Wal-Mart Supercenter along your route, and, prepare to spend a little money. Get the Bars Leaks Block Head Gasket Fix that is usually $26, at least 1-2 bottles of 50/50 Dex-cool approved coolant, and, at least 2 large bottles of water. While you're in the Wal-Mart parking lot, follow the instructions on the Bars Leaks bottle closely, except, after you've allowed the engine to cool on the last step, start the engine and relieve the air in the coolant system via the bleed screw. This may take up to 5 minutes before you see coolant. Add coolant and water as necessary to get home.
After either method, consider gasket replacement immediately. Take note that a blown head gasket in the case of the 3400 DOES NOT MEAN the death of your engine, and, can be rather easy and in-expensive to replace especially if you order them from eBay with everything, including the bolts.... There is a YouTube video on the entire replacement from start to finish.
It's not a bad tear down at all really, and, it gives you a chance to clean everything. Here's the video:
I hope this helps some on this silent but deadly issue....