Forte Bio Degreaser & Cooling System Flush: every question customers actually ask

Forte Bio Degreaser & Cooling System Flush is a £20 cooling system cleaner for a car with oil in the coolant reservoir, brown or rusty coolant, sludge in the header tank, a heater blowing cold or a temperature gauge that wanders, used to clean the system out before fresh antifreeze goes in. It is safe and biodegradable and important for wet-belt engines; it is not a leak sealer, and it will not fix the head gasket or oil cooler that put the oil in the coolant in the first place.

By Phil Clements, BMW Tool RentalUpdated 16 September 2026

What is it and what does it fix?

A cooling system clean-out, done before a coolant change, so the new antifreeze goes into a clean system.

Bio Degreaser & Cooling System Flush is a cleaning treatment that removes oil, grease, sludge and contamination from the cooling system before a coolant change. It circulates with the engine running and breaks down heavy oil, grease and sludge deposits in the radiator, heater matrix, hoses and coolant passages so they can be drained away with the old coolant. It is formulated as a safe, biodegradable mixture.

The symptoms Forte list are oil and coolant cross-contamination (oil in the reservoir or expansion tank), brown or rusty coolant that points to corrosion, sludge in the header tank, cold air from the heater because the matrix is blocked, temperature fluctuations and overheating. It is also the fix for a neglected system or one where different antifreeze types have been mixed and reacted. It is a fluid treatment, so no fault code is tied to it.

The benefits are the reason it matters: it restores cooling system and heater matrix efficiency, and it stops fresh antifreeze mixing with the acidic remnants of the old coolant, so the new coolant works at full effect from day one and aluminium engine parts do not corrode early. Reviews and the order button are on the Bio Degreaser & Cooling System Flush product page.

How do I use it?

Forte's process. It sits inside a coolant change you were going to do anyway. It is often used alongside Forte Radiator Stop Leak & Cooling System Conditioner; the Forte Radiator Stop Leak & Cooling System Conditioner page explains when that pairing makes sense.

Add the product to the cooling systemDirectly into the system through the expansion tank or radiator cap, with the engine cold and the old coolant still in.
Run the engine up to normal operating temperatureLet the thermostat open so the flush circulates through the radiator, heater matrix, hoses and passages and breaks down the oil and sludge. Turn the heater to hot so the matrix gets a share.
Drain the system completelyLet it cool, then drain everything. The oil, grease and sludge the flush has loosened leave with the old coolant.
Flush with clean waterRun clean water through until it comes out clear, then drain again. This removes the last of the loosened contamination and the old coolant residue.
Refill with fresh antifreeze, then add Stop Leak if you have a weepFill with the correct fresh antifreeze and bleed as normal. If you are treating a minor coolant leak, Forte recommend cleaning with this flush first and then adding Radiator Stop Leak & Cooling System Conditioner to the fresh coolant.

When will it not help?

It cleans the system. It does not stop a leak or repair the part that caused the contamination. The Forte Radiator Stop Leak & Cooling System Conditioner page covers the companion product and when to add it.

It does not stop a coolant leak. That is a different product. Minor seeps and weeps from the radiator, hoses, gaskets or water pump are what Forte Radiator Stop Leak is for, and the recommended order is flush first, then Stop Leak. See the radiator stop leak page.
It does not fix what put the oil in the coolant. Oil in the reservoir has come from somewhere: a failed head gasket, an oil cooler or a transmission cooler in the radiator. The flush cleans up the contamination so the new coolant works; if the source is not repaired the oil comes back.
Head gasket failure. White exhaust smoke, overheating or a rapid drop in coolant level means the cooling system needs mechanical repair, not an additive. Forte are explicit that no cooling system product of theirs fixes a head gasket.
Leaving it in the system. It is a flush, used and then drained. It is not a conditioner to run in the coolant between services; that job belongs to Radiator Stop Leak & Cooling System Conditioner.

Forte Bio Degreaser & Cooling System Flush

The cooling system flush for oil in the coolant, rusty or sludged coolant and a blocked heater matrix. Add it, run to temperature, drain, water-flush and refill. Safe, biodegradable, and the flush to use before Radiator Stop Leak. Ships worldwide from the UK; UK delivery free on orders of £35 or more, postage and any duty shown at checkout for the USA, Canada, Australia, New Zealand and Ireland.

£20Free UK delivery on £35+ orders

Order Bio Degreaser Full product page

Forte Radiator Stop Leak & Cooling System Conditioner

The follow-on for a minor coolant leak. Natural fibres plug hairline cracks and pinholes in the radiator, hoses, gaskets and water pump, and it conditions the coolant against limescale and corrosion. Compatible with all MEG and OAT coolants; will not fix major cracks or a head gasket. Ships worldwide from the UK; UK delivery free on orders of £35 or more, postage and any duty shown at checkout for the USA, Canada, Australia, New Zealand and Ireland.

£20Free UK delivery on £35+ orders

Order Radiator Stop Leak Full product page

Bio Degreaser questions, in the order people ask them

There is oil in my coolant reservoir. Will this flush clean it out?

Yes, that is its first job: it breaks down heavy oil and grease in the radiator, heater matrix, hoses and passages so they drain out with the old coolant. Find and fix the source as well, because a head gasket or oil cooler that is still leaking will put the oil straight back.

What does oil in the coolant mean?

Oil and coolant are being cross-contaminated somewhere: usually a head gasket, an engine oil cooler or an automatic gearbox cooler built into the radiator. The flush deals with the mess; a pressure test or a garage finds the cause.

My coolant is brown and rusty. Is that a problem?

Brown or rusty coolant means the corrosion inhibitors have worn out and the system is corroding. Forte list it as one of the symptoms this flush is for. Flush, water-rinse and refill with fresh antifreeze so the new inhibitors go into a clean system rather than mixing with acidic old coolant.

My heater blows cold but the engine is warm. Can a flush fix a blocked heater matrix?

Cold air from the heater is on Forte's list, and restoring heater matrix efficiency is a stated benefit. The flush breaks down the sludge that blocks the matrix so it can drain away. Set the heater to hot while the engine runs to temperature so the flush circulates through the matrix.

My temperature gauge goes up and down. Would a cooling system flush help?

Temperature fluctuations and overheating are listed symptoms, because sludge and contamination stop the system moving heat properly and can upset the thermostat. If the gauge still wanders after a flush and fresh coolant, the thermostat, water pump or a head gasket needs checking.

The coolant looks like mud. Do I need this or a new radiator?

Start with the flush. Sludge in the header tank and a neglected system are exactly what Forte list it for, and it is far cheaper than a radiator. If the radiator is still not passing heat after a proper flush and water rinse, then it is blocked beyond cleaning.

I mixed two types of antifreeze and it has gone thick. Can I clean it out?

Yes. Forte name mixed antifreeze types as one of the cases the flush corrects. Add it, run to temperature, drain completely, flush with clean water until clear and refill with one correct antifreeze.

How do I use Forte Bio Degreaser and Cooling System Flush?

Add it to the cooling system, run the engine to normal operating temperature, drain the system completely, flush it through with clean water, then refill with fresh antifreeze. The full method is on the Bio Degreaser product page.

How long do I run the engine with the flush in?

Forte's instruction is to run the engine up to its normal operating temperature so the thermostat opens and the flush circulates everywhere, then drain. There is no stated long soak; the point is a full circulation at temperature.

Do I have to flush with water afterwards?

Yes. Forte's process is drain, flush with clean water, then refill. The water rinse carries out the loosened oil and sludge and the acidic old coolant so the fresh antifreeze goes into a clean system.

Can I use it before a routine coolant change even if the coolant looks fine?

Yes. It is designed for use prior to a coolant change, and the benefit Forte describe is that fresh antifreeze does not mix with the broken-down remnants of the old, so it works at full effect from day one and aluminium parts do not corrode early.

Should I use it before Radiator Stop Leak?

Yes. Forte highly recommend cleaning the system with this flush before adding Radiator Stop Leak & Cooling System Conditioner when you are treating a minor coolant leak. The stop leak then goes into fresh coolant in a clean system. Both are £20; the radiator stop leak page covers what the sealer will and will not fix.

Is it safe for aluminium radiators and engines?

Yes. It is a safe, biodegradable mixture, and one of Forte's stated benefits is preventing the accelerated corrosion of aluminium engine components that happens when new coolant mixes with old.

Is it safe for a wet-belt engine?

Yes, and Forte single out wet-belt (timing belt in oil) engines as a case where it matters. Those engines need a clean cooling system to control temperature accurately; contaminated coolant can upset thermostat operation and damage the belt. A clean system is part of looking after a 1.0 EcoBoost or 1.2 PureTech.

Can I use a normal degreaser to flush a radiator instead?

A workshop degreaser is not made to circulate in a cooling system or to be safe with its seals, plastics and aluminium. Forte's product is formulated for the job, is biodegradable, and is drained and rinsed out as part of the process.

Will it fix a head gasket?

No. No additive of Forte's fixes a head gasket. If you have white exhaust smoke, overheating or a rapid drop in coolant level, the cooling system needs mechanical repair. The flush is for cleaning the contamination up, before or after that repair.

Does it stop leaks?

No. It is a cleaner. For minor seeps and weeps from the radiator, hoses, gaskets or water pump, Forte Radiator Stop Leak & Cooling System Conditioner is the product, used after this flush.

Which antifreeze should I refill with?

The type your handbook specifies, and do not mix types. The point of the flush is that the correct antifreeze goes into a clean system and is not diluted or attacked by the old coolant's remnants.

Does it set or clear any fault codes?

No. It is a fluid treatment with no OBD fault code association. An overheating warning needs its own diagnosis.

How much is Forte Bio Degreaser and where do I buy it?

£20 with free UK delivery on orders of £35 or more from BMW Tool Rental's Bio Degreaser page, dispatched from Lichfield. With Radiator Stop Leak for a weeping system it is £40 for both.

Do you deliver outside the UK?

Yes. UK delivery is free on orders of £35 or more. Orders to the USA, Canada, Australia, New Zealand, Ireland and the rest of Europe are sent tracked from the UK, with postage and any duty shown at checkout before you pay. There is a dedicated page for US customers.