Timing belt in oil: which oil for PureTech and EcoBoost engines?

We are used to the timing belt living under a plastic cover at the side of the engine and having nothing to do with oil: if oil turns up on the belt, look for the leak before it eats the belt away. That is how most engines were built, the ones more than one generation of car owners grew up with, from the Lada Samara to diesel Volkswagens. But that does not mean yours is built the same way: many European cars made after 2010 have engines in which the timing belt runs right in the oil — and for them oil matters far more than usual.

The best-known examples are the 1.2 PureTech, fitted to Peugeot, Citroën, Opel and DS, and Ford’s 1.0 EcoBoost; some VW and Toyota engines have a belt in oil too. Plenty of these cars reach Ukraine second-hand from the rest of Europe, and sellers at the used-car market as a rule keep quiet about this feature. Often they do not know about it themselves.

What a “wet” belt is

A “wet” belt is a toothed timing belt that runs inside the engine and is constantly bathed in engine oil. Its material is special, but it is designed for oil of a particular make-up — the oil the engine maker tested it with, not whatever liquid happens to be in the sump.

A rubber boot serves for years in water, because that is what it was made for; stand in it for an hour in solvent and see what is left of it. The belt is the same: in “its own” oil it lasts its full term, in “someone else’s” it gives up much sooner.

Why the designers went for this solution is a question for them. What matters to us is something else: these are mostly small petrol engines, often turbocharged, with direct injection, and it is exactly these engines that suffer most from petrol in the oil.

What kills it

There are essentially three causes, and all of them are about oil.

1. The wrong oil. A different base, a different additive package, a different viscosity — and the belt material may start to swell. The oil the seller called “universal, for all engines” may turn out to be that very solvent for this one.

2. Fuel in the oil. On short trips an engine like this does not have time to warm up, and some of the petrol finds its way from the cylinder walls into the sump. In one laboratory test of such an engine, fuel made up as much as 9% of the oil, and its viscosity dropped noticeably. That mixture is no longer the oil the belt designers were counting on.

3. An overdue change. The longer the oil works, the more fuel, moisture and combustion products it carries, and the less protection it gives.

What happens next? The belt swells and starts to delaminate, small bits of rubber sink into the sump and clog the oil pick-up strainer. The oil pump can no longer move the oil, the pressure drops, and the engine is left without lubrication — with everything that follows, up to a full rebuild. Do not smile — a belt you have never seen can kill the engine without even snapping.

Which oil it needs

I will say it once more: oil is chosen by the car manufacturer’s requirement — approval, class, viscosity — and in exactly that order. How to find that requirement is explained in the article on the manufacturer’s recommendations. And for an engine with a belt in oil, the approval is also a question of whether the belt will survive.

For the 1.2 PureTech the documentation shows several families of approvals — the former PSA ones and the current Stellantis ones that replaced them:

PSA B71 2312 — a 0W-30 oil; its place in the new system has been taken by the Stellantis approval FPW 9.55535/02.

PSA B71 2010 — a 0W-20 oil with low sulphated ash, phosphorus and sulphur (for engines with a particulate filter); it was succeeded by FPW 9.55535/01.

FPW 9.55535/03 — a 5W-30 oil, roughly equivalent to the former PSA B71 2290.

Which of them your engine needs depends on its generation and year of manufacture. And for some of these engines the manufacturer later revised the requirements, so where 0W-30 was once allowed, the current maintenance plan may already say 0W-20. That is why I will not give you one “correct” approval “for the PureTech” — it simply does not exist. What decides it is the service book and the current Stellantis schedule for your engine code; when in doubt, the brand’s official dealer will name it from the VIN.

A separate word for Opel owners: if there is a 1.2 PureTech under the bonnet, the dexos approval you know from Opel has nothing to do with it — these engines require PSA/Stellantis approvals (more on that in the article on Opel without GM).

For the 1.0 EcoBoost with a belt in oil, Ford at the time introduced a separate approval, WSS-M2C948-B (a 5W-20 oil). What exactly is specified for your engine — again, look in the service book. And make sure your car really has a belt: in newer versions some of the engines got a chain.

One more thing. “Meets B71 2312 requirements” or “recommended for…” on the can is a statement by the oil maker itself, not an approval. The difference between the two has already been covered in the article what oil approvals are. Here you are better off with an oil the car manufacturer has tested itself and put on its own list.

Change interval and advice for owners

The oil change interval in the service book is a maximum, not the norm, and with these engines it shows especially. If you drive mostly in town, in short hops and traffic jams, petrol builds up in the oil faster, and it is worth changing it more often. More on that in the article on the factors that shorten the service interval. The belt is not eternal either: for some PureTech engines the manufacturer has also revised its replacement schedule, so look at the current plan, not just the old entry in the book.

What else is worth doing:

1. When buying a used car, find out what engine is under the bonnet, when the belt was changed and which oil went into it. No answer — budget for a new belt and an oil change in the purchase price.

2. At the service station, name the approval you need and look at the can they are pouring from. “Decent synthetic, we put it in everyone’s car” is no answer for this engine.

3. If the belt has already been replaced because it delaminated, ask whether they cleaned the sump and the pick-up strainer. A new belt will not save you from bits of the old one left in the sump.

4. Oil pressure warning light on — switch the engine off straight away, do not limp on “to the garage”.

5. No additives. An oil with an approval has passed the car manufacturer’s tests for exactly this engine; what yet another “miracle” additive will do to the belt nobody knows, its seller included.

A belt in oil does not forgive what an ordinary engine puts up with for years: the wrong oil, petrol in the sump and an overdue change. Put in what is specified for your engine, change the oil on time — and the belt will have every chance of lasting its full term.

Further reading:

• LSPI: why does a small turbo engine need a modern oil?
• Oil change interval today: Longlife, the indicator and short trips