Test Drive after Reinstall, Purge Requirement and Oil Up Belt Issues
Finishing up on the oil pressure switch unit change, on a short initial test drive, as there were no leaks in the sender, I decided to go to a close-by car wash to clean the engine bay then change back to Mobile 0W-20 and do an air purge.
Surprisingly, in only 2 miles, along with an uncharacteristic few minutes warming up while idling to check for leaks first before setting out there was a problem detected by the OBDCIIC&C Gauge, whereas the car's instrument cluster reflected nothing at all in blissful ignorance.
Day was warm, but nothing out of the ordinary on the car's gauge was noted. Suddenly, and to my consternation, the OBDIIC&C warning tone was heard and the Red LED warning lamp suddenly indicated.
WoT NeXt! was the reaction at the initial alarm! A litany of possible fails fleeted across the mind, but not too hard to guess the issue, recalling the perceived coolant loss estimated from the TB.
Observed: ECT [engine coolant temp] on the 'Gauge was spiking for only a second or so from airlocked superheated steam pockets. Still, cluster gauge totally normal and non-indicative.
Digital temp spiking was 'on-again-off-again' but more or less persistent. ICT on The 'Gauge was up bobbling 5-6 degrees C over the 'normal 91-92' when the red light and tone was 'Off', but spiking up and indicating to a reading of 186-187C quickly, then back down to a high 95C-97C.
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Seeing the above, stopped immediately at roadside & popped hood and checked: the rad cap was cool to the touch. Airlock in the coolant system no question. Car wash was only a few blocks away, so glided in to commence cleaning the blown out oil and grime coating the engine and surroundings and let things cool a bit.
Sprayed the areas most affected on the warm block avoiding the head as best as could; but by necessity getting some of the engine bay parts wet that likely shouldn't have been*.
Firewall and right side of the block was so much heavily coated from the prior AirTek/Wells oil pressure sensor spew; that in the spraying and rinsing process, the oil and crud loosened by the pre-soak got inadvertently splashed back onto the fan belt with predictable slipping/squealing right away at anything over about 1800 rpm. Chirped rapidly like a failing ball bearing in a small way.
Not totally convinced it was just a wet belt after it persisted longer than just plain water on a belt, and with my mind thinking it could be a bearing after the low oil issue, or the maybe even water pump gone on account of the steam pulses, I pulsed and glided with shutting the engine off intermittently for the bulk of the way home. btw: What a great method to save fuel and wear

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[*Anecdotally, I likely got some water/presoak stuff or the spotless wash stuff on the injector connections as well: for at the very same time, the first letter of the 'INJ' parameter disappeared while the Red LED and Alarm Tone were doing their thing.
Got no codes thrown on the OBDIIC&C fwiw, but the gauge wasn't liking the signal from all the wet in the engine bay and I got the two solid bars on the 'Gauge whereupon pulled the plug and reinserted after restarting. All was again normal save the still spiking temperature on the OBDIIC&C.
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Anyways, … got home and posthaste changed out the mix of oils that were in there from the emergency top-ups earlier, back to try Mobile 1 brand, 0W-20 again.
Displacing the air from the coolant as planned following the oil change, …level was down about half a liter, …which amount of air in the coolant system is obviously more than enough for this small engine's 4L total capacity to form an airlock and cause spikes seen only by the OBDIIC&C.
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[Again, don't forget to turn that
Climate Control Heater Temperature Knob to Maximum Hot during the air purging process! I was already primed for that to be sure. lolz]
Temperature level after purging as registered on the 'Gauge was totally rectified after air purging the coolant, but the squealing from the pulley area persisted. Ditto for cluster happily reading normal.
Not totally sure if it was the water pump taking a hit from the spikes of hot steam, or maybe some other component failed because of potential oil starvation so .....went out for another test drive on the ring roads. I thought that belt sounded a bit like a dry bearing and wanted to be sure it was just the belt, by trying to dry it out.
Having in mind the credo:
"Drive it until it Breaks!" [from the same workplace mechanic, who said
"If it Ain't Broke Don't Fix it."], and more or less figuring it _
had_ to be due to the car wash, I took a moderate speed cruise at low rpm, with a few spurts of acceleration into higher rpm to test the squeal again.
No probs: The noise was diminishing more and more with further travel and time.
Ramped up the speed on the ring roads, 'in the wind, so to speak' and in only about 15 minutes, the high pitched squealing required even more rpm to duplicate. The sound was hardly discernible at a half hour, only at full song, with a few peeps only at [near/towards] the top rpm then, with nothing to hear an hour later.

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In the case of an oil leak from the oil pressure sensor, this would be one case where 'drive it until it breaks' would surely be a false economy; and change it out ASAP for as best as you can obtain to get back on road and stay there as RedJellyBean recommends.
[EDIT -
refilled coolant overflow container prior to next cold start. Check for a few days and top up as required for an air free coolant system]