Car shakes when coming to a stop?

You weren't clear whether you are referring to an engine shake when the vehicle is stopped, or there's a pulsation in the brake pedal while braking but still moving. If you're referring to a pulsation in the brake pedal, your mechanic is likely correct. The mistake is insisting on having the new rotors replaced under warranty.

When we make parts out of cast iron, we set them aside for 90 days to "age" before they get their final machining. Most new rotors today come from China. There's nothing wrong with their quality, but they machine them right away, then they age on your vehicle. That commonly results in minor warpage and that pulsing brake pedal. This even happens quite often on brand new cars. The proper fix is to give those new rotors a light machining to true them up, then they'll be fine. If you insist on new ones under warranty, they will likely do the same thing.

Most auto parts stores will do the machining for you at no charge, if you bought them there. If a shop did the work, it is common practice for them to do the machining, also at no charge. This rarely happens a second time as long as you wait long enough for all the warping to take place. This usually shows up after three to four months.

Talk with the folks who installed and sold you the new rotors. This warping after around three months is extremely common with an easy solution. It has been addressed here quite a few times. When you pay a shop to do a standard brake job, or any other work, for that matter, it is understood they are going to stand behind their work and take care of related problems not caused by you. 99 percent of shop owners are conscientious and will do whatever they can to keep you satisfied. As for new parts, they mark them up a little, then part of that profit goes toward ordering or running to get replacements when one of them fails under warranty. That's why you are not expected to pay again, usually including labor to replace a defective part, but the mechanic can be expected to be paid to do the job a second time, again, as long as he didn't cause the problem.

One wrinkle to my story that you might be running into is a lot of shops that do brake work no longer have a brake lathe for machining drums and rotors. Back in the 1980s, it was quite common to pay over $100.00 for a slip-on rotor for a little front-wheel-drive car. Today that same rotor can run as low as $20.00. Given that low cost, it doesn't pay economically to machine old rotors. Consumables, including the cutting bits and vibration damping equipment, plus the labor time, adds up to more than the cost of new rotors. Also, all manufacturers have published legal limits to how much a rotor can be machined to, and a slightly thinner limit to how much more it can be allowed to wear to. Everything on cars today is made as light as possible to save weight. Rotors often wear down to the "discard" spec just from normal driving. That means it is worn well past the "machine to" spec that is the legal limit. No mechanic will risk his reputation or a lawsuit by machining a rotor when it will end up too thin. Since many rotors are already too far worn and can't be machined, there's little point in having a brake lathe.

If the shop you're working with doesn't have a brake lathe, they had to buy those rotors from somewhere. That's the place they should take them too to have them machined.

Keep in mind there are other things we can accidentally do to cause pulsations and noises, and there are things we do to prevent those problems, but in those cases the problems we cause show up right away on the test-drive. When everything is fine for the first month or two, it's a pretty good bet the job was done properly.

To add another chapter to this exciting story, when I worked for a very nice Chrysler dealership, the manufacturer had a policy that any repair done to a part under warranty was not allowed to decrease its life expectancy. As such, we were not allowed to machine brake rotors since that made them thinner. I suspect other manufacturers have a similar policy. Remember, this is just for parts and vehicles under warranty when the manufacturer is paying for the repairs. Well, it happened that the rotors on almost all new cars were warping in the first few months. Their analysis showed taking a light cut on the brake lathe solved the customers' complaints, and the rotors would still be in specs, but need to be replaced at the first normal brake job. Doing that machining didn't cost the car owner anything, it saved the cost of replacing hundreds of thousands of rotors, and it insured the pulsation wouldn't return. Everyone came out ahead.

Talk with the people at some of your local auto parts stores to see how they handle warped rotors. We have about a dozen stores in my city, and all of them have light machining equipment in house, including brake lathes. The only rotors they charge for are old ones being cleaned up as part of a competent do-it-yourselfer's normal brake job.

If no one will give you satisfaction up to this point, look for a nearby community college with an Automotive program. Talk with the instructor who teaches the "Brakes" class. In my school we were always looking for live work to give our kids real-world learning experiences. In fact, we had about two dozen community members who would sit on a broken car until it fit what we were teaching at the time, because they knew the value that had for the class. You can bring the rotors in yourself or they can take the vehicle and pull the rotors off for you. We did charge ten dollars per hour for what the job was supposed to take, but part of each day is spent in the classroom, and when in the shop, other projects may be in the works, so it can take longer than normal to get your vehicle back. Also, we only did brake work for the eight weeks of the year we were teaching that class. To do that work, say, in Electrical class, would take work away from the shops that hired our graduates, so we avoided that.

If this was in my class, we would remove the new brake pads, clean them up, then clean and lube all the appropriate points just like we were doing a regular brake job. You would be getting a full, regular brake job, but without the expense of new parts.

Let me know how this works out for you.