First some info:
2000 F150 4.2 V6 162,000 miles.
Only problems since new purchase: radiator flushed 1 year ago.
Lower intake manifold gasket replaced at approx. 155,000 miles.
(They did a compression/leakdown test after the replacement: all was normal. They said the engine appeared to be in great shape.)
Other than that, a perfectly trouble free engine / vehicle.

FWIW, I changed the oil 3 days ago.
The CEL came on a few weeks back. I reset the code and it stayed off about a week, then came back on. I believe it to be an O2 sensor but haven't got the code read yet.
The engine runs smooth as silk, MPG is as high as it has ever been. No stumble, no miss, no hesitation, nothing. In short, it runs great.


While driving to work this morning I stopped to buy a Coke.
I got back in, cranked up and drove off.
1 mile down the road the Oil Warning light came on.
"What the heck!?" Thinking I might have a leak (but knowing better) I pulled over, shut the engine off, and popped the hood to check.
Engine oil level was fine - dead in the middle of the good zone on the stick - right where it was after my oil change. No leak.

BUT...the coolant overflow/de-gas tank was bubbling like mad! I knew that was bizzare because my temp gauge had been right where it always sat - in the middle of the normal range.
The coolant didn't seem to be boiling. It looked like it just had air blowing through it: big bubbles of air.
I was puzzled.
I checked the oil level again. Not high. Not low. Just right.

I got back in and started the engine. The oil light did not come on. The temp gauge was normal - all was fine.

I drove on to work (10 miles) without incident. I rechecked under the hood.
Oil level is fine.
Coolant was not bubbling at all (I did loose some coolant that blew out of the tank while it was "erupting" earlier).
Engine did not appear to be running hot or abnormal in any way.
Everyting seemed as normal as it has for the last 162,000 miles.

I'd suspect maybe a head gasket leaking, blowing hot exhaust gas into the coolant, but I'd think maybe SOME sort of indication otherwise. Also, If it was a blown head gasket, I'd expect it to repeat the bubbling coolant episode.

Anyways, that's what happend.

ANY ideas?
Thanks in advance guys!

 

Update:
Drove to and from Auto Zone - no incidents:
Code:
P0420 Catalyst System Efficiency Below Threshold (Bank 1)

Looks like it is a bad sensor.

Still looking for any ideas on the oil light and erupting coolant episode.
Thanks.

 

Sensors seldom cause P0420 codes. Worn out cats do. MIL eliminators would be suggested.

Oil light: Do you have an oil "pressure guage"? If so, what did it do? If it dropped in conjunction witht he OIL light, I'd suspect the oil pressure sending switch as the most likely fault.

As for the coolant, I'd replace the radiator cap and see what happens.

Steve

 

Steve,
Thanks.
The oil pressure "guage" (idiot light with and arm rather than a bulb) stayed right where it always has.

I do not think the coolant cap is the problem. I have run the truck with the coolant cap completetly loose and never boiled at all.

I'm perplexed because neither of these two events ahs ever happened before and the without warning, both occured at the same time, both stopped and now it's as if nothing ever happened.

 

I wonder if a cat is partially clogged and causing too much backpressure. It may be loose inside and clogging sometimes and not others.
I would do a coolant pressure test and a leakdown test to check for an ailing head gasket.
Another possibility could have been an air pocket in the cooling system that may have burped out. Is it warmer than usual where you are?

 

Thanks Jimmy. Indeed I do have a cat whose guts have broken up. It has been like that for quite sometime and I figure that is why I am now getting the 420 code. (Converter efficiency problem). I have notied no effect on engine performance. I had a truck long ago whose cat clogged. It was very noticeable.
Assuming that the cat is part of the picture I still am left to wonder why my temp gauge didn't spike AND why the Oil Warning light came on.
I just can't put everything together to make the pieces fit.
Drove home today with the odometer in test mode.
Engine ran fine. Thermostat kept the temp at 89-91 Deg C with or without the AC.
I checked the OIL readout and got 000, which, I believe indicates oil pressure is in the safe zone (above 6 psi).
I am baffled by this one.
I know the cat is bound to be a part of the equation, but the rest just doesn't add up.

 

I wonder if the oil pressure gauge problem is just a bad sender and the timing was just coincedence.
I think I'd start getting ready to do head gaskets soon