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Hi all,

I replaced my diff with a 4.1 FGR LSD diff recently, and I've noticed that when in 6th gear my engine seems to do this very subtle 'on-off-on-off' when cruising at fixed speeds under slight throttle application. This is only noticeable when I'm in 6th gear.

Has anyone else experienced this and may know what could be wrong?

Thanks in advance!
Wilson

Auto/manual?
What was old diff?

project.r.racing wrote:Auto/manual?
What was old diff?

Manual & Old diff was a 3.7 LSD.

Problem might have nothing to do with the diff change, but not really sure where to start troubleshooting.

I tried the battery unplug thing to see if the ECU (fuel trim?) recalibration made any difference and the car almost kept on stalling at idle for the first 5-10 minutes until it sorted itself out after driving around for a bit.
Car is smoother now funnily enough, but the pulsating throttle is still there at 6th gear -> it's almost as if the car is oscillating back and forth to maintain the rpm, but it's input step-size is too large (If that makes any sense - I don't really know the terminology).

wleung wrote:Problem might have nothing to do with the diff change, but not really sure where to start troubleshooting.

Might be worth checking the Mazda Technical Service Bulletins for the relevant NC series at the following: https://www.miata.net/garage/tsb/index.html

I'd check all the air intake piping, hoses etc, looking for a vacuum leak. Has there been ANY work done in the engine bay, that would be my starting point.

Did you check the PPF alignment after the diff install? If it's truly only happening in 6th gear, then it shouldn't pipes and hoses, etc shouldn't be the concern.

I don't quite see how a miss-aligned PPF could cause it either if I'm honest, but it's a mechanical connection between the gearbox and the diff and it can easily miss-align during a diff job.

Thanks for the responses guys.

- I'll have a skim over the TSBs this weekend.
- Don't think I did anything in the engine bay -> I don't believe I even opened up the hood.
- NC PPF installation didn't seem to have as complicated of a system as the NA/NBs: Service Manual indicated that I only had to ensure the gap between the PPF and the butterfly brace was wtihin spec? (So single axis alignment only).

I downloaded FORscan onto the laptop, and will spend some time logging data whilst reproducing the issue over the weekend.
Will see if I can match the oscillation up with anything obvious from the data.
The oscillation felt more like something was intentionally was cutting power -> so getting ABS and PCM data should cover most mechanisms that do that hopefully

What sort of LSD diff? Clutch Pack (1, 1.5, 2 way)? Helical/Torsen?

Magpie wrote:What sort of LSD diff? Clutch Pack (1, 1.5, 2 way)? Helical/Torsen?

The diff is just a US or JDM version of the NC diff -> just different FGR from the AU one.

Tochigi Fuji Super LSD from googling. Cone Clutch type? (I don't really know the different types).

Well went for a short drive and had a look at some of the the PCM and ABS data.
ABS wheel speeds were never off by more than 1km/h so it's not DSC or anything.
Nothing else looks crazy (At least I know the engine is fine).

So perhaps the problem isn't control related, and it's the drivetrain -> I don't suppose it's possible that my clutch is slipping?
Thinking back - I guess I did have to move the drive shaft a little, and had to jack up the transmission a little to align the PPF.
Maybe I shifted or hurt something in the process?

I do recall one point last week where I smelled something burning when I got home, but I was outside and assumed it was from the neighbour's chimney.
In one of my older cars when I overheated the clutch (reversing up an extremely steep hill slowly) it smelt like burning garbage, not like burning wood, so I assumed that wasn't the case.

I'll have to drive the same route again and see if the burning smell comes back.

Anyhoo - it doesn't seem like engine's gonna get hurt, so I'll just keep on hooning around in 1st - 5th gears until something gives me more clues.

Thanks for everyone's input. If I ever find out what the problem is, I'll be sure to post it up.

Just to close this off properly. I acquired a Tactrix 2.0 recently and was able to get some logs at increased resolution over my bluetooth one.

I managed to have resolved the issue, but as usual, I've changed too many parameters at once, so can't really tell what 'fixed' the problem.

logs confirmed that whilst cruising at 76kmph my APP (accelerator pedal position) was pretty much constant, but the TP_REL (relative throttle) was oscillating a bit. Not knowing what the ECU was doing, I noticed it was somewhat following the LOAD values. Having a glance at the MAF (mass air flow) values, they were oscillating strangely as well.

Well, here's what I did:

1) Wiggled the MAF sensor cables
2) unclipped, repositioned,& reclipped my intake air filter box
3) Using RomDrop, reprogrammed the ECU with the 4.1 Final Drive Ratio instead of 3.7

As for 3) - I don't even know if the ECU even uses the Final Drive Value for TP_REL calcs. Writing the new data probably cleared some other learned parameters which may have inadvertently fixed the real problem.

I also found a copy of the ROM from a US-specced NC2 with the 4.1 FGR (Cal ID: LF9VEB) vs mine (LF9HEE) - and there was no difference in any of the fueling or load maps.
Only significant difference was the spark advance IMRC-only table, and I was logging the IMRC switch the entire time - it never changed.

Anyway - final run - the MAF values were all smooth, and the problem seems to have gone away. I'm thinking I just cleared some learned value that couldn't be cleared via a battery disconnect, but who knows. Just gonna enjoy the car now, and maybe play with some throttle map tweaks to suit my own preference.

Attached some pics of logs before and after

Wow, that is an impressive amount of sleuthing to fix. Can you imagine how much a shop would have charged you for all those hours.

Wow, that is an impressive amount of sleuthing to fix. Can you imagine how much a shop would have charged you for all those hours.

Heh - I lived with the irritable 6th gear cruising for about 2 years tho (lucky it's just a weekend car) - but yes, I'm extremely glad this is sorted out.

I bought the Tactrix for a different reason (thought it'd be fun to peek & muck around with the ECU values in my car), but then realised I could actually troubleshoot this problem again after seeing higher resolution logging ability & the FGR value exposed for changing.

On that note -> Changing the FG ratio in the ECU tables might have some merit -> speeps (the RomDrop creator) suggested that perhaps the value is used in some torque load calculation, but he couldn't say for sure (wasn't worth investigating, given it takes less than a minute to flash the ROM with their tool -> might as well just change it).

A few posters in the same romdrop thread mentioned they also changed their FGRs in the maps when they changed the diff.

Drive by Wire (DBW) is way more complicated than I thought - but yeah 'torque request' is the term I should've used.
A gross simplification from an expert: https://forum.miata.net/vb/showpost.php?p=9859303&postcount=2464