Why So Many People Hate Wet Timing Belts

Timing belts are a part we love to hate. They put a very real deadline on the life of your engine unless you spend many hours changing the timing belt; fail to do that in time and your machine is likely dead. But if you thought regular timing belts were bad, you haven’t met the disastrous technology of wet timing belts. They’re the ire of Europe, in particular, and they’ve won many enemies over the last two decades.

I’d never heard of wet timing belts until I came across this video on YouTube. What I found shocked me. The concept is simple enough: just build an engine where the crank pulley for the timing belt is actually in the oil bath of the sump. It’ll get nicely lubricated and run smoother and quieter, with a minor improvement to efficiency, which everybody loves. All good, right?

Well, reality has proven that’s not always the case. Wet belts have become a bugbear for many owners and mechanics. It’s one of those cases where many wish automotive companies had just stuck with the old way of doing things. They’re not just as bad as traditional timing belts—they may, in fact, be even worse. All thanks to one unexpected failure mode that can really ruin your day.

Why Use A Wet Belt?

The wet timing belt promised multiple benefits over a traditional dry timing belt. Lubrication from the engine oil was supposed to reduce wear and noise. More importantly, though, it reduced friction, which would provide an efficiency benefit, with fuel savings of around 1%.

The first mainstream engine that boasted a wet timing belt was the Ford 1.8-liter TDCI turbodiesel, as seen in the Ford Mondeo and Transit Connect. The original version of the engine used a timing chain from the crank to the diesel injection pump, and then a regular dry timing belt from the injection pump up to the cams. However, in 2007, Ford changed all that, using a wet timing belt to replace the timing chain. Traditionally, automakers had recommended that timing belts should never come into contact with oil, as it would degrade the rubber of the belt. However, these belts were manufactured out of special Hydrogenated Nitrile Butadiene Rubber (HNBR) that was more resistant to heat and oil, and also coated with Teflon for added resistance.

By design, the lower timing belt would come into contact with oil at the bottom of the engine. The wet lower belt was supposed to actually last longer than the upper dry timing belt, since it had the benefit of lubrication. Ford quoted a 100,000 mile, 5 year change interval for the dry belt, and a 150,000 mile, 10 year interval for the wet one.

The design quickly spread from there. In 2012, a wet timing belt made its debut on Ford’s 1-liter 3-cylinder EcoBoost engine. Renault also got on board, as did Volkswagen, Peugeot, Vauxhall and Opel. In the vast majority of cases, the wet timing belts were used on small displacement engines of 1.5 liters or less. Almost all are inline-three or inline-four engines.

Why Many Don’t Like Wet Timing Belts

First off, just google “Wet Timing Belt” and you’ll come across lots of stuff like this:

Yes, while there are plenty of examples of wet timing belts working just fine, despite the promised benefits, public opinion on wet belts is largely negative. You won’t find too many people singing their praises, but you can find a lot of gearheads raging against them. The reasons are simple. People are finding that they don’t last as long as promised, and they also introduce a significant additional problem that traditional rubber timing belts never had. We’ll get to that in a moment.

As you probably already know, a timing belt (whether wet or dry) syncs valve movement with piston movement and makes sure the valves are retracted from the combustion chambers as the pistons reach the tops of their strokes to prevent the pistons from colliding with the valves. If the timing belt fails on an “interference engine,” the engine will often die a very complete death as the pistons and valves collide. And so, it’s always wise to follow the manufacturer’s suggested interval for timing belt replacement and thus avoid the costly broken-belt scenario.

The problem is wet belts sometimes fail quite early. Why? Based on what I’ve found around the web, it seems the special compounds and coatings that make the belts more oil-resistant are not actually perfect. In many cases, they’re simply not able to keep a belt alive as long as automakers might have hoped. Sitting in hot oil just isn’t good for belt life, there’s no two ways about it. Using the wrong grade of engine oil or skipping oil changes can also potentially harm belt life. Contaminants in the oil and buildup of acidic content over time could make a difference, here.

This Peugeot engine had a wet timing belt rated for 112,000 miles or 10 years. This belt was looking ragged after just 75,000 miles and 6.5 years in operation.

Replacement is a sizeable job. 

Wet belt engines are typically found in small European market vehicles, so dealing with wet belts has become a common problem for European mechanics. These vehicles have also been sold elsewhere, but much of the content discussing wet belt problems comes from Europe and the UK.

For example, it’s easy to find videos on the early Ford TDCI engines from British YouTubers, like this replacement video from mechanic Alan Howatt.  “I know a lot of ya have had your engines trashed because these belts have failed,” says Alan. He’s eager to sound the alarm for Mondeo and Transit Connect owners. “I would seriously look into it and have it replaced if it needs doing,” he warns.

More Ways To Die

Even if a belt doesn’t fail, it can harm the engine in other ways. For example, if the timing belt starts coming apart and shedding particles, these can end up blocking the oil pickup, starving the engine of lubrication. This can kill an engine even if the timing belt is still largely intact.

Make no mistake: This failure mode is entirely unique to wet timing belts. This never happened with traditional timing belts because they exist completely outside of the engine’s oil system. It’s a special and terrible feature of the wet belt design. You can find mechanics decrying this problem all over the place.

Yes, it’s really that bad. You don’t just have to worry about your timing belt snapping. You also have to worry about it shedding gunk and cutting off your engine’s oil supply. Instead of just one ugly failure mode, wet timing belts give you two! How lovely.

What Is To Be Done?

Knowing whether your wet timing belt is failing is key to avoiding major engine damage. However, it can be difficult to determine. A visual inspection, if possible, is a good start—if swelling or cracking is apparent, the belt should probably be changed. For Peugeot, Citroen, Vauxhall, and Opel models, a tool is commonly available for checking belts—it’s a simple width gauge. If the timing belt doesn’t fit in the gauge, it’s swollen enough that replacement is indicated.

The belt can be checked in-situ in some engines by inserting the tool through the oil cap.

Beyond that, it’s worthwhile looking in the sump if possible to make sure the engine oil pickup isn’t fouled. Simply changing the oil won’t solve this problem. The debris must be removed from the sump and pickup if present.

It’s difficult to understand why these belt designs even entered production. The benefits seem relatively minor, and the risks seem almost obvious. For my money, as an engineer, I’d speculate that the problems with these wet belts weren’t obvious in manufacturer testing. Automakers might put millions of miles on mule vehicles, but they don’t test a new engine design for five or ten years prior to release. They test on accelerated schedules. There are ways to simulate longer-term degradation and wear, but they’re not perfect. I can only guess that the belts seemed fine, only to start falling apart and causing grander problems when they got out into the real world.

Thankfully, these wet timing belts aren’t everywhere. Japanese automakers largely haven’t touched them, and vehicles with bigger engines have mostly stuck with traditional timing chains or dry belt designs, too. If you do have an engine with a wet timing belt, though, you really need to stay on top of things. Keep doing your oil changes on time, and inspect your timing belt regularly—well ahead of the manufacturer-recommended interval. If it needs changing, change it, and check your oil pickup, too.

It’s just another one of the weird ways automakers have changed vehicles, and not for the better. This happens from time to time, but usually, lessons are learned and things get better again. Here’s hoping wet belts go the way of the PowerShift transmission and cease darkening our engine bays forever more.

Image credits: Alan Howatt via YouTube screenshot, driving 4 answers via YouTube screenshot, Ford