Overheat at interstate speeds after numerous new parts & HG

2014 Prius
236k miles
iCarsoft CR v3.0 diagnostic tool

I have a mild overheat issue at highway speeds and hills. Around town with the A/C on, the temperature stays below 205F with the radiator fans on low speed from the A/C signal. At 60 mph plus, the temperature sometimes hits 205F and the diagnostic tool shows the fan going to high speed. On the interstate at 80 mph at 75F ambient, the temperature is around 210F and spikes to 220+ when the cruise control accelerates going up mild Florida panhandle hills on I-10. I am perplexed.

Work done:

2023, 190k miles – Replaced water pump impeller on original pump when I had intermittent overheat at highway speeds. Also cleaned the EGR cooler and manifold, and replaced the thermostat, coolant temperature sensor, and spark plugs.

Aug, Sept 2025 – 800 mile round-trip on flat, Florida roads with a light trailer, no issues, no spitting coolant, but did not have diagnostic tool attached for real time temperatures.

Nov 2025, ~233k miles – Started seeing slight disappearance of coolant. Suspected head gasket, but no combustion gases in cooling system. Found light coolant seepage on radiator between fin core and side tank. Replaced radiator (Nissens) and radiator hoses (Gates, Dayco).

Feb-Mar, 2026, 235k miles – Continued to have disappearing coolant and worsened to an intermittent stumble on start, progressing to more regular stumbling and misfire codes on #1. Borescope in #1 cylinder showed a glistening, steam-cleaned piston and cruddy, carbon crusted #2-4. Replaced head gasket with Felpro. Replaced water pump impeller and thermostat (Motorad) and cleaned EGR cooler and manifold. Also new spark plugs and coils.

Early Apr 2026 – No cooling issues or lost coolant in town, no stumbling. Attempted interstate road trip, but found coolant on and around the reservoir at highway speeds. Replaced reservoir and cap (Dorman) out of desperation to try to continue the trip, but still spitting coolant at highway speeds.

Recent – Test drives with diagnostic tool with real-time data at 80 mph on I-10. Temperature spikes to low 220+F and spits small amounts of coolant from reservoir at 80 mph and hills when cruise control accelerates, water pump rpms high 4k to low 5k with actual slightly higher than commanded, fan to high speed at 205F. Replaced water pump (Autozone Duralast) with no change in behavior at speed. Research showed possible thermostatic control valve problem on the exhaust heat exchanger. Bypassed exhaust heat exchanger down near the exhaust. Still no change in behavior at speed at 75F ambient, with the exception that the high temperature recovers to below 200F when slowing down much, much quicker than with the exhaust heat exchanger connected.

Never had coolant system sealant in it and the antifreeze is fresh and proper mix. System purged of air per manual after refills.

To recap, new radiator, radiator hoses, thermostat, head gasket, water pump, reservoir and cap, bypassed exhaust heat exchanger, still have temperature spikes to low 220+F and spitting coolant from the reservoir at 80 mph and hills. Also had a hybrid system fault and a weak 12VDC battery, and the battery was replaced between the water pump replacement and the exhaust heat exchanger bypass.

I never hooked up the diagnostic tool and ran at interstate speeds prior to this month to check real time temperatures when initially looking for disappearing coolant, and from what I recall, I wasn’t spitting coolant. I am perplexed. I feel like I’m looking at either having messed up something during the head gasket repair (but what??) or I have a defective radiator or thermostat. If anyone has ideas, I am more than game to listen.

 

I watched the original water pump speed with the new impeller and it was about 5000 rpm. Same speed as with the aftermarket pump with its impeller. Unless the new radiator or thermostat are defective out of the box, there's no nothing to block coolant flow. I cleaned the condenser when I replaced the radiator a few months ago, so it's clean.

There are threads discussing the issue with the original water pump being the impeller, not the electric motor. The impeller fixed my problem a few years ago as it had cracked and swollen. The car has done a couple 3000 mile road trips and a number of 1000 mile since then.
P0117, intermittent overheat, and water pump rotor

 

The water pump was not the problem as the commanded and target speeds for both the original with aftermarket impeller and the Duralast were very similar at similar temperatures and loads. Perhaps the longevity of the Duralast will bite me, but a)I kept the original, and b)there's the Autozone lifetime warranty.

No codes since the head gasket misfire codes. None for cat or O2 sensors.

I've used Motorad thermostats in several other cars, including an RV and a race car without incident. I did order an Aisin in case I get desperate and feel like lighting another $40 in fire.

Thanks for the diagram and the info on the exhaust heat exchanger. It's been difficult finding information on it. I'll check that in the morning.

 

Previous owner only had it to 135k miles and was a helicopter pilot and reasonably well off, so I don't think he was shadetreeing the cooling system with gunk. And with the EGR cooler clean out a few years ago and the recent radiator, etc work, I've seen no evidence of gunk.

 

Condenser is clean. I was hoping to see a plastic shopping bag stuck against it, but no luck.

 

Fixed! Removed the thermostat, water pump, and radiator hoses and reverse flushed the system. No debris caught in the thermostat, water pump, or block, and none noted from the radiator inlet, though hard to monitor. Decided to burn the cost of the OEM Aisin thermostat since the internals looked different between it and the aftermarket Motorad.

I could immediately tell before getting to the interstate for high speed runs that the Aisin thermostat had fixed it. Coolant temps at 60 were lower, as were water pump speeds. Cruising on flat ground at 80 mph and 75F ambient, the temperature was about 190-194F (was 205-210) and the water pump speed was about 2600 rpm, half(!) of previously. On hills, the temperature went down because the water pump would increase rpms and the system was allowing the flow to increase cooling. No more 220+ and coolant reservoir spitting.

I'm going to reattach the exhaust heat exchanger and call it good.

Also, here's a more complete diagram of the exhaust heat exchanger.

 

Agreed. I've been working on cars since 1976, including a number of years professionally, and I have very rarely been burned by bad parts. Certainly not right out of the box. After some googling, it looks like Motorad, a company established in 1958 and was an OEM supplier, was acquired by a private equity firm in 2015. We've heard that story before.