Thermostat and Cooling System Repair on the 4.3L Astro

Halfway up a long grade on a warm day, coolant temp on the live data crept to 108°C. That's my "pull over now" threshold on this engine. We nursed it home and got to work.

The suspects

On a 187,000-km 4.3, the usual list:

• Sticking thermostat
• Weak radiator cap losing pressure
• Aging hoses letting go internally
• Water pump on its way out
• Radiator gunked up

What we replaced

Thermostat + gasket. Stant OEM-spec 195°F unit. The old one was corroded and cycling slowly.

Radiator cap. 16 PSI OEM-spec. A weak cap lets coolant boil at a lower temp — an easy $15 fix people forget.

Upper and lower hoses. Both original. Squeezed one, it collapsed with a sad little noise. Replaced.

Coolant flush. Full drain, rinse, refill with 50/50 Dex-Cool. About 12 L capacity on this system.

What we inspected but left

• Water pump. No weep, no play in the shaft, no whine. Left it — for now.
• Radiator. Fins clean, no bent tubes, back-flush ran clear. Left it.

Result

Same climb, same day of year, temp settled at 94°C and stayed there. Fixed the way it should be fixed.

Cost

• Thermostat + gasket: CA$28
• Radiator cap: CA$15
• Upper + lower hoses: CA$48
• Coolant (2 gallons): CA$65
• Total: CA$156, one afternoon.

Logged in our Service History.

The lesson

Cooling systems on high-mileage vans are a package deal. Don't just do the thermostat and call it done. Do the cap and the hoses at the same time — every one of them is a $15 part that could ruin a $6,000 engine.

What's next

With the mechanicals sorted, we started on the interior. Chapter 9 — Camper Build 1.0.