What is a marine gearbox cooler and why does it need cleaning?

A marine gearbox cooler regulates transmission oil temperature using seawater. Over time, salt, debris, and marine growth can restrict flow, leading to overheating. Regular cleaning ensures proper cooling and protects the gearbox.

What problems can a clogged gearbox cooler cause?

A clogged cooler can cause high transmission temperatures, slipping gears, contaminated oil, and premature gearbox failure. Early cleaning helps avoid expensive transmission repairs.

What does gearbox cooler cleaning include?

The service includes removing the cooler, descaling the raw-water passages, pressure testing it for internal leaks, inspecting the fittings and reinstalling it with new crush washers and clamps.

Why is pressure testing the gearbox cooler important?

Pressure testing checks for internal leaks that could allow seawater into the transmission oil. Detecting leaks early prevents oil contamination and serious gearbox damage.

How often should a gearbox cooler be cleaned and tested?

Follow your engine and gearbox maker's schedule. In salt water, have the cooler checked at least once a year, and cleaned and pressure tested sooner if the transmission runs hot or the oil looks milky or dark.

Can this service extend the life of my marine transmission?

It helps. Clean cooling passages and a leak-free cooler reduce heat stress and the risk of water in the oil, two common causes of early gearbox failure.

Do you inspect transmission oil during the service?

Absolutely. We inspect transmission oil for signs of water intrusion or overheating and provide recommendations if fluid replacement or further service is needed.

Breeze Marine technician performing a hydrostatic pressure test

Breeze Marine technician performing a hydrostatic pressure test (2)

Marine Gearbox Cooler Cleaning & Pressure Test

Heat kills transmissions. We remove your gearbox oil cooler, acid flush the raw water channels, and pressure test it with air under water to check that the water and oil sides do not leak into each other.

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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

Transmission Protection

Cool Under
Pressure.

Your gearbox cooler is a small part that protects a costly transmission. Neglecting it can lead to overheating or water in the transmission fluid. We test whether it is sound.

Heat Kills Clutches

Marine transmissions rely on friction plates. If your cooler is clogged with scale or debris, fluid temperatures spike, glazing the plates and leading to transmission slipping or total failure.

The "Strawberry Milkshake"

If the internal tubes corrode through, seawater can get into the transmission fluid, often while the engine is stopped, turning it into a pink sludge that quickly damages the gearbox.

Pressure Testing

Cleaning isn't enough. After marine transmission cooler cleaning, we bench-test every cooler under pressure to check that the internal tubes are sound and do not leak between the oil and water sides.

Bench Service Protocol.

From clogged and risky to clean and pressure-tested.

Remove & Inspect

We extract the cooler from the engine, capping hydraulic lines to prevent leaks, and inspect the end caps for corrosion.

Acid Flush

We circulate a descaling solution (like Barnacle Buster) through the raw water side to dissolve calcium, salt, and marine growth.

Pressure Test

We cap the water ports and pressurize the oil side (or vice versa) with air, submerging the unit to check for even the tiniest bubbles.

Paint & Install

Once passed, we prime and paint the exterior (if metal) and reinstall with new crush washers and hose clamps.

System Integrity.

A small service that protects a costly gearbox.

Pressure testing a marine gearbox oil cooler

Gearbox cooler core before and after cleaning

Reassembling cooler lines on a marine transmission

Gearboxes Serviced.

We service coolers on these marine transmissions, from a Velvet Drive cooler to ZF and Twin Disc units.

Velvet Drive 71 / 72 / 5000 Series

ZF Marine Hurth & ZF Geared Units

Twin Disc Large Capacity Coolers

Common Questions

How do I know if it's clogged?

You often won't know until you notice boat transmission overheating or poor shifting. If the cooler inlet hose is cold but the outlet is hot, flow is restricted.

Can you repair a leaking cooler?

Generally, no. If the internal copper/nickel tubes are perforated, the cooler must be replaced. A patch repair is too risky for the transmission.

Save Your Trans.

Don't wait for the "milkshake." Test it now.

Service My Cooler

Cooler Service vs Marine Transmission Repair

Many boat gearbox repairs trace back to the cooler: scale restricts water flow, the oil runs hot and the clutch plates glaze. A cooler that fails the pressure test can also let seawater into the oil. Cleaning and testing the cooler is a bench job, while marine transmission repair after overheating or water contamination means taking the gearbox apart. If shifting feels slow or the fluid looks darker or milky, have the cooler checked before the damage reaches the clutches.