Shunting

[Steve P]

Good evening,

The car is experiencing shunting to a point where it is almost un-driveable in urban areas. On the motorway its fine. While under acceleration it is fine. But get under 40mph in any gear 4th, 3rd, 2nd, 1st when coasting or coming off the gas shunting occurs at every opportunity.

 

Now I have been on a long term investigation into this matter. I had a rear wheel bearing gone, a whining diff and input bearing on the gearbox was shot. So I've fitted new rear drive shafts, the gearbox has been rebuilt, overdrive overhauled and diff has been rebuilt. The diff mountings are solid and the propshaft has no play at all.

 

The only area where there is play in the drive train is the diff. My thoughts are that the shunting is symptomatic of play in the drive train.

 

In the past I've been told by an expert in Cheshire "mate they all do that". Now I don't have experience of being driven in or driving an other TR6 but I have seen comments on this forum and in particular one member posted that after fitting new drive shafts and a new diff:

 

"Result , excellent !
Nice smooth drive, no clonks or other noises."

 

And the other issue is diff noise. I've had this diff rebuilt twice the second under warranty but the result is the same a whiny diff. each time a new crown wheel was fitted so not cheap.

 

So I don't know what to. Any advise will be appreciated.

 

Thanks

Steve

[MostEasterlySteve]

Are you confident the throttles are in balance? They can be a pig to drive on small throttle openings if the balance is out.

[elclem1]

Clutch release bearing sleeve.

[elclem1]

Or tight throttle cable?

[Steves_TR6]

It sounds like engine to me, as above a throttle issue?

[Steve P]

Good evening Steve, I have EFi installed with one throttle at the manifold inlet.

 

Mr Elclem1,

Can you elaborate please on why the two points you mention might result in shunting?

 

Thanks

Steve

[Steve P]

Hi guys,

Interesting that a couple of issues regarding the throttle have been raised. So might the issue be that the throttle is allowing air past kind off slightly revving the engine?

 

Steve

[elclem1]

I have a friend who had a shunting problem, e.g. He touched the throttle and the car would kangaroo but just in 1st 2nd he couldn't poodle along.

[elclem1]

Sticking release bearing sleeve results in letting out the clutch and the car shunting or snatching, for example you can't ride the clutch it's on or off.

[Steve P]

Clutch is fine. It can be rode all day long

 

So it could be the fueling at no throttle?

 

Steve

[Steves_TR6]

Interesting re efi.

 

Do you have a throttle position sensor?

 

If so these can cause hunting if badly adjusted or failing, had it on a ducati once.

 

Steve

[elclem1]

That could be it over fueling.

[Steve P]

Yes a TPS is installed.

 

At the moment the idle settings are not good. So it looks like i should do some work in that area.

 

Thanks guys

[Nick Jones]

Shunting could be caused by "lean hitch" from the engine which is most likely in the driving areas you mention. This could be just down to it being mapped very lean at high manifold vacuum/small throttle openings or, if you do have a separate throttle per cylinder, poor throttle balance.

 

There may also be backlash in the drive train, but it normally takes something to set shunt off and if the engine is not running clean that will do it. My Vitesse does it at very small throttle openings up to 1800 rpm or so because I have a rather extreme cam and a single throttle body.

 

Nick

[Spit_2.5PI]

Lean Hitch - is that what it's called? Thanks Nick.

 

After years of my Spit PI running very rich (low 20's mpg & mid 20's on M'way - very sooty plugs) I bit the bullet and adjusted the metering unit. Turning the largest ring to lean off right through the vacuum range, then slightly richer so it responds to blipping the throttle. Now the exhaust doesn't reek of unburnt petrol. This is only last Saturday/Monday so I'm still assessing the results. Now when I pull out the choke fractionally I see a rise in idle rpm when warming up and a fall when hot. Previously it never needed choke except to start.

 

However, I now get a little bit of shunting as described at parking speeds - or Reading/Wokingham traffic speeds these days. I'll try the choke trick next time it does this.

 

Cheers, Richard

[peejay4A]

At low speeds and throttle off overrun it's the small adjuster that has control especially if you have banging in the exhaust on overrun. Richen it a touch on that one and see if that helps.

[Ragtag]

Do you have any form of Air Idle Control valve fitted? Shunting in SAAB 900s which have single point EFI is almost always down to this sticking.

[Peter Cobbold]

My single butterfly is about four feet from the ivs, and the engine runs smooooothe (almost all the time). The difference is the whole length of the indcution path including the plenum chamber is full of mixture. With injectors most of the plenum chamber after the TB will be filled with air until fuel is injected. This will be fine at faster rpm. But at lower rpm there will be enough time for fuel vapour to mix backwards into the incoming fresh air in the plenum. With bigger squirts of fuel this wont matter, the mixture although leaned will still burn. But at small throttle openings at cruise or over-run and small fuel squirts the dilution may well be making the mixture too lean to burn. So the mixture entering the ivs can go very lean: Nick's 'lean hitch'.

