Can a water pump go bad with no check engine light?

I "was" using a ScanGauge for this, till it started causing problems: kept getting brake/traction warning lights, "Check Hybrid System", and work-to-rule brakes. The latter kinda scary: at first it feels like NO brakes. Dealership mechanic suggested I stop having a constant OBD plug-in, and that did the trick.

Yeah, there are workarounds, but they can backfire. And it used to be the norm to have a coolant gauge, in the dash, from the factory.

I wouldn't wait for trouble, just replace them both, at least every 100k miles. Like an oil change. Changing the inverter water pump is a chore...

 

Don't they have noticeable misfires, especially upon cold starts when head gaskets blow?

 

People who's not mechanically inclined or never experienced a misfire don't know or care. They see there's not CEL and it was intermittent so they figure that everything is OK; because they know a mechanic will cost them money. Can't argue with that logic, it's just going to be a larger bill when that CEL eventually lights up.

PS; an overheat isn't recorded or sets a CEL on the Prius. Your temperature lamp may just blink; take it to a mechanic after it cools down, <248F - no CEL and no temp lamp.

 

Yes you are correct. An intermittent water pump can cause mild overheating without reaching 248f. It might only reach 235f for short periods like high rpm hill climbs in hot weather. Knowing what is normal and having a 210f audible alarm is the proactive step I setup years ago with an obd2 display.


(Gratuitous use of Ai to impress the younger crowd)

If I saw the coolant temperature running 10 degrees higher than normal and breaking my maximum observed temperature of 204f on high load, water pump degradation (not failure) is number one priority. At this point there would be no reservoir pressure cap releases or coolant loss.

But the normal hg failure mechanism starts losing coolant very occasionally, maybe once a month for many months.

A gen3 hg leak fools a lot of mechanics as well who don't understand how it passes a combustion leak detector test (combustion gasses in the coolant).

Initial gen3 hg coolant leaks typically create coolant drops into a cylinder when the engine is cold but coolant pressure still exists. Maybe so few drips that the spark plug fires on startup and there is no misfire. The cylinder head quickly heats up and seals the gap. Not enough time for the combustion gasses to enter the coolant and chemically change the combustion leak detection test fluid.

There's almost never coolant in the oil.

Much later with plugs that are older there is enough coolant on an occasional cold start to cause a misfire that lasts for a few seconds. The mg1 electric motor has spun the engine to 1000 rpm so the old school slow stumbling start never happens. Instead the spring loaded damper between the engine and the transaxle slips due to uneven rpm caused by the temporarily lost cylinder. This creates an unmistakable severe metallic rattling and engine shake some blame on cold weather intake manifold condensation entering the cylinder.

Things are serious now. The parts cannon is loaded. Catch can in pcv line. Plugs are replaced. The new plugs fire right through the hg coolant leak for a few days. The owners raise their arms and declare mission accomplished.

The misfires return and they often start coding as misfires on random cylinders. The cannon is reloaded. A thousand dollars later the coolant reservoir finally is noticeably low.

The five stages of Gen3 Prius grief might begin.


(Old school humor attempt with manual photo text editing)

 

Loved how you edited the 5 stages of grief...