Quick answer: A coolant expansion tank pressure alarm means your engine’s cooling system isn’t holding the pressure it should. The most common and cheapest first check is the expansion-tank cap — a worn cap that won’t seal is a frequent cause. But pressure can also escape through a thermostat, a cooler, a hose, or — on the MAN R6-800 — a cracked, coolant-cooled turbocharger housing. A pressure test confirms the system is losing pressure; finding where it’s going is the actual diagnostic work.
Start With the Cap
The expansion-tank cap holds the cooling system at its designed pressure. When the cap’s seal wears out, the system can’t build or hold pressure and you get a low-pressure alarm. Replacing the cap is cheap and quick, so it’s always worth ruling out first. If new caps don’t clear the alarm, the pressure is escaping somewhere else.
Where a Coolant Expansion Tank Pressure Alarm Comes From
- Expansion-tank cap — a worn seal that can’t hold system pressure, the most common cause
- Coolers and heat exchangers — a breach that lets pressure or coolant escape
- Hoses and connections — a weak or loose connection bleeding pressure
- Turbocharger housing (MAN R6-800) — the exhaust housing is part of the cooling circuit, so a crack in it bleeds coolant pressure and throws this exact alarm
The One People Miss
On the MAN R6-800, the turbocharger exhaust housing is coolant-cooled. When it cracks, coolant pressure escapes through it — and the symptom at the helm is a low coolant-expansion-tank pressure alarm, with nothing obviously wrong at the usual cooling components. A shop seeing that alarm for the first time might work through thermostats, hoses and coolers for days before suspecting the turbo. Knowing the platform is what points straight to it.
A Real Example
A 44-foot Riva with MAN R6-800 engines had a low coolant-pressure alarm. The captain had already replaced the expansion-tank caps — same alarm. From experience with the R6, we pressure-tested the system, removed the turbocharger cartridge, and found cracks in the exhaust housing. Replacing the housing cleared the alarm. Read the full case study →
How We Diagnose It
We rule out the simple causes first, then pressure-test to confirm the loss and trace where it’s going — using factory diagnostics and direct inspection rather than replacing parts until the alarm stops. The difference between the causes is the difference between a cap and a turbocharger housing, so confirming the real source matters.
If You Have a Coolant Pressure Alarm
Don’t ignore a cooling-system alarm — running an engine that isn’t holding coolant pressure risks overheating and further damage. If you’ve already tried the easy fix and the alarm is still there, we can find where the pressure is actually going. A second opinion from an authorized MAN and John Deere dealer can find what was missed.
Miami: 786-717-6471 · Fort Lauderdale: 954-763-9804 · 24/7 emergency line available.
Frequently Asked Questions
What’s the first thing to check on a coolant expansion tank pressure alarm?
The expansion-tank cap. A worn cap that won’t hold pressure is a common and inexpensive cause, so it’s always worth ruling out first.
Why would a turbocharger cause a coolant alarm?
On the MAN R6-800 the turbocharger exhaust housing is part of the cooling circuit. A crack in that housing lets coolant pressure escape, which shows up at the helm as a low coolant-expansion-tank pressure alarm.
Can I keep running with this alarm?
A cooling-system alarm should never be ignored. Running an engine that isn’t holding coolant pressure risks overheating. Have it diagnosed before continued operation.
The caps didn’t fix it — now what?
If new caps didn’t clear the alarm, the pressure is escaping elsewhere — a thermostat, cooler, hose, or on the R6-800 a cracked turbocharger housing. A pressure test and inspection will find the actual source.