A strange shudder

[rust]

Here's a weird one. This issue has only just started happening in the last 250km.

Somewhere in-between the point where any gear begins to hit peak cruising rotation and full throttle, the motor ever so slightly feels as if it's somehow "shuddering".

There's no other way I can describe it except to say it's not a hardcore shake, it's more like a very subtle tremble and it keeps going for as long as I hold a particular speed. If I drop the acceleration back a bit or increase the speed, that strange feeling usually stops until I reach the trigger point again. There's no sounds, bad smells or excessive heat.

Any ideas?

[rar110]

Prop shaft or gear box mount may need to be tightened.

[rust]

Cheers, I'll check them out - hopefully they're simple jobs???

[rar110]

Tightening is just a matter of tweaking up the bolts:
* at each end of the prop shaft;
* holding the gearbox mount brackets to the chassis.

I was getting a horrible noise underneath on occasions. It was loose gearbox mount bolts following a trip which saw lots of corrugations.

Loose prop shaft bolts also have caused some noise from underneath in the past.

[rust]

Thanks heaps - that's awesome. I'll be looking underneath when the rain clears.

[rust]

I've had a go at the tightening and everything feels good... BUT the shuddering seems to be getting worse, especially at what feels like around 4000RPM in any gear.

I'm now beginning to think it may be related to something that's not the prop shafts or gearbox...

Does anyone have any other ideas???

[Bearman]

If you are hitting 4K rpm there is the problem. The Isuzu motor variant in the Perentie is governed to 3200 and the truck version (as in the 6X6) to 3500. It doesn't like it!
Seriously, it could be fuel starvation in the higher revs area which will cause a shuddering effect. Unless you have done it recently I would change all fuel filters and don't forget the steel gauze strainer under the domed nut on the fuel pump. It is a common offender if it hasn't been cleaned for a while and gets clogged up with crap. Also look for fuel weeps around any fuel lines, if fuel leaks out air can get in as well under suction. Carefully tighten any connections that show evidence of weeps.

[rust]

Thanks Bearman - You just gave a perfect example of perception vs reality. I thought 4000 RPM seemed midrange when in fact it was WAAAAY over the reality. The only device I have with a tacho is an outboard motor that goes to 5400 RPM so that was my imaginary baseline sound.

Anyway - The problem is almost solved! A mate got underneath after a test drive and discovered that the UNI joint on the back end of the front prop shaft is loose through metal fatigue.

Turns out I need to replace the UNI JOINTS on the front propshaft - Where would I be able to find a part number or a high quality OEM?

[isuzutoo-eh]

Should be a 1310 size uni, from any of the usual suspects-even Repco etc stock them, though the brands and quality of the brands can be variable.
I bought my last set of genuine unis from Karcraft in Silverwater, Sydney.

[rust]

For those with similar issues the U-Joint kit part number is RTC3346 and it's an 82mmx27mm - About $26