BVD

I recently saw a thread somewhere on one of these forums about a guy who had the problem of 'hunting' or surging when driving at very low speeds.

If this happened at low speeds you'd probably ending up kangarooing down the road like a 90 year old having her first driving lesson.

Unfortunately I can't find it again. When searching, I can't seem to find whatever word he used to describe it.

"Hunting" used to describe a situation whereby an engine goes into a repeated cycle of revving and dying down again - usually because of some fuel problem. Not sure if it's still used.

I seem to recall that someone suggested that this can also happen with drive by wire systems - whereby you are not directly controlling a carburettor or whatever through a direct mechanical linkage, but are just sending signals to a computer which then does its interpret/nanny thing and does what it thinks is best for you. Apparently, sometimes the computer can get a little confused, and keep rapidly changing its mind about what's needed.

Or did I misread that? Unfortunately I must have forgotten to put my mind into Gomez mode in order to actually retain what I was reading.

Anybody had any experience of this, either with their RX8 or any other car? ( I mean the hunting, not the forgetting... )

Seenitall

Try
Why does the Renesis "Lope" at idle

Tech garage forum.

I would guess that this tendency would translate to Bucking at low engine speeds. I have noticed some bucking, but only at just off idle an dlittle or no throttle opening. Past experience says that this is present in a lot of high performance engines

BVD

Thanks Seenitall,

That thread certainly does cover similar ground, but it wasn't the one that I can't seem to find again.

Interesting that you have noticed some 'bucking' at idle or little or no throttle opening. That's exactly what the guy in the post I read was experiencing, only a bit more pronounced.

It was a reply that tied it to the way that the management system responds to that situation that interested me.

One of those "Hey, I must go back and read that through properly" situations, where I didn't do it quick enough.

I must learn to pay attention the FIRST time!! :D

Choppy

Maybe it was just a cold gear box?

Regards
Phillip

Gomez

Hmmm, haven't read that post BVD. Timbo is the man if you want to know about surging on cruise control though.....he had problems way back with it. However, I don't recall him ever getting a satisfactory response back from the dealer.

Homework for Timbo: Tyre thread sticky.....updated fuel graphs....Update on cruise control problem... :D

Regards, Gomez.

timbo

Yes, mum

Actually I wasn't sure if BVD was asking about the surge effect with the cruise control, which has been well covered by me and elsewhere. Like most things mechanical, I haven't been able to replicate that with the service tech on board
But it IS there!

The 8 does "buck" a bit a low speed, but much less than my first car, the mighty (******) Hillman Minx. The solution with the 8, as it was with the Minx, is not to *****-foot too much :D I mention this all for the benefit of BVD, who will no doubt recall with pleasure the curious traits of 50's cars. (of course, you had to ***** foot the Minx quite a bit, as its brakes were the latest 'drive by jelly' technology, with the added 'which will lock up first' feature.

But I digress, and just noticed I have added nothing of value to this thread...

Back to my homework....!

BVD

On the contrary! You just made someone ( who is currently pining for the loss of his car) laugh out loud. A valuable therapeutic service!

I still have occasional dreams in which I'm driving somewhere and I press the brake pedal and nothing much happens... A throwback from all those cars of my student days when that was pretty much standard.

My brother had an early 1930s MG which had a foot pedal (short of course) and a handbrake (long vertical lever coming up from the floor) which both rotated the same bar, which in turn operated either rods or cables to the brakes (can't recall the details).

Naturally, pressing the foot pedal produced almost no effect at all, on a fairly clapped out car. Hauling on the handbrake gave much better mechanical leverage, and did slow the car slightly.

The same car also had an interesting habit whereby the steering wheel came off in your hand. I can still recall my brother handing me the wheel while he attempted to steer with the small stub of shaft (no chance) and we both hauled on the handbrake lever! :D

The wheel game off in his hands 3 times before he got round to finding the right sized bolt and clamping it on firmly.

Ah, the immortality of youth...

Gomez

labrat has inspired in me the need to do some research..so I did. Cheap, nasty research......as befits one like myself!

Went and idled in first gear up a slight incline and on the flat.....900rpm or so in first without any input on the accelerator. No, surge/stumble....DSC off, same same. DSC 1/2 off, same same. All at 7kph.

Slight increase to 2000rpm, 14kph.....no drama's. The car was up to temperature, though......and for the record, I have never experienced Timbo's cruise problem.

Gomez, Apprentice labrat.

BVD

I do love an experiment!

Might try that myself Gomez, and see what happens.

Perhaps I should point out here that my own car does not seem prone to 'loping', 'bucking', 'hunting' or 'surging' - despite its other little foibles.

It was just part of a newly provoked interest in the whole business of how computer controlled management systems run modern cars, and what quirks and limitations they might or might not have.

My knowledge more or less stops at coils, distributors, setting points with feeler gauges etc.

Time to get some 21st century information...
:p

rotarenvy

the only bucking at low speed I have felt was caused by 'lugging' the motor. not as bad as a piston motor as the rotars tend to be smooth.

Hymee

iTrader: (1)

I think the phenonemon BVD is reffering to is different to Timbo's Cruise Control surge.

How to reproduce BVD's god-dam bucking problem:

The most common place to get this situation on the road is when is a slow crawl of traffic. When you are at slow speed in 1st or 2nd gear, and to maintain the speed, you need to give a tinsy bit of throttle, just off idle. My theory is this triggers a small feedback loop that magnifies unless you are on the ball. You touch the throttle - just enough to open it slightly, and the engine make more power - acellerating the car forward. The inertial on the driver tends to hold your body back, which takes a slight bit of pressure off the pedal/throttle. That slows the car slightly, and the inertia of the driver comes closer to the front, putting more pressure on the throttle again. And the cycle repeats.

I think it only really happens when you are "pusyying around" with the pedal, as was mentioned previously.

Timbo's Cruise Surge, I have only experienced when I "Resume". I might have been set on $1.10, and had to cancel it for whatever reason, then when I resume it takes off. The furtherst I have seen it "overshoot" is up to $1.20. But if you leave it alone, it does come back town to the preset speed eventually.

Cheers,
Hymee.