So I've got my VVT swapped NA driving like I want it to for the most part, except I think I've got some play in my rear diff, or maybe an axle. How would I go about diagnosing/fixing this?

My drivetrain:

VVT engine - MS3 PnP
6 speed transmission - when I bought this component, I moved the input shaft myself and saw no play between it and the output shaft regardless of gear. Installed with diff and axles at the same time.
3.63 Torsen 2 diff - bought here from a UK distributor. I did not inspect this part myself prior to putting it in, but had my local shop put it in for me. They replaced the seals prior to putting it in. Installed with trans and axles at the same time
1.8 driveshaft and axles - bought with the diff. Installed with trans and diff at the same time.

The symptom:
When I'm driving and I go from acceleration to suddenly letting completely off the gas, the car will jerk/shudder a small bit. Also, when the car is in gear but no throttle (engine braking), and I apply the throttle, the car will jerk/shudder even though my AFMs don't show anything scary. This was very simlar to when I had my old 5 speed that I felt I had to baby, and when I took that out and put in the 6 speed, I noticed there was several degrees of play between the input and output shaft. This also means that there has always been this problem with the car, and my dad (the prior owner) even said this felt "normal" to him, but again the old transmission had play in it, and this doesn't feel "normal" to me. It makes me be very, very careful when powering out of a turn, because I have to apply throttle oh so gently or else the car will shudder.

Any ideas?

 

Check your engine mounts? If one has split separated this could contribute to shudder

 

Engine mounts and diff mounts were replaced with the transmission. I got the stiffer mazdaspeed mounts. I haven't inspected them since, but I don't believe them to be the cause.

 

Check engine mounts, perhaps bolt torque if they've been replaced - sounds like the most likely culprit.

You also want to check PPF bolt torque, and check your diff mounts as well. The whole drivetrain kind bolts together so play in any of those items could cause that behavior.

If you've not checked your PPF alignment - you may wish to do so as well.

 

So earlier, the reason I wasn't driving the car was because of a fueling issue. Turned out it was 2 bad sending units, specifically the wiring. The one that was in the car, and the one I got to replace it. When my local shop replaced it with their own sending unit, I also asked them about this question. They took a look and tightened all the PFF/diff bolts and everything.

How would I verify their work? Just get underneath the car with a wrench and see if anything is loose?

 

They’re upside down bolts, they’ll fall out if they’re loose. Torque for the big ppf bolts is 90 ft/lbs, same as your wheel lugs, so if you have a torque wrench for those, it’ll work with a 17mm socket. The 4 longer ones take a lot of stretch before they reach 90, so it turns more than you think before clicking.

You might try looking into your AE settings, they can cause jerkiness if not tuned well.

but to check play, grab the driveshaft and rock it back and forth by hand very slightly. You’re looking for The driveshaft to move when the axles aren’t. That’s play in the ring gear, a little is normal. Check the axles too, there shouldn’t be any rotational play in them.

 

My AE is set up so that when I blip the throttle or go from low to WOT, the AFMs get to 11-12 before settling. It's not tuned that great, but I don't think it would be causing this problem. I also don't have this problem if I'm cruising and then go WOT.

How much play in my diff is normal? Last time I was underneath the car, I checked driveshaft versus axles for play and got some. It's hard to say how much since I don't exactly have a protractor or anything under there, but it was a good bit of wiggling the driveshaft with zero result on the axles. Maybe as much as 5-10 degrees, but that could be wrong. That's why my first thought was differential.

 

Quick math says that .30-.60”, end play spec is closer to .003-.006”. Not sure how accurate your measurements are though

 

my measurements are not accurate. Just a hand wiggle. Also, I don't understand your comment, or even the specification. How are inches a measurement of rotation?

 

 

Did you ever find the solution? I'm having the same issue with my car.

 

I would also like to know if a solution was found for this. My car does this in low gears (1,2,3), at low rpm. I need to super carefully finesse the gas pedal either on or off, in order to not get the car to buck a bit. Its like I touch the gas and the car throws me back a touch so my foot comes off the pedal, then the car decels and I get thrown forward so my gas hits the pedal again, and back and forth until the oscillation evens out. Could just be me used to driving a slightly more refined 2023 civic type R and forgetting how to finesse the ol' miata.

 

When my NB had similar issues I got a
Beatrush PPF brace and replaced the stock diff bushings with polyurethane.

 

Interesting, I'll give it a look. I do already have some poly diff bushings I can slap in so maybe that will be my first move. Did it fix the issue for you?

 

The diff moved so much with soft stock bushings. With the brace it was better. There will be some increase in gear noise with poly bushings. It was not too bad in my opinion.
I also got Mazda competition motor mounts. Together, this made it okay to autocross.

 

Thank you! I did some innovative motor mounts back in the spring. Just installed super pro diff bushing inserts today, wicked easy. We'll see how that does this weekend. Although I have a sneaky suspicion my problems stem from, what I can only assume is, the original clutch. Feels like maybe on decel, when the car has enough time to "unload", the old crusty springs spring back to full length and cause the herky jerkiness. While the car is loaded up, the springs stay compressed and cause no issue. About 120k on my baby so she's long past due but I've had very few clutch hiccups. Supermiata organic planned for this offseason.