Cant Figure This One Out

[GTR_STEALTH]

Car is a S2 R33 GTS-T

Completely standard except for pod filter.

Problem:

Cruising at 80km p/hr in 5th gear there is a slight vibration going through the car and felt through the seat (not steering).

If i go to 90+km it lessens but is still very slightly there. Go to 100+km and i cant really tell to be honest, so its very minimal.

What ive tried:

To rule out speed i did 80km in 4th (and in neutral and with just clutch in) and problem is not evident.

To rule out revs i mimic the same revs in every gear and problem is not evident.

Spark plugs have been changed to Iridium and Coil Packs changed.

Tried disconnecting battery and discharging thinking maybe the computer needed a reset.

Clutch has already been changed by GZM in Thomastown. (although the vibration was still there beforehands)

Gearbox oil was changed when i first got the car (but i doubt this is the prob)

Airflow meter has been cleaned.

Side note: When cleaning the afm i took off the tps from the top and bottom screw to inspect, but i replaced in exact same position. Just incase i was a mm off i have tried over the last few days going 1mm left or right but it has not made a difference.

4ws has already been done by Centreline. (Although it cant be this coz if i go in Neutral at 80km there is no vibration.

Not sure if this is related, but when cold and i first startup the revs are a lil hesitant but if u tap the accelerator then it goes up as per cold start, and comes down as it warms up. ...so I believe the IAC needs a clean.

It is almost asif the engine retards a lil only in gear (5th) whilst cruising at 80km.

Apart from that:

It idles smoothly

It cruises smoothly

Boosts smoothly and holds

Accelerates very well and goes to top speed 180km+ quickly. I know this coz i tested it on private property

No temp problems

No oil pressure problems

What should i be looking at next?

[mad082]

don't think it's going to be related to the engine running rough. it could possibly be an engine mount or a gearbox mount of something like that. the harmonics from the engine at that certain speed is what is causing it and that is why it won't do it at lower speed or different revs in a higher gear.

might be worth getting your wheels balanced to see if that helps.

[Tony de Wonderful]

You need to always drive at 100 km/h then.

Could try putting on a torque damper.

Edited November 5, 2010 by Tony de Wonderful

[GTR_STEALTH]

don't think it's going to be related to the engine running rough. it could possibly be an engine mount or a gearbox mount of something like that. the harmonics from the engine at that certain speed is what is causing it and that is why it won't do it at lower speed or different revs in a higher gear.

might be worth getting your wheels balanced to see if that helps.

Makes sense.... thanks dude