NHC MOTORS · SERVICE RECORD

2015 Bentley: Over 4 L of Engine Oil in the Cooling System, Oil Cooler Replacement and Flush

A 2015 Bentley in which an internal fault in the engine oil cooler let more than 4 L of engine oil into the cooling system. We lowered the engine to replace the oil cooler, then flushed the cooling system and cleaned the expansion tank.

Engine and drivetrain lowered out of the body, with surrounding lines

2015 Bentley

If you see brown oil in the coolant expansion tank, is cleaning or replacing the tank enough? First you need to find where the oil is coming from and how far it has spread through the cooling system.

In this 2015 Bentley, an internal fault in the engine oil cooler had let more than 4 L of engine oil into the cooling system. To reach the cooler, we lowered the engine out of the body and replaced the oil cooler, and in the same job we flushed the cooling system and cleaned the expansion tank.

01 / NHC MOTORS

The brown contamination in the expansion tank was inside the lines too

The oil was not confined to the expansion tank. Brown deposits coated the tank's inner walls and internal parts, and light-brown contaminated fluid ran out of the open end of a disconnected line. This means the oil had traveled along the coolant path and spread beyond the tank.

Cleaning only the expansion tank, the part you can easily see, would leave oil clinging to the lines and other components. So we planned the job in two parts: fixing the point where the oil was crossing over, and removing the oil that had already spread.

Brown deposits on the expansion tank walls and internal parts

Light-brown contaminated fluid running out of a disconnected line

02 / NHC MOTORS

The cause was inside the engine oil cooler, and we replaced the cooler

The engine oil cooler cools the engine oil by passing its heat into the coolant. Inside the cooler, oil and coolant flow separately on either side of internal partitions; if something fails inside, the two fluids can mix. In this Bentley, an internal fault in the cooler had let engine oil pass into the cooling system.

With the cooler removed, brown contamination was visible in the passages and on the mating surface of the engine-side mounting, and in the cooler's own passages. The cooler was replaced with a new part. Cleaning without fixing the cause would let oil cross over again; replacing the cooler without cleaning would leave the oil that had already spread in the system. That is why we did both in one job.

Engine-side cooler mounting with the cooler removed; brown contamination in the passages and on the mating surface

Contamination left in the passages of the removed oil cooler

03 / NHC MOTORS

Reaching the cooler meant lowering the engine

Because of where this cooler is mounted, a wide range of surrounding components has to come off before it can be reached. We removed the front end to expose the area around the cooling components, then lowered the engine and drivetrain out of the body to get at the cooler and its surrounding lines.

Because a single access to this part takes this much disassembly, NHC's view is that on this model, if the engine is coming out for another job, a preventive oil cooler replacement is worth considering.

Front end removed for access to the cooling components

Front components exposed after lowering the engine

04 / NHC MOTORS

More than 4 L of oil took considerable work to drain and flush

More than 4 L of engine oil had mixed into the cooling system. The fluid drained from beneath the engine was a murky brown, with contamination floating on the surface.

Draining the coolant alone may not remove all the oil clinging to the line walls and the inside of the tank. So we flushed the cooling system, and removed the expansion tank to clean it separately on cleaning equipment. With this much oil mixed in, it was not an easy clean.

Brown fluid drained from beneath the engine

Removed expansion tank being cleaned on cleaning equipment

05 / NHC MOTORS

After reassembly, the coolant still needs watching

We reassembled the area around the new cooler and reconnected the hoses and lines around the engine. Because the engine had been lowered and refitted, it matters that every connection that was separated is refastened in its proper place. After the cooling system was flushed, new coolant was used.

After a repair like this, please watch three things as you drive: whether the coolant level drops, whether any connection is leaking, and whether an oil film or brown contamination reappears on the surface in the expansion tank. Oil left deep in the system can surface later. If oil does reappear, we will check whether it is leftover contamination coming up or new oil crossing over.

Reassembly and fastening around the cooler

Hoses and lines around the engine, reconnected

New coolant and product containers prepared for the job

How oil crossed into the coolant, and the order of the fix

Normal

Engine oil and coolant flow separately inside the cooler, divided by partitions

This fault

Internal cooler fault let engine oil into the cooling system · more than 4 L

Fixing the cause

Engine lowered, oil cooler replaced

Removing oil that had spread

Cooling system flushed · expansion tank removed and cleaned

Replacing the cooler stops new oil from getting in; flushing the system and cleaning the tank remove the oil that had already spread.

Frequently asked questions

If there is oil in the coolant reservoir, can I just clean or replace the reservoir?

First we need to look at what let the oil in and how far it has spread. In this Bentley, an internal fault in the engine oil cooler had let oil into the cooling system, and contamination also came out of lines away from the tank. So we replaced the oil cooler and flushed the system and cleaned the tank in the same job.

Does the cooling system still need flushing after the oil cooler is replaced?

Yes. Even with a new cooler, oil that has already spread can stay in the lines. This car had more than 4 L mixed in, so we flushed the system and cleaned the expansion tank as well. Afterwards, keep an eye on the coolant level and whether an oil film reappears.

Do I have to replace the oil cooler whenever the engine comes out?

It is not necessary on every car. This Bentley's cooler was replaced because an internal fault in it was the cause. If the engine is coming out for other work, NHC's view is that preventive replacement is worth considering on this model. We explain the current condition of the part, the service history and how much disassembly it would take to reach it again, and decide with you.

If you see oil in your coolant, we start with the cause and how far it has spread

If you see an oil film or brown contamination in the expansion tank, we will check separately where the oil is coming from and how far it has spread through the cooling system. Then we will explain the repair to the faulty part and how far the flush needs to go. If the job requires lowering the engine, we will discuss in advance which other parts are worth looking at while it is out. When you book, please tell us when you first noticed the oil and whether you have topped up the coolant.

We explain the work and estimate before proceeding with your consent.

Call for service advice Book via Naver

031-258-1617 · NHC Motors, Suwon · By appointment

Published 2026-09-27