What a timing belt actually does

The belt with the ribs is the other one. A serpentine belt drives the accessories by the friction of those ribs. A timing belt has teeth, and the teeth keep the camshaft sprockets in step with the crankshaft. A timing chain does that job in metal. Gates’ 2010 interval guide, catalog 431-1448A, puts the timing belt inside the engine, which is why the belt people can see is so often the wrong one.

The ribs are a different belt

The toothed belt and the ribbed pulleys can sit in the same bay.

The file identifies this as a Nissan RB30E with the timing covers off. Red bodywork is at the right, and the ground is crumpled paper with print on it. A black toothed belt runs down the middle, meshed with a large rusty brown toothed metal sprocket at the top. Below the belt are a smooth silver idler pulley and a round hub with protruding studs. To the right are ribbed metal pulleys, one above another, and a smaller ribbed pulley near the bottom. Black ignition leads running across the top are printed BOSCH. At the lower left a gray housing has a connector marked PUSH. The toothed belt is the timing belt. The ribbed pulleys are the accessory drive.

Teeth, then the cam, then the valve

The belt in the guide is built in layers the outside does not show. Gates describes those belts as an aramid-reinforced nylon jacket, high-grade HNBR rubber, and high-strength tensile members. A black loop on a bench is still just a black loop.

The file calls this a toothed timing belt and does not name an engine. It lies in an oval on an off-white background. The teeth are on the inner face. The outer face is smooth. No markings are visible.

On one engine the path is written out. Audi self-study 920253, August 2015, is the 1.4 TFSI in the A3 Sportback e-tron, the first use of that EA211 engine in North America. The camshafts are driven by a toothed timing belt. Audi gives that belt a Teflon coating and names a long service life, lower friction, lower mechanical stress on the timing gears, and a smoother, quieter engine. Those are the results Audi states for this engine. A guide pulley and the crankshaft sprocket keep the belt running smoothly. An automatic tensioning roller guides it with raised flanges. A three-piece cover keeps dust and dirt off the belt, and the aluminum center section of that cover is also an engine mount.

The belt turns the sprockets. It does not open the valve. On this engine the valves are operated by roller-lever cam followers, which is the cam’s work. The intake sprocket is not a plain wheel. Audi calls it a vane adjuster with a 50-degree range. The belt turns that sprocket. The adjuster is a different part.

The file calls this a toothed rubber timing belt and does not name the engine. The cover is off the sprocket. A black toothed belt is meshed with a silver-gray metal sprocket, and a hex bolt sits in the center. Stamped numerals on the sprocket, including 3, 4, and 6, and the characters R70 are stamped on the face. Another numeral is partly hidden. At the lower right a ribbed metal pulley sits in front of a finned housing. The teeth are in the sprocket. The ribs are on the other wheel.

Clearance, or a collision

The same Gates guide sorts engines by what happens if that step is lost. In a free-running engine, with the crankshaft still moving, there is enough clearance between the valve and the piston even if the cam stops with a valve fully open. An interference engine does not have that clearance. The guide says those engines usually sustain damage: the piston and the valve collide, and the parts it names are valves, pistons, or heads. Its grouping, in that 2010 edition, is that most higher-compression import engines, and most car and light-truck diesels, are interference. Interference engines are marked with a star in the listings. The star is a mark in the book. It is not printed on the belt.

A lost belt is not, from the driveway, a way to tell which class the engine is. The guide says to follow the automaker’s mileage recommendation, and on that same page it says Gates recommends replacement at around 60,000 miles. The cover of the guide says the recommended interval or every 72 months, whichever comes first. The rows under those lines are per engine, and they are not all 60,000 miles. The interval is the row with the engine on it.

A chirp is a test, and a damp rod is a look

Acura bulletin 14-025, September 18, 2020, version 6, is a list of Acuras. It covers the 2014–2018 MDX, the 2019 MDX trims inside the VIN ranges it prints, the 2016–2018 RDX, the RLX years and hybrid splits it prints, and the TLX V6 years it prints, including the 2019 VIN cuts. The symptom is a chirp from the timing-belt area after the engine warms up. The cause the bulletin names is the timing belt contacting the back edge of the crankshaft pulley. The test pulls the drive belt and listens again while the engine is hot. If the chirp is gone, the bulletin does not apply. If water on the back edge of the timing belt lessens the chirp, changes its pitch, or stops it, the bulletin continues, and the repair it prescribes is an idler-pulley shim once the inspection confirms the contact. If the chirp does not change, the bulletin does not apply. A chirp, on these cars, is not yet the timing belt, and it is not a belt that has already let go.

Subaru bulletin 02-142-13, August 9, 2013, is the hydraulic adjuster on 1.8, 2.0, 2.2, and 2.5-liter EJ engines, for the tensioner part numbers the bulletin lists. Normal, in the bulletin’s photos, is no oil past the rod seal beyond a slight residual film. Failed is a significant accumulation that has leaked past the seal. The rod is compressed slowly, and it is held vertical. Too fast, or horizontal, can weaken it. A damp rod is the inspection. The wetness alone is not the failed one.

The smooth back, and the pump that may not be on this belt

Gates lists automatic, manual, and hydraulic tensioners, and idler pulleys, as separate parts. The hydraulic tensioner in that guide is for applications with high loads or angular vibration, where a mechanical automatic tensioner cannot provide the damping. On two GM cars the service note is narrower. Bulletin PI0923, March 2013, covers the 2011–2014 Chevrolet Cruze and the 2012–2014 Sonic with the 1.8-liter, RPO LUW or LWE. When the timing belt is replaced because the maintenance interval is up, the tensioner and the idler pulley are replaced with it.

The file identifies this as a timing belt being replaced on a car, and it does not name the car. A black toothed belt runs over a blue-gray metal sprocket held by three hex bolts, and the smooth back of the belt rides a smooth pulley with a bright blue ring. Two hands in gray knit work gloves, one printed NITRAS, hold the belt. Black hoses are at the left, and daylight is in the background. The teeth are on the sprocket. The back of the belt is on the smooth pulley. The photograph does not show whether that pulley is a tensioner or an idler.

The water pump is not automatically on this belt. Gates says a water pump that is driven by the timing belt should be replaced when that belt is replaced. The sentence is about pumps the timing belt drives. On the Audi 1.4, the coolant pump is a different toothed belt, running off the exhaust camshaft, on the flywheel end of the engine. Audi calls that pump drive maintenance-free, and the belt is replaced if the pump is replaced. The oil pump on the same engine is a chain, with no tensioner, and its drive sprocket is fixed to the crankshaft. The cam belt turns neither of them.

Oil or antifreeze on a timing belt is on the Gates warranty list of damage, along with tension that is too high or too low, a belt installed wrong, and a belt used on the wrong engine. The list is what voids that warranty. It is not a way to name a noise from the driveway.

The teeth keep the cam in step with the crank. The ribs belong to the other belt. A cover, a chirp, and a damp rod are not the job.