# Diesel Engine Thermostat Replacement in Upland > Diesel slow to warm up or running hot? Route 66 Auto Repair in Upland tests the thermostat with scan data before replacing it and bleeds the system right. Source: https://route66autorepairshop.com/diesel-engine-services/diesel-engine-thermostat-replacement Updated: 2026-09-17 Business: Route 66 Auto Repair, 1775 W Foothill Blvd, Upland, CA 91786, (909) 608-7699 Hours: Monday to Friday 8 AM–5 PM, Saturday 8 AM–2 PM, Sunday closed **TLDR:** A thermostat stuck open wastes fuel and heat; one **stuck closed** overheats the engine. We **confirm it with live data** before replacing it. A diesel thermostat is the valve that holds coolant in the engine until it warms up, then opens to send it to the radiator. Route 66 Auto Repair in Upland replaces stuck or lazy thermostats on diesel pickups and vans after confirming the fault with live temperature data. Many diesels use two thermostats, and some housings sit under other parts, so our ASE-certified technicians explain access and parts in a written estimate before starting. ## How do you know a diesel thermostat is failing? - Gauge that stays low on the highway, even after twenty minutes - Weak cab heat on a winter morning in the foothills - Temperature that spikes quickly, then drops when the thermostat finally pops open - Poor fuel economy and extra smoke from an engine that never fully warms - A coolant temperature code on the scan tool ## Stuck open versus stuck closed Stuck open is the common failure on a diesel. The engine runs cool, the heater is weak, and the engine computer keeps adding fuel as if it were still cold. It is annoying, not an emergency, though long-term it can build soot. Stuck closed is the one that hurts. Coolant cannot reach the radiator, so heat climbs fast, especially towing up the Cajon Pass. If the gauge rises quickly, pull over. For the wider system, see [diesel engine cooling system repair](https://route66autorepairshop.com/diesel-engine-services/diesel-engine-cooling-system-repair). ## Our diesel thermostat replacement process 1. **Watch it warm:** We log coolant temperature from a cold start and compare the opening point to specification. 2. **Rule out look-alikes:** A bad temperature sensor or low coolant can mimic a thermostat, so both are checked. 3. **Replace the thermostat:** The housing is opened, the old thermostat and seal come out, and the new parts are installed with the correct orientation. 4. **Fill and purge air:** Coolant is topped up with the type your engine specifies and trapped air is bled out before a road test. ## What's included in diesel thermostat replacement - Cold start temperature log - Temperature sensor reading check - New thermostat or thermostats and housing seal - Housing surface cleaning - Coolant top-off and air purge - Road test to full operating temperature ## Is it a need-now or need-later repair? **Need now:** - Rapid temperature climb or overheating - No heat at all plus a check engine light in cold weather **Need later:** - Engine runs a little cool on the highway - Weaker heater output with a normal gauge in town ## How pricing and timing work Some diesel thermostats sit right on top of the engine. Others need the intake tube, alternator or fan shroud removed first. That access, plus how much coolant must be drained, sets the labor. If the coolant is dirty, we will say so and you can decide on a [radiator flush](https://route66autorepairshop.com/cooling-heating-ac-services/radiator-flush) while it is apart. The written estimate comes first. ## Aftercare The gauge should now settle in its normal spot and stay there. Check the coolant cold after a few drives, since small air pockets can work out. Weak heat after a fresh thermostat usually means air, and a quick [coolant system bleeding](https://route66autorepairshop.com/cooling-heating-ac-services/coolant-system-bleeding) clears it. ## Frequently asked questions ### Should I replace both thermostats on a dual-thermostat diesel? Usually, yes. They age together, and the labor to reach them overlaps. We tell you if your engine has two and quote both. ### Can I just remove the thermostat to stop overheating? No. Without it the engine runs too cool, burns more fuel and builds soot. Some engines also overheat without it because coolant flow is wrong. ### Is the gas-engine thermostat job the same? The idea is the same; the parts and access differ. Our [thermostat replacement](https://route66autorepairshop.com/cooling-heating-ac-services/thermostat-replacement) page covers cars and gas trucks. --- Call (909) 608-7699 or visit https://route66autorepairshop.com/contact for a written estimate.