Guides  /  2010 Lexus RX350

2010 Lexus RX350 Serpentine Belt, Tensioner & Idler Pulley Replacement (120K Service)

If your 2010 Lexus RX350 is rolling up on 120,000 miles, the serpentine belt — and very likely the tensioner and idler pulleys spinning behind it — are due. A dealer will charge you a small fortune for a belt-and-pulley service. The parts run about $150, plus maybe $50 in tools you’ll keep forever.

Fair warning: this is the hardest job in the RX350 maintenance series — call it a 3 out of 5. The real gotcha is that once you tear into it, the car isn’t drivable until it’s fully back together, so block out the time and don’t start on a day you need to drive. Here’s how to do it right, including the holding trick and the pulley mistake that cost a full week.

Before you start — get the right parts and tool

Go genuine Lexus on the belt and pulleys if you can. Once these are in, you don’t want to be back under there anytime soon. For the tool, skip the temptation of the $18 bar — see the tool note below.

The pulley trap: Two identical-looking pulleys were ordered, but the one coming off the car had a different inner washer setup. That specific pulley is only available as part of the full idler/tensioner assembly (~$250) — not sold on its own. Identify exactly which pulley you have before ordering, or you’ll burn a week swapping parts like this video did.

Step 1 — Release the belt tension

1. Get comfortable. Lay a blanket or creeper down — you’ll be working on your back.
2. Put your 14mm on the tensioner bolt. The tensioner is right behind the accessory pulleys.
3. Crank to loosen. Pull on the bar to rotate the tensioner and create slack. You’ll notice you have to swing it surprisingly far before the belt frees up.
4. Slip the belt off the main pulley. With slack created, reach in and roll the belt off by hand.

Step 2 — Lock the tensioner with an Allen wrench

This is the trick that makes everything after it sane.

1. With the tensioner cranked over, insert an Allen wrench into the lock hole on the tensioner.
2. This holds the tension in place, so you can take your 14mm wrench off and free up both hands to remove the pulleys.

Step 3 — Remove the idler and tensioner pulleys

Pay attention to thread direction — it bit this job more than once.

1. Top pulley: the bolt comes out counterclockwise like a normal bolt.
2. Bottom pulley: here it’s reversed — because counterclockwise loosens the tension, the bolt actually comes out clockwise. Make a mental note before you wrench on it the wrong way.
3. Mind the inner washer. One pulley has a washer that sits down inside it. Cradle the pulley with two hands, find the hole by feel, and thread it carefully so that inner washer stays seated.

Step 4 — Compare old vs. new

Lay the old belt next to the new one. At 120K the old belt was honestly not badly cracked — a little dried out, but impressive for the mileage. The new belt looks almost fabric-like by comparison. Either way, while you’re in this deep, replace it.

Pro tip: New belts ship coiled tight. Let it relax / lightly route it before installation so it’s easier to wrap around the pulleys.

Step 5 — Route the new belt

1. Wrap the belt around every pulley except the last one — leave the final pulley (the one nearest the tensioner) for the tension step.
2. Double-check your routing against the diagram on the car before tensioning. The path runs through the AC/alternator pulley, the tensioner pulley, and the main drive.

Step 6 — Tension it (the hard part)

This is where the cheap tool fails and the right one earns its money.

1. The $18 bar doesn’t give enough leverage, and its half-inch drive needs a hard-to-find half-inch 14mm crowsfoot — and a two-sided crowsfoot can slip off the bolt. Use the GearWrench ratcheting kit with the proper 14mm socket instead.
2. Crank, reset, crank again. A single pull doesn’t reach far enough. Pull the ratchet up, reset it, and pull again — repeat until you’ve got enough swing.
3. Hold your progress between pulls. Jam a bar in to hold the tensioner where it is, reset the ratchet, then keep going. (Rope was tried first to hold it and did not work — it had to travel too far.)
4. You’ll end up cranking the tensioner roughly 120 degrees from its rest position before there’s enough room to roll the belt onto the last pulley.
5. Slip the belt over the last pulley, then slowly release the tensioner back onto the belt. Ease it — a wound-up tensioner under that much spring tension wants to fling.

Step 7 — Verify and button up

1. Double-check the belt is fully seated on every pulley and matches the routing diagram.
2. Start the engine. It should sound noticeably quieter at idle with fresh pulleys and belt.

That’s it

This one’s a project, not a five-minute swap — but you’ve just done a real 120K service item for about $150 in parts versus a hefty shop bill. The two make-or-break lessons: identify the exact pulley before you order, and buy the proper ratcheting belt tool so the tension step doesn’t beat you. All the parts and tools used are linked in the box above.