Shunting in a higher gear at low speeds

[Taxi King]

I have heard this is a common on the Chimp 4.0l and was wondering whether anyone has a fix or could give me an opinion on what i think might fix it.

Is it worth cleaning the stepper motor, checking or replacing the potentiometer?

My brother has a 1986 Rover SD1 which has a Rovercraft 4.5L lump in it and fitted with a MoTec programable fuel/ignition system, he has a similar problem and his investigations has led him to the potentiometer. He is finding it really hard to buy a replacement as Rovercraft don't exist anymore.

The Potentiometer on the Rovercraft engine is much bigger than on the TVR so would anyone have any idea who to contact for a replacement.

The Rovercraft engine sends his car up the road at an incredible rate of knots - he thinks it is about 380BHP at the flywheel. Anyone else heard of Rovercraft?

[jr6yam]

My personal opinion is that the 'shunting' problems (on standard cars)
are down to tickover control, so yes cleaning the stepper motor may help.
BUT, there are a lot of variables in the tickover control system, so there
may be other issues.
However, get a rock steady 900RPM and the shunting disappears

John

[Taxi King]

Cheers John

[The Dodger]

jr6yam said:

However, get a rock steady 900RPM and the shunting disappears

John

Not true. Mine idles perfectly but still has the low speed 'shunting'. I'm quite sure it's fuelling-related, either the ECU programme or maybe dirty injectors. As mine has been the same for the past 8 years I'm inclined to think it's the ECU fuel map. I now just drive around the problem.

[CarlosV8]

In reply to your SD1 query, is the car still using a flapper air flow meter? If so sounds like you've got the standard SD1 throttle pot, which is a common cause of problems now the cars are getting on a bit. They're easy enough to test and adjust, assuming it uses the standard Rover settings.

If you need a replacement they are generally listed as NLA, but you can get a Jag version which is exactly the same for around £80. Alternatively you can make an adaptor plate and buy a generic throttle pot from someone like RS Components.

[Taxi King]

There is no air flow meter, its all taken care of by the Motec system and a sensor off of a Vauxhall Vectra bolted to the panel under the windscreen but inside the engine bay.

BTW - the trumpet into the plennum chamber is twice the size of the TVR Chimp one. It gets so much air - I think thats where it gets the increased power from.

[Pupp]

If the engine has been breathed on but still using a single plenum, the chances are it's shunting due to having a cam with long overlap. Drive round it or fit throttle bodies to properly fix, although it can be masked to some degree by reducing advance and adding air in the afflicted rev range. Will still stumble 'though due to charge robbing and the confused signal in the plenum.

Afraid I don't believe the power claim

[Pupp]

Taxi King said:

There is no air flow meter, its all taken care of by the Motec system and a sensor off of a Vauxhall Vectra bolted to the panel under the windscreen but inside the engine bay.

BTW - the trumpet into the plennum chamber is twice the size of the TVR Chimp one. It gets so much air - I think thats where it gets the increased power from.

Best have some pictures of this lot then please

[SILICONE KID]

I had shunting problems on my 5l.Now the megasquirt is fitted i can drive in 4th gear at 25mph with zero shunting

[spend]

Is it only me that knows what he will have posted even before reading it

[SILICONE KID]

Well that is the cure for shunting .... Fuel and wasted spark and you know it !

[spend]

You should review some of your old posts

[Number 7]

Taxi King said:

IThe Rovercraft engine sends his car up the road at an incredible rate of knots - he thinks it is about 380BHP at the flywheel. Anyone else heard of Rovercraft?

Yes, Rovercraft used to be just outside Maidstone. They built the BV heads and installed Motec on my car about 10 years ago. Unfortunately, Nick left the business completely (and his brother was killed at the Brighton Speed Trials a few years earlier driving a car powered by one of their engines). The engineering side was bought out by a chap who still operates outside Maidstone, but really concentrates on machining.

No idea on the TPS though - I've often wondered what to do for a replacement should mine start to play up.

7.

[SILICONE KID]

That was a faulty lambda controller..how come i know what your going to say before you have said it tt

[domV8]

SILICONE KID said:

Well that is the cure for shunting .... Fuel and wasted spark and you know it !

...in your case, but not in mine.

Don't think it's the be-all-and-end-all cure, Daz...

Dom

[Meatball]

One of these ? http://www.parkersofcroydon.co.uk/carsforsale.php?...
You should try these guys theres lots of guys running SD1`s on there http://www.v8forum.co.uk/forum/

[blitzracing]

There is an inherent issue with running a plenum chamber, log duration cam and catalyst fuel map, where multiple factors combine to produce the effect. You can overcome in some cases by making the mixture richer at low RPM, but the Stock ECU just forces it back to Lambda 1 to keep the emmisions low, so its not easy. The non catalys fuel map is much better, but running that can cook the catalysts, it will fail an MOT on emmisions, and the stock TVR catalyst chip has only a Range Rover non cat map, thats not really suitible for TVR use. Personally I think it should be possible to overide the catalyt fuelling map just at the point it shunts, but let it run as should be the rest of the time. Im currently working on making the low cost chinese Bosch AFM replicas work as a plug and play replacement for the lucas 5AM with better breathing and the added benefit of being able to tweek the fuelling to reduce shunting. I have a prototype set up running on my car at the moment, and its stopped all the shunting, and Im currently making 2 more test units, but its early days. If there is anyone near