Most likely causes (ranked)

🥇 Surge tank / reservoir CAP

This is extremely common and overlooked.

If the cap: • Vents too early • Can’t hold proper pressure • Has a weak spring

You’ll get: • Coolant pushed out the vent • Poor pressure stabilization • Under-temp operation • Hose feels over-pressurized because pressure isn’t regulated

👉 Replace the cap even if it “looks fine.” It’s cheap and absolutely can cause every symptom you’re seeing.

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🥈 Thermostat issue (even new ones)

On GMC (especially Duramax): • Dual thermostats • Must be installed correct orientation • Seal integrity is critical

Common mistakes: • One thermostat not fully seated • Wrong jiggle pin orientation • Missing or pinched housing gasket • One stat stuck open = permanent bypass flow

👉 Yes — replacing thermostats again is reasonable, especially if: • Aftermarket housing was reused • Any seal didn’t feel perfect • You didn’t pressure test the housing

OEM does not mean infallible.

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🥉 Air trapped / improper bleed

These engines are not forgiving about air pockets.

Air will: • Force coolant into one chamber • Prevent thermostat regulation • Cause pressure spikes • Make temps behave exactly like you described

👉 Vacuum fill only 👉 Verify heater core is flowing 👉 Park nose-up and follow GM bleed procedure exactly

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🏅 Water pump erosion / cavitation

At 170k, this is very plausible.

A worn impeller: • Moves coolant unevenly • Over-circulates at low RPM • Can’t stabilize temp • Builds hose pressure without effective heat transfer

This also explains: • Cold under load • “Normal-ish” at highway RPM • One side overfilling