lurching on neutral throttle

[v-lux]

Ok firstly, Mods: ive put this in Syncro as our gearbox mounts are different...

On neutral throttle, at low speeds when trying to maintain constant speed i get a lurching/bucking motion. Its kind of always been present, but has been getting worse of late. Its now at the point where its making driving the van arduous on anything other than constant speed motorway driving.

It also makes it self known during gear changes as a dull 'thump' when re applying power after lifting the clutch.

I figure there are quite a few things that could contribute and it may be a combination of multiple things. Engine mounts, gearbox mounts, worn CVs, worn driveshaft splines, slack in the gearbox itself etc etc.

Anyone got any informed advice on what might be the most likely culprit so i can start working through it?

[silverbullet]

Knackered springs in the clutch friction plate hub? Could allow oscillations to build up and cause kangarooing on a light throttle. Like a DMFW on the way out...
Do any of you chaps with hi-po diesels fit a torque arm/stabilizor to the engine or do they have one as standard, this might help as a damper to the engine.

[v-lux]

Knackered springs in the clutch friction plate hub?

I would hope not, the entire clutch set up was brand new when the engine went in a few months back.

torque arm/stabilizor

What's one of those then?

[sonic23]

Interesting, I thought this was just a quirk of DJ powered vans.....and not diesels.
Mine has always done this......and i have never got to the bottom (and boy have I tried all the obvious), other than suspecting it was old cabling that might be causing the problem....particularly between hall sender and ECU, as the ignition is always hunting a little.

Sorry....I can only offer sympathy!

Rich

[ajsimmo]

Hmmm...interesting.

Mine did exactly the same thing for a while (but only whilst prop is off - doesn't do it when 4wd). Couldn't find any reason, checked all over, had box out & checked clutch (was like nearly new) & 'box mounts, engine mounts good & solid, very little backlash in diff....then it went away again.

Knowing others have the same is (sort of) reassuring

Maybe it's a 1.9 thing...

[v-lux]

Mine still has the prop on, thought about trying it with the prop off, but couldnt see how it make any difference.

[silverbullet]

Al - a torque arm or stabilizor is a bit like a double-ended arb link, rubber bushed at each end. Transversely mounted, connected to the block at one end (usually top of block/head) and to the body shell the other by a bracket and reinforcement to prevent fatigue cracking.
The idea is that it helps to stop the torque reaction of the engine when under load (and springing back afterwards), putting uneven loads on the rubber mounts and if correctly placed can be an effective way of restraining a very torquey motor and/or one with low engine mounting points.

[v-lux]

Interesting, no i certainly dont have one of those and dont think i've ever seen one on anyone else's van.

Mind you, i'm not sure where you manage to mount one to be honest, what with the engine being over at 56 degrees. Wouldn't something like that transmit quite a bit of vibration into the body of the van also?

There have been some developments today though....

Ive been meaning for ages to reset my Injection pump timing an finally got round to it today. It needed more advance as i have bigger injectors (Mr Simpson informed me that the larger nozzles would lower injection pressure, and the pump would need more advance to compensate)
Having done this, the van drives extraordinarily different.

Lots of improvements, but most noticeably the surging has changed massively. It does still do it, but far less. Feels much easier to apply power without the effects of the bucking coming into play.

If i try to do it, i can still make the dull 'thump' when bringing the clutch up from a gearchange, but this also seems to have been affected by the pump timing change also!

[lloydy]

torque arm or stabilizor

They make something like that for beetles with T1 T4 engines
http://www.geneberg.com/cat.php?name=Tr ... s&cPath=11

[toomanytoys]

Funny how all you derv donk upgraders always blame anything else before the engine....



If there is a "lurching" its either got to be something quite mechanical in the transmission driveline (so would get worse pretty quick) or the engine loading and unloading through a missfire/missfueling scenario I would say...

My petrol 5 pot Audi 100 used to do a slight "fluff" on a light throttle, most likely down to the injectors having done 300k km (as all ignition parts replaced..).... always seemed to be a bit better after a dose of injector cleaner and a good thrash...

Yes, sloppy engine/box mounts will be responsible for clunks and bumps when going on and off the power..

[v-lux]

engine loading and unloading through a missfire/missfueling scenario I would say...

I reckon you might be right on the money there. I only did the pump timing as i knew i had to do it and was pleasantly surprised when it had an effect on the surging.
The engine performed so well otherwise that i hadnt really considered it might be a culprit. You live and learn!

The 'Thump' however, i think probably is to do with engine and or gearbox mounts, it feels 'mechanical' when it happens, trasmits through the chassis.

[HarryMann]

Knackered springs in the clutch friction plate hub? Could allow oscillations to build up and cause kangarooing on a light throttle. Like a DMFW on the way out...

Been suggested before for the bang/thwack/thump/clunk problem before... will find out on mine in a day or two, when new clutch goes in, don'texpect any revelations though

Do any of you chaps with hi-power diesels fit a torque arm/stabilizer to the engine or do they have one as standard, this might help as a damper to the engine.

Been considering one of those to inhibit prop vibes and or wind-up torque... really a damper is needed rather than a rubber mount, a bit like some timing belt strut dampers.

====

Al, I expect like me you could change every engine & gbox mount to clear that thump, but rather than write a tome, maybe we have a chat sometime, to cover all angles.

My thoughts on wind-up were engine/ throttle response rather than transmission.. the only case would be excessive play say in a totally shot gearbox or diff, which you don't have... Quite a few diesels have this throttle control problems, drove an Astra 1.7D lately, great car, but has this problem at 'very' light throttles... almost frightening, probably susceptible to absolutely micro amounts of fuel and timing mismatch.
Call us sometime..?

PS. I also call this problem 'shuttling', or neutral throttle instability. If we could go back about 8 years to the early Red/Black ezBoard 80-90 forum, you'd find a thread on 'shuttling on a neutral throttle' in the Syncro section (containing about 1 post a week) - times change unfortunately ezBoard was monumentally hacked and we lost 90% of everything