What are the signs of a worn impeller, a bad thermostat or a failing circulation pump?

Watch for overheating, especially at idle, a weak stream of water at the exhaust or telltale, swinging temperature readings or alarms. A worn impeller or scored pump housing moves too little water at low rpm; a stuck thermostat makes the engine run hot (stuck closed) or never warm up (stuck open); a failing circulation pump may leak at the weep hole or make bearing noise.

Can you test the circulation pump and thermostat without replacing parts right away?

Yes. We perform full diagnostic testing before recommending replacement. This includes temperature verification, flow testing, thermostat opening temperature checks, and inspection for leaks or internal wear. Only faulty components are replaced, helping you avoid unnecessary costs.

What happens if I keep running the engine with a bad thermostat or pump?

Continuing to operate the engine can lead to severe overheating, warped cylinder heads, blown head gaskets, or internal engine damage. Early testing and repair of the circulation pump or thermostat can prevent major and expensive engine failures.

Do you test both raw water and closed-loop cooling systems?

Yes. We service both raw water–cooled and closed-loop (freshwater) cooling systems. We verify proper coolant circulation, thermostat operation, hose condition, and overall cooling efficiency based on your engine type and manufacturer specifications.

How long does an impeller and thermostat service take?

An impeller and thermostat service usually takes 2-4 hours of labor, depending on engine access. On sterndrives with the pump inside the drive, the drive has to come off, so the boat must be out of the water. Labor is $150 an hour plus the trip fee for work at your boat, and you get a written estimate before any work starts.

Do you replace marine thermostats and circulation pumps if needed?

Yes. We replace marine-grade thermostats and circulation pumps using OEM or high-quality equivalent parts. All installations follow manufacturer torque and sealing specifications to ensure proper cooling system performance and long service life.

How often should the raw water impeller be replaced?

We recommend a new impeller once a year or every 100 hours, whichever comes first, even if the boat sat unused. The rubber vanes take a set and crack with age, and broken pieces can block the heat exchanger.

Marine technician using an infrared laser thermometer to check the temperature of a thermostat

Marine technician using an infrared laser thermometer to check the temperature of a thermostat (2)

Cooling System: Impeller & Thermostat Service

Overheating is one of the most common causes of serious marine engine damage, so cooling checks belong in every marine engine service. We replace worn rubber impellers and test thermostats so your engine runs at its normal operating temperature.

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$150 /hr

Labor estimate Usually 2–4 hours

Book sends us a ticket; nothing is charged online. Before any work you get a written estimate for labor, parts and the trip fee, and nothing is done without your OK.

Trip fee By distance, one way: $30 in San Pedro, $50 in Long Beach

San Pedro$30
Wilmington, Terminal Island$40
Long Beach: Shoreline, Rainbow Harbor$50
Long Beach Marina, Alamitos Bay$60
Redondo Beach$60
Seal Beach, Huntington Harbour$75
Marina del Rey$85
Newport Beach$110

Farther away: $25 plus $2.50 a mile, one way from our San Pedro shop. No trip fee when the boat comes to our shop, or for plan members. The work of a visit is at least $150; the trip fee is on top.

Registered in California: Breeze Marine Boat Service LLC, San Pedro
Where we work: At your slip from Marina del Rey to Newport Beach, or in our San Pedro shop
Questions first: (213) 693-0224 or submit a ticket

About this service

Prevent Overheating

Stay Cool
Under Pressure.

Heat is the enemy. A small rubber impeller is often all that stands between your engine and serious overheating damage. We service the whole cooling system to keep the water flowing and the temperature steady.

Impeller Service

The "heart" of your raw water pump. Rubber impellers dry rot and lose vanes even when sitting still. We replace them annually to prevent broken rubber bits from clogging your heat exchanger.

Thermostat Testing

A stuck thermostat can cause rapid marine engine overheating (closed) or prevent the engine from reaching operating temp (open). We remove and test them in a heated bath to verify opening temperatures.

Circulation Pump

The engine's internal water pump. We inspect the weep hole for leaks and listen for bearing noise using a stethoscope to catch failures before the pump seizes and throws the belt.

The Cool Down Protocol.

Verification before replacement.

Infrared Scan

We run the engine and use a laser pyrometer to measure temperatures at the thermostat housing, risers, and manifolds to identify hot spots.

Pump Inspection

During raw water pump service, we open the pump. If the impeller vanes are set (curved permanently) or cracked, we replace it. If the housing or wear plate is scored, we replace it.

Thermostat Check

We inspect the housing for salt corrosion (common in saltwater boats) which can jam the thermostat. We install new gaskets and seals.

Flow Verification

After assembly, we verify water flow at the exhaust. A strong, steady stream indicates a healthy pump and clear passages.

Vital Components.

Keeping the flow moving.

Worn impeller with broken vanes next to a new impeller

Cleaning salt scale from a marine engine thermostat housing

Infrared laser temperature check on a marine engine thermostat housing

Service Intervals.

Typical intervals; your engine maker's schedule comes first.

Raw Water Impeller Every 1 Year / 100 Hours

Thermostat Every 3 Years / 300 Hours

Coolant Flush Every 2 Years (Closed Cooling)

Common Questions

My engine gets hot at idle but cools down when running. Why?

Boat overheating at idle is the classic sign of a worn impeller or a scored pump housing. The pump can't move enough water at low RPMs but catches up at high RPMs. Have it serviced before the next trip.

What happens if an impeller breaks?

The missing rubber pieces travel downstream and get stuck in the heat exchanger or oil cooler, blocking flow. We have to "go fishing" to find all the pieces, which adds labor.