Heat is what kills diesel engines. A temp gauge climbing toward the red isn't a 'watch it' situation — it's a shut-it-down-safely situation, because a cooked head gasket or a warped head is a very expensive day.
Why it's overheating
- Low coolant from a leak — hoses, water pump, radiator, or the EGR cooler
- A stuck thermostat that won't open
- A weak water pump or a slipping fan clutch not moving air
- A plugged radiator or CAC packed with bugs and grime — no airflow
- Head gasket / EGR cooler failure — coolant going where it shouldn't
Reading a coolant leak
A puddle and a sweet smell is an external leak — a hose, the water pump weep hole, or the radiator. Coolant disappearing with no puddle is the worrying one: it's going internal, often a head gasket or (on many modern diesels) a failed EGR cooler, and you may see white smoke or coolant in the oil. Steam or a hot sweet smell in the cab points to a heater core or a hose up front.
Why you stop, not push
Running a diesel hot even for a few miles can warp the head, blow the head gasket, or crack it — turning a $150 hose into a five-figure repair. If the gauge is climbing and you can do it safely, get off the road and shut it down. 'Making the next exit' is how a cheap fix becomes an engine job.
What we do
We pressure-test the cooling system to find the leak, check the thermostat, water pump, fan clutch, radiator and CAC airflow, and — if coolant is going internal — test for a head gasket or EGR-cooler failure. We fix the cause and verify it runs at temp under load, so you're not watching the gauge on every hill.
Need cooling system diagnostics? Long Road Repair handles it in-shop and mobile across the South Puget Sound. See our cooling system diagnostics service or call and talk to a real tech.
FAQ
What should I do if my truck starts overheating?
If you can do it safely, get off the road and shut it down — don't try to 'make the next exit.' Running a diesel hot can warp or crack the head and blow the head gasket, turning a small leak into an engine job.
Where does coolant usually leak from on a semi?
Hoses, the water pump, the radiator, or the EGR cooler. An external leak leaves a puddle and a sweet smell; coolant vanishing with no puddle is going internal — often a head gasket or EGR cooler — and needs diagnosis fast.
Why is my coolant low but there's no puddle?
That usually means it's leaking internally — commonly a head gasket or a failed EGR cooler on modern diesels. You may see white smoke or coolant in the oil. Get it tested before it does more damage.
Can you find a coolant leak that only shows up when hot?
Yes — we pressure-test the system and check it under load, which reveals leaks that hide when the engine is cold and unpressurized.
Truck down? Let's get you rolling.
Book your truck or trailer in, or call and talk to a real tech.
Book a repair →Call (425) 900-6212