NC 6th gear rpm 'wobble'

[keito]

Hi all,

I'm taking my NC back into the mechanics this week for the third time in a month, but I thought I would pose the question to the brains trust to see if anyone had experienced something similar.

Two months ago on a long trip home I lost clutch hydraulic pressure, making it very difficult to change gears. Managed to limp the car home almost 400km (not fun) and decided it was time to do some major work to make sure I was set for summer. Drained and replaced the hydraulic fluid, bled and tested the master/slave cylinder. Everything seemed fine and the clutch was working again.

Recently had my diff replaced with a 4.1 lsd from the states (thanks Boyracer) and installed a GWR midpipe and SQ exhaust. Everything seemed fined until I hit sixth gear, at which point the car would lurch quite noticeably, at any speed and on any road condition. Using an OBD2 sensor I could see that the revs were dropping about 100-150 every second or so before picking back up, causing the car to slow down then speed up. Thought to myself that something wasn't right, booked it in for another inspection, and the mechanics replace the clutch and flywheel. I last replaced the clutch 70,000kms ago, and drive fairly 'spiritedly', so I agreed that it was the probable cause.

Picked the car up again, and lo and behold it is still exhibiting the same issues in sixth gear, albeit much more gently. Now it is dropping 30-40 revs once to three times a second. I have trawled through all the forums and looked for similar issues and haven't found any that weren't resolved by replacing the clutch. I have also read through the entire workshop manual and found nothing (which is worrying).

Things I've noted:
- Only happens in sixth gear, but occurs at any speed 60km/h to 140km/h.
- It occurs regardless of whether cruise control is engaged or not
- Isn't noticeable when accelerating or decelerating
- There is no unusual noise coming from the transmission in any gear

A question I have which the mechanic may not pick up on - with a higher final ratio in the diff, could this throw something off in the ECU? I've reset the ECU multiple times with no change.

[Regie]

never herd of this happening. not really too sure how you notice it dropping 30-40rpm, thats nothing.

[keito]

I agree, and it does the same in all other gears, except in sixth gear it happens a few times a second. The car basically feels like it is travelling against a strong blustering headwind in sixth gear.

The other thing I noticed was that the recorded 'boost' pressure (I presume this is measuring pressure of exhaust gasses) varies wildly in sixth gear, whilst staying fairly stable in other gears.

Finally, in sixth gear, when holding at a set throttle position the cars speed does vary. I found that at 100km/h it jumps down 1km/h and back up every second or more.

[bruce]

Sounds like it is all related to your original problem. Something is not quite rightly fixed. Maybe give it to a clutch specialist to take it for a drive?

[hks_kansei]

The other thing I noticed was that the recorded 'boost' pressure (I presume this is measuring pressure of exhaust gasses) varies wildly in sixth gear, whilst staying fairly stable in other gears..

Not sure on the NC, but the documentation for the NA/NB relates "boost pressure" to intake vacuum and from memory the fuel tank purge valve.


I doubt it's the clutch, a clutch will usually slip, or not, never heard of a clutch causing speed variations (unless it's off balance and vibrates)

I'd be checking vac hoses etc.
a split intake pipe (downstream of the AFM) can cause revs to behave strangely at cruise, since un-metered air can get in through the split making the mixtures go lean.
At which point, since it's in closed loop at cruise, the ECU sees a lean reading on the O2 sensor and will add fuel, then cut some fuel, then repeat it (that's pretty standard behavior for narrow band o2 sensors, but the split may exaggerate it)

So yeah, i'd be seeing if you can trace a vac leak or something.

[keito]

Thanks guys for all your input. Something that bugs me though is that this only happens in sixth gear. It it was something wrong with the clutch, or vac hoses, or the driveline, surely it would exhibit the same problems in other gears?

I think I also forgot to mention that whilst in sixth gear and cruising at say 100km/h (with cruise control on or off) the throttle position fluctuates wildly and constantly. It might hit a low of 18% and in a second be as high as 25%, then instantly dropping down again.

On Thursday I will be driving the mechanic around in the car, and making sure he's clear on the problem and what needs to be fixed.

[hks_kansei]

are NC's drive by wire? or cable throttle?

if it's an electric throttle it sounds like it's starting to play up, and probably needs replacing.

just need to confirm if it's the trhottle end, or the pedal end that's causing it.

[JBT]

are NC's drive by wire? or cable throttle?

Fly by wire.

[keito]

And if it was something wrong with the throttle, shouldn't the problem be present in 1st-5th gear?

[keito]

Right. So. The mechanic is stumped.

He has put it through the full diagnostic and come up with nothing. When he disconnected the MAF sensor he saw some improvement, bit he reckons that the MAF sensor isn't faulty. And at $500 a pop it's an expensive bit to replace just on the off chance it fixes the problem.

However, he reckons that the newly installed exedy flywheel may be causing the problem. How? Dunno. But he suggests replacing it with a stock flywheel.

[bruce]

Tis logical. Problem didn't exist before, changes made, now the problem exists. Look at the changes.

[keito]

Nope. Problem existed (and was worse) before the clutch and flywheel were replaced.

[JBT]

Maybe its bent. Have you had a thorough check of alignment - suspension and body?

[keito]

Maybe its bent. Have you had a thorough check of alignment - suspension and body?

If all else fails I'll out a new realignment on the list of things to do (was done 6 months ago).

At this stage it looks like I'm going to have to get the clutch pulled out and a oem flywheel reinstalled. If that fails, take it to a tuner and see if they can iron out the kinks.

Or maybe I'll go to the tuner first.

[Red_Bullet]

Maybe the tuner first, stick it on a dyno in 6th gear and see what happens? Lot less work than pulling the gearbox, clutch and flywheel. Drive it up a steep hill in 4th gear and check for clutch slip? Weird.