Bongo Fury

We've owned our 4WD pop-top Freda for about eighteen months.

Since early this year we've had an occasional graunching noise accompanied by mild steering vibration when pulling away from intersections. It sounds/feels like a faint hammer-drill effect that skips in and out, or a growley magnification of the normal drive-train noise.

If you can imagine a pair of gears bouncing into and out of true engagement with each other, that's the sort of sound -- though I don't suppose that's the actual cause!

Detailed symptoms/conditions:

* Sound seems to be coming from front/underneath us; not too clear whether one side or the other

* Only occurs under rapid acceleration from fully stopped, NEVER at speed and only rarely under gentle acceleration

* Mostly when making a right or left away from an intersection, but can happen on straight-line accelerations

* Always more likely if road surface is uneven, as if that sets up the aforementioned bouncing effect

* Never when cold -- always needs a few km to warm up before we hear it

* Always more likely if van is half or fully laden, though not when towing (possibly because of more cautious accelerations then)

* Once it kicks in, it tends to persist for 1-5 seconds of (straight) driving - longer if road is uneven - then gone as RPM settles or speed increases or acceleration eases (hard to say which)

* Very hard to repeat with a mechanic in the van!


What we've tried after reviewing previous topics here:

* Replaced both outer CV joints

This may (or may not) have improved things a bit, but it's certainly not gone.

* Reconditioned P/S rack for unrelated reasons

* Put some expensive lim-slip-diff additive in diffs

* Looked for broken/loose brackets in front

* Closely examined tyres and wheel wells for rubbing

Our mechanic has no suggestions at this point other than taking it to a Mazda specialist.


I think the symptoms are pretty distinctive. Do they ring a bell with anyone in the know?

Thanks!

If it's 4WD it could be the VC (viscous coupling) on it's way out, or extremely sticky.

To test it you need to spin the back wheels VERY slowly with the front wheels locked.
The best bet is a rolling road, but don't try jacking up the rear wheels unless you can
ensure it's impossible for the van to move in either direction.

Hi

We have pretty much the same symptoms - got our van about 6 weeks ago. Only happens when warm.

W are due to take it back to the garage that sold it to see if they can figure out what's going on.. will let you know ig there is any progress.

Regards

Neil

I'll go with 2 sticks on this one - simplest way to diagnose is to remove the front propshaft and see if the noise disappears.

We had a problem with our '97 Freda last year - a vibration from the front end when pulling off from a junction whilst turning right. It turned out to be the passenger side front brake caliper was partially seized and rubbing against the disk - hence the noise.

Many thanks for the suggestions folks!

dandywarhol wrote:I'll go with 2 sticks on this one - simplest way to diagnose is to remove the front propshaft and see if the noise disappears.

Yes, I see the possibilities if that's fairly easy to do. However, because I'm more software than hardware -- if we do that, and the behaviour stops, what will that tell us?

2sticks wrote: To test (the viscous coupler) you need to spin the back wheels VERY slowly with the front wheels locked.

What will be I looking for when I do that, to tell me its the VC needing service?

chris-m wrote: ...vibration from the front end when pulling off from a junction whilst turning right. It turned out to be the passenger side front brake caliper was partially seized and rubbing against the disk.

I suspect that's not it, because our problem sometimes manifests in a straight line, or turning left. Coincidentally, our ABS ring came adrift a couple of nights ago; some slightly scarey sound and behaviour but at least the brakes still functioned. Easily fixed by our mechanic (I suspect it may have been a hangover from the CV replacement they did, and I suspect they thought so too, as they didn't charge for sorting it out). Anyway, the graunching problem is still there afterwards...

And Neil, looking forward to finding out how you get on!

Will keep y'all posted.

If the combined speed of the front wheels is not EXACTLY the same as the combined speed
of the rear wheels then the VC has to slip. This must happen with left or right turns and
possibly less so under heavy acceleration. A sticky VC will lock up under tension and suddenly
release in a series of sharp knocks, in fact almost exactly the same symptoms you described.

If you can find an empty car park, then drive slowly on full left or right lock continuously, the
VC must slip because of the difference in wheel speeds. Normally the VC should slip smoothly
without any noise. Bear in mind that worn CV joints will produce knocks and "graunching" so
you have to discriminate between the two effects.

As Dandy says, the best way is to remove the front prop shaft and drive it to see if the noise
dissapears. I wouldn't be absolutely sure that this is the problem, but a simple turn test
should prove it one way or the other.

2sticks wrote: As Dandy says, the best way is to remove the front prop shaft and drive it to see if the noise dissapears. I wouldn't be absolutely sure that this is the problem, but a simple turn test should prove it one way or the other.

Thanks. Initially I'll give the turn test a go and see what ensues. Empty carparks aren't that hard to find here

(BTW, in case it gets to that, I assume that removing the front prop shaft is relatively easy done, and has no consequences other than not having 4wd for the duration, yes?).

Never having done it I can't say for sure, Dandy will no doubt advise. I would think it's just a matter of removing
the engine tray, undoing about 8 bolts and possibly removing a splined shaft. If you're not spanner friendly then
it may be better to get a spanner man to do it.

As far as the turn test is concerned, you have to remember that the VC has to transmit around 50% of the power
to the front wheels and requires a bit of power to cause it to slip. If you do the test at tickover speed, you should
feel the engine labouring slightly or even stall. You may have to increase the throttle opening to compensate, it's
all a matter of knowing what's going on and getting a feel for the effect. You need to use a flat dry surface to
ensure that the tyres have a good grip.

If it does turn out to be the problem, it could have been caused by a long period of inactivity. With a bit of luck, you
may find that the problem may cure itself if you spend half an hour going round in circles fairly quickly, unless of
course you're subject to giddyness.

Another test before throwing up through giddiness is to jack one of the front wheels in the air after running around in circles for a while - if the jacked up wheel revolves (unwinds) then the VC is at fault.

I've never done it but it looks just like 8 bolts secure the front propshaft to the VC.

Update: I warmed the van up and ran the circling test in both directions for several minutes apiece yesterday and heard nothing except a clear/normal-sounding faint whine when the power was on just a little. I'd say the VC is almost certainly ok and not the cause of the graunching symptom that concerns me -- which I didn't hear at all yesterday, before or after the test.

dandywarhol wrote:Another test before throwing up through giddiness is to jack one of the front wheels in the air after running around in circles for a while - if the jacked up wheel revolves (unwinds) then the VC is at fault.
I've never done it but it looks just like 8 bolts secure the front propshaft to the VC.

Thanks. I should try the jack test just to be sure.

That aside, I think my best bet just now is to see what Neil's garage reports, in case it is the same basic fault in both vehicles. And if not, maybe try the prop shaft disconnect.

Whatever it is, it clearly seems more of a worrying-noise rather than a "your driveshaft is going to fall apart at 60mph" kind of issue at this point; let's hope it stays that way.

You say you've added some special additive to the diffs, but you don't say whether you changed the diff oil first. I assume you did, but just in case you didn't......I had very similar symtoms which were cured as soon as I changed the diff oil.

Rob

RobnKathryn wrote:You say you've added some special additive to the diffs, but you don't say whether you changed the diff oil first. I assume you did, but just in case you didn't......I had very similar symtoms which were cured as soon as I changed the diff oil.

Rob

Our local mechanic did the job so I can't be sure of the precise detail. But, judging by the price of the job, they replaced all the existing diff oil with enhanced LSD diff oil.

The problem has been less noticeable in the past few days (not quite nil, but very close) so perhaps the tightening up of things they did when they fixed the minor ABS failure we had last week has helped. Or a delayed result from the diff oil change. But I'm keen to hear Neil's result, because I haven't really been able to give the van enough of a run to be sure.

Update: it's still there, though certainly seems rarer than it was.

Neil, did you have any joy getting your similar symptom sorted?