What is a wet belt, or a belt in oil system (BIO)?

The timing belt explained

Timing belts, which run within the engine oil, made their first appearance around 20 years ago. They have grown in popularity since and many common cars are fitted with Belt in Oil (BIO) systems. Yet, they are causing owners headaches. Should you be affected, this blog may prove useful.

Introduction to timing belts and their purpose

An engine timing belt runs on a series of engine pulleys. Its main purpose is to keep the crankshaft and camshaft(s) synchronised. Should the belt fail and a pulley rotates independently, engine pistons and valves are very likely to collide. The result is a ruined engine that may even be unrepairable.

Most timing belts run dry and are positioned outside the engine crankcase, within a dedicated cover. This facilitates periodic replacement, dependent on the mileage and time intervals that are stipulated by the car manufacturer's service schedule. However, to improve fuel economy and carbon dioxide exhaust emissions, this situation is changing...

What is a wet belt?

The Belt In Oil (BIO) system transfers the timing belt from outside to inside the engine's crankcase, where it is lubricated by engine oil.

The impact of BIO timing belts on engine efficiency and emissions

Compared with those outside the crankcase, BIO belt systems reduce drag by 30%. This provides CO2 emission benefits for the carmaker and performance/fuel

economy benefits for the driver. The BIO drive is also lighter and quieter than equivalent chain drives.

Which cars are fitted with wet belts?

The Ford Motor Company and the brands incorporated within the PSA Group (now Stellantis), especially Peugeot and Citroën, tend to be the main users of BIO technology, at the time of writing. Some Toyotas, which employ PSA/Stellantis engines, are equipped also with wet belts. Volkswagen Group cars rely on a single wet belt for the oil pump drive. Should you be unsure whether, or not, a car has a BIO, the belt tends to be visible through the oil filler, once the cap is removed.

What materials are used in timing belts, designed for oil baths?

Dry timing belts are attacked by engine oil. As BIO belts reside inside the engine, they are constructed with special oil and heat-resistant materials. These include Hydrogenated Nitrile Rubber, Aramid fibres and PTFE. We understand that the formulas continue to evolve.

Are BIO timing belts more reliable than traditional timing belts or chains?

GEM has come across instances of engine timing chains and 'dry' timing belts failing before their official replacement intervals. More recently, reports are surfacing about modern BIO belts not lasting as long as their manufacturers envisaged, too.

Belt In Oil (BIO) timing belts were introduced first on Ford's 1.8TDCi diesel engine in the late 2000s. Earlier iterations of this unit used a duplex chain assembly, which lasted the engine's lifetime. Yet, this is not the case with the belt, at least, on that engine.

Belt in oil timing belt reliability

Not only have we heard cases of belts failing prematurely but also some car manufacturers have even reduced their replacement intervals. Reports indicate that Stallantis's PureTech's original BIO replacement schedule of 120,000 miles/ten years has been cut to 60,000 miles/six years, with an annual inspection added, requiring a special tool.

What problems can occur with BIO timing belts?

Common problems with BIO timing belts include swelling, delamination and tooth shredding. All of these parts end up trapped within the engine and must be removed manually.

Can a BIO timing belt cause oil pump damage?

While a failed BIO timing belt can have the same ramifications as a failed dry timing belt, in that engine damage is almost certain, a BIO can cause more harm, even before it fails.

Fragments from the BIO belt are known to break off and find their way into the oil pump. This can cause a serious loss of oil pressure and potential turbocharger and engine seizure.

A flickering oil light, or a similar low oil pressure warning, is an obvious warning sign. Yet, there is also a safety problem. On some models, particles from the BIO belt can reduce the vacuum pump's effectiveness, resulting in a potentially dangerous loss of braking assistance.

Our next technical blog on BIO timing belts looks in closer detail at how you can avoid expensive engine damage.

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