1997 Mazda MX-5 Miata: Rattle Patrol, Driveshaft Edition
June 27, 2012
Sometimes you can locate and solve a weird noise in a car with a torx bit, a ratchet and three minutes. Then there are the noises that are more elusive.
*Updated* - more accurate intel inside
A few weeks ago Project Miata developed a very subtle skrik-skrik-skrik in certain driving conditions. In the space of a week or so it grew obnoxious, and the conditions more obvious -- the noise was happening in gear, clutch engaged, on the overrun (or while in reverse), and was not coincident with wheel speed. To me this was the telltale of a bum u-joint. U-joints on Miatas are not serviceable, so the whole driveshaft would need to be swapped.
Replacing a Miata's driveshaft is cake -- drop the underbody brace (six bolts; it's already removed in the lead shot), remove the O2 sensor, drop the exhaust, then it's just four bolts at the diff and the driveshaft practically falls out.
Our boneyard driveshaft came from a 2004 Mazdaspeed Miata.
The MSM driveshafts apparently are identical to those of non-MSMs of equivalent model year. (Flyin Miata reports that MSM driveshafts are unique relative to non-MSM units. -JK)
Once I popped the new driveshaft in and tried to install said four bolts, it became immediately obvious that something wasn't right. The replacement driveshaft would rotate about a quarter-turn before its front u-joint bonked into the powerplant frame (or PPF, Mazda's brace thingamajig that connects the back of the gearbox to the nose of the differential).
Upon inspection it could be seen that the later driveshaft was beefed up compared to our original '97 unit, and that this beef had beefed itself right into the top of our PPF.
The later driveshaft (top, in picture above) has a larger u-joint and more prominent bosses on the yokes than the '97 driveshaft (bottom). Initially, the later driveshaft's protruding bosses contacted the top of our PPF.
That shiny spot is the witness mark where the later u-joint has a property dispute with the PPF. Also note on the PPF the dirt-caked grease that flew out of our old u-joint, which no doubt accelerated its demise.
Before doing anything else I called Flyin Miata to double-check whether additional components related to the driveshaft (like the PPF) had, too, changed in later cars.
Nope, not that they knew of (and they've seen pretty much everything related to Miatas). (they've since learned that MSM driveshafts are different from non-MSM ones. -JK) But, it was suggested, try taking up the slack in the PPF mounting to buy some clearance. So I simply loosened the three big bolts at the front of the PPF and re-torqued them while prying. That did the trick -- no more interference.
Buttoned it up and confirmed that, indeed, all was well and the noise was now gone.
It's not clear when Mazda started installing the bigger u-joints on the Miata's driveshaft but I suspect this change came in 2001. (seems to be an MSM thing -JK) Anyway, a later driveshaft can go in an earlier car (well, this earlier car, anyway) as long as you crank the PPF out of the way. It's an easy fix.
--Jason Kavanagh, Engineering Editor @ 137k miles.