Torn CV Boot: Can You Keep Driving, and What Happens If You Don’t Fix It?

A torn CV boot makes no noise. The silence is what makes it expensive. The boot is a rubber sleeve that holds grease around your CV joint. Once it tears, the grease slings out and road grit gets in, and that grit grinds the joint apart from the inside. Caught on a lift, it is a boot. Caught by ear, it is an axle.

We pulled the rear axles out of an ’87 Porsche 944 Turbo (951) to show you what that looks like when nobody catches it.

What is a CV joint, and what does the boot actually do?

CV joint stands for “constant velocity joint” and they can be found on each end of the drive axles in all front wheel drive cars (FWD) and in many rear wheel drive applications as well (such as the example 944 Turbo in this post). Their function is to transfer the torque from the transmission to the drive wheels at a constant speed (hence the name) while accommodating for the variations in relative wheel position due to the up and down motion from suspension travel as well as the camber, caster and toe set-up of the alignment geometry.

Every drive axle has two. The inner joint sits at the transmission or differential end and absorbs suspension travel as the wheel moves up and down. The outer joint sits at the wheel hub and handles steering. The complete assembly, axle shaft plus both joints, is what a technician calls a half shaft. Which of the two is failing changes what you hear from the driver’s seat.

The joint itself is packed with grease specifically formulated for this application to keep the joint lubricated. This grease is critical for the operation of the joint; without it, the heat and friction generated by the unlubricated metal-to-metal contact would quickly damage the joint and cause it to fail. To keep that all-important grease contained where it should be, each joint is enshrouded by a protective rubber boot, or “CV boot.” As long as that boot remains intact, it is not uncommon for CV joints to last well over 100,000 miles or more. If the boot becomes ripped or torn, the grease contained within will get slung out from the joint while road grit, debris and water will be allowed in, which further accelerates the wear and tear on the joint.

So the boot is the wear item, not the joint. A sealed CV joint will usually outlast your interest in the car. A boot is rubber, it lives six inches off the pavement and it has a finite life.

 

Can you drive with a torn CV boot?

Briefly, and only far enough to get it looked at. There is no safe mileage number, and anybody who gives you one is guessing.

A boot that tore this morning is still holding most of its grease, and the joint underneath it is fine. A boot that tore four months ago has been packing grit into that joint every mile since, and the joint may already be finished. From the driver’s seat, those two cars feel exactly the same. Both are quiet. Both drive normally. The only way to tell them apart is to get the wheel off the ground and look at it.

Failure, when it comes, is not subtle. A CV joint that lets go completely stops sending power to that wheel. The steering goes vague or erratic. You get heavy vibration through the front of the car and loud clunking or grinding from that wheel. In a sharp turn or at highway speed, that becomes a handling problem rather than an inconvenience.

So: torn boot and a quiet car, get it inspected soon. Noise plus new vibration, do not drive it in to us. Call for a tow.

The quiet stage is the expensive stage

CV boots cost people money because the cheap window is silent, and it closes without telling you.

1. The boot tears. No noise. No change in how the car drives. The only tell is grease flung on the inside of the wheel or across the suspension nearby. This is the stage where the repair is a boot.
2. Grease out, grit in. What grease is left mixes with road dirt and sand into something abrasive. The joint is now grinding itself down. Still often completely quiet.
3. The joint wears loose. The internal parts start moving where they should fit tight. Now you finally hear it: clicking on turns, or a clunk when you accelerate.
4. The decision gets made for you. By the time it is audible, the joint has wear in it. You are buying an axle.

Replacement boots are significantly less expensive than CV joints or half-shaft assemblies.

What happens if torn CV boots aren’t replaced? It won’t take long for enough of the CV grease to be replaced with harmful grit and debris that damage to the joint is unavoidable. That’s exactly what happened to the 944 in these pictures. Look at the gouging and pitting in the stub axle, and how gritty and dirty the grease in the joints became. In this instance, the damage to the joints was exacerbated by a broken transmission mount as well as the torn boots.

That transmission mount is the part worth noticing. The torn boots did most of the damage, but a broken mount was letting the axles move in ways they were never designed to, which accelerated all of it. You do not find that by looking at the one part the customer called about. You find it by looking at the car.

“We find torn CV boots pretty regularly during inspections on cars that came in for something completely unrelated, and a lot of times the customer has no idea, because it hasn’t started making noise yet.That’s exactly why we do a visual inspection with routine maintenance: we’d rather catch a torn boot now than replace the whole axle later.”  – Chris Gustafson, Production Lead, Fifth Gear Automotive Repair.

 
Top: Torn CV boot
Bottom Left: View of CV joint exposed by torn boot. Bottom Right: Badly worn and dirty CV joint caused by exposure from torn boot.

What the noise tells us: inner joint or outer joint?

If your CV joints reach this state, there will be some definite signs and symptoms that they need to be replaced, including an audible clicking or popping noise while turning, excessive vibration from the axle while driving or a clunking/clattering from the joint while driving, which can be intermittent.

Those three symptoms are not interchangeable. They point at different parts of the axle, and knowing which one you have tells a technician where to start.

• Clicking or popping when you turn: almost always the outer joint. Loudest on slow, tight turns, the kind you make in a parking lot. It gets louder and more frequent as the joint wears.
• Clunking when you accelerate or let off: usually the inner joint, the one that handles in-and-out movement as the suspension travels.
• Shudder or vibration under hard acceleration at highway speed, fading when you coast: also the inner joint. A symptom that appears under load and disappears when you lift off points at the axle rather than a tire or a wheel balance problem.
• Grease on the inside of the wheel and no noise at all: the boot, and you are still in the cheap window. This is the one you want to catch.

One distinction worth having before you call any shop: a hum or growl that gets louder as your speed climbs is usually a wheel bearing. CV noise follows the steering wheel, not the speedometer. A bad CV joint generally does not vibrate when you are driving straight down the road.

Why we won’t quote a CV axle over the phone

We get this call constantly. “How much for a CV axle?” We don’t know until we’ve seen your car, and we would rather say that than hand you a number we have to walk back later.

“CV axle” describes a dozen different jobs.

• Boot or joint? Two different repairs at two very different price points, and nobody can tell which one you need over the phone. It takes eyes on the part.
• Which wheel, and how many? Front-wheel drive cars have two axles up front. All-wheel drive has four. Plenty of rear-wheel-drive and AWD cars, including the 944 in this post, run CV axles at the back, which is a different job to access.
• OE or aftermarket? On some applications a quality aftermarket axle is the right call. On others it is a false economy, and we will tell you which is which for your car.
• What else is going on? The 944 above needed a transmission mount nobody was asking about. Quote an axle over the phone, then find the real cause on the lift, and we have wasted your time and set a bad expectation.
• What the car actually is. The badge changes the job more than most people expect.

So bring it in, or call and describe what the car is doing. Describing the noise genuinely helps us. Naming the part is our job, not yours.

Pictured: Scoring and gouging of the stub axle caused by damaged CV Joints.

Why the marque changes the job

CV axles are not one part. On European and performance cars the differences run deeper than most owners expect.

• Spline counts fragment inside a single model line. On 2007-2013 BMW 1 and 3 Series cars, the 335i, 135i and xDrive models use a 30-spline rear hub while base 328i and 128i cars use 27 or 28, according to Go-Parts’ CV axle guide for those chassis. Fit the wrong axle and you’re replacing the hub and knuckle too. That’s a VIN-level parts lookup, not a year-make-model guess.
• The ABS sensor ring is built into the axle on some models. When it corrodes, the symptom is an ABS or stability control warning light, not a noise. Diagnose that car on a front-wheel-drive assumption and you’ll never find it.
• On rear- and all-wheel-drive European cars, “CV axle” usually means a rear half shaft. Different access, different labor and a different failure pattern than the front-wheel-drive scenario most online advice is written for.
• Torque and tooling are specific. On those same BMW chassis the axle nut torques to 420 Nm (310 ft-lb) and must be staked after installation, with E12 Torx differential hardware. Get it wrong and it doesn’t announce itself. It comes back six months later.

We specialize in European auto repair for BMW, VW, Mercedes, Porsche, Audi, Volvo, Jaguar, Land Rover and most other popular European marques. But we service all makes and models, domestic and import, and most of the cars in our bays aren’t European. The diagnostic process doesn’t change with the badge. A Tahoe with a clicking front axle gets the same treatment as a Cayenne. The parts research just gets easier.

When to have your boots inspected, and what to ask

To avoid more expensive repairs to the CV joint or axles, we recommend inspecting the CV boots at every routine maintenance interval and replacing them at the first signs of damage or wear.

Boots are a visual inspection item, not a mileage one. A sealed CV joint routinely runs well over 100,000 miles. The boot is what sets the clock, and no mileage number covers it, because a boot on a car that lives on chip-seal county roads in Argyle will not age like a boot on a garage-kept car in Southlake. Conditions set the clock, not the odometer, so you look at it while the car is already on the lift. A boot on a car that lives on chip-seal county roads in Argyle will not age like a boot on a garage-kept car in Southlake. Conditions set the clock, not the odometer, so you look at it while the car is already on the lift.

Four questions worth asking whoever inspects yours:

1. Is the boot torn, or is the joint already damaged? These are different repairs.
2. Can you show me the grease? Splatter on the inside of the wheel is the visual confirmation.
3. Is a reboot viable on this application, or is it replace-only? On many modern cars the labor to fully disassemble, clean and inspect a joint approaches the cost of a complete axle, which is why replacement is often the better value.
4. OE or aftermarket, and which is right for this car?

If you suspect that your CV joints might be damaged, or if you’re unsure of when the boots were last inspected, give us a call today to have one of our certified auto repair technicians take a look. Fifth Gear Automotive Repair has ten locations across DFW: Allen, Argyle, Aubrey-Cross Roads, Autosports, Castle Hills, Frisco, Lewisville, McKinney, North Fort Worth and Southlake. A CV boot inspection is quick to add to a visit you were already making.

John Miller

John Miller is the Director of Business Development at Fifth Gear Automotive.
Fifth Gear Automotive Repair provides automotive service and repair. We service all makes and models – both domestic and import. We specialize in European auto repair for BMW, VW, Mercedes, Porsche, Audi, Volvo, Jaguar, Land Rover and most other popular European marques.

Fifth Gear Automotive has 10 locations – Allen, Argyle, Aubrey-Crossroads, Castle Hills, Frisco, Lewisville, McKinney, North Fort Worth, and Southlake, Texas.