The mixture will go leanest when the throttle is first closed, then richens as the plenum space before the injector gradually builds fuel content, restoring the mixture to acceptable. This allows engine to speed up but that pulls in more air and the mixture leans again: hence the oscillatory 'shunting'.

Some engines given over-lean mixture will misfire one stroke and then fire the next: 'eight stroking'.

Peter

Edited October 14, 2016 by Peter Cobbold

[Nick Jones]

The hitchiness I get is purely self inflicted and the consequence of running a 300º duration cam (on the inlet side at least) with a plenum manifold and single throttle. The exact same system was flawless with the OE 270º cam. It's purely a very light throttle + low rpm phenomenon - the engine will pull cleanly from 900 rpm with more throttle. I've spent alot of time trying to tune it out (moderated but not gone) and presume it's a reversion effect. Separate throttles would sort it. Sequential injection (rather than current batch-fire) might help. I should have picked a less aggressive cam! It's only a problem in traffic.......

 

Nick

[GT6M]

re TPS, just cos its on TPS 1, DONT mean its actually on /in TPS 1

 

baffled, well look at it this way, there a top an a bottom to a TPS settin

if the TPS is set at TPS 1, and its at lower end of its scale {which cant be seen } then fuel settings on v v low

throttles will be effected, same too if its the other way, to high.

 

how to fin oot if its adjusted reet is to alter the TPS read oot,

say its reading TPS 20, and 1 clik, alters it to TPS 21, then its at top of scale

Butt, if 2 clicks is needed to alter the settin frae 20 to 19 then its deffo at top end of its scale.

1 click will have no effect on TPS , BUTT will ev an effect of the VE MAP

 

Also, the TPS will alter wid heat, esp at idle for long periods

when it alters, there a change in fuel load on the VE Map, of 7 on mine

7 is alott of fuel to either put in or tek oot at idle /off idle speeds

can be seen by the auto idle adding or tekn fuel away.

NOTE, VE map values are different for engin running,or just wid IGN on by about 4

 

this is enough t,mek yer engine run ruff {hitchy as Nick puts it } at low throttles /speeds

 

if not on auto idle, then if the setting is too low,it,ll run weak

too high,it,ll run richer.

So need t,have a look at yer TPS window, alter it, see what effect it has on the TPS readout for the VE map or read oot in parameters.

or if on a separate screen on there,

 

My VE read oot, and TPS read oot screen

meb,e of some help

 

Ohh, a clonk is generally too much play in the planets when on over run at lower speeds.

can be seen by prop turning a long way befoer it teks upplay

 

M

 

 

[Steve P]

Morning gents,

Thanks for all the input. I recently fitted a fast road cam and didn't have time to get the system tuned in. I had a problem with idle and reduced the VE which helped but I guess that has added to the shunting effect. Anyhow all the comments posted have given me ideas of what to try.

 

P.s. it's also pinking a bit. Do I advance or retard?

 

 

Thanks Steve

Edited October 15, 2016 by Steve P

[hot-growler]

reading this with fascination as i have a peugeot 205gti 1.6 that does this and is a real bitch to drive in start stop traffic. thankfully my TR6 is very well mannered under all driving conditions. I know this doesn't help, but felt the need to comment. Hope it gets sorted Steve, Steve.

[Peter Cobbold]

Morning gents,

Thanks for all the input. I recently fitted a fast road cam and didn't have time to get the system tuned in. I had a problem with idle and reduced the VE which helped but I guess that has added to the shunting effect. Anyhow all the comments posted have given me ideas of what to try.

 

P.s. it's also pinking a bit. Do I advance or retard?

 

 

Thanks Steve

Steve, Retard to kill pinking. A couple of crank degrees at a time. Peter

[Steve P]

Hi Peter,

I re-set the timing which sorted the pinking.

 

Neil,

I set the VE to 100 in the area of the table where coasting occurs. It has helped a lot. But it still wants to shunt while in 4th, 3rd gear under 30mph coasting. Also wants to shunt in 2nd gear slow corners. I'm not sure what going over 100% VE will do. Perhaps I will look a reducing the AFR is that area as well.

 

Cheers

Steve

[Spit_2.5PI]

At low speeds and throttle off overrun it's the small adjuster that has control especially if you have banging in the exhaust on overrun. Richen it a touch on that one and see if that helps.

 

Thanks Pete. Yes, adjust A3 for minimum fuelling. I adjusted A1 last week (got my Lucas manual next to me today!) reasoning it was rich right through the vacuum range. After thinking I would try the choke trick, it wouldn't shunt yesterday afternoon or today's run. It was only slight before.

 

Sorry Steve, didn't intend hijacking your thread, but thought it interesting that I was experiencing a mild version of what you had, albeit on a PI, not an EFI.

 

Cheers, Richard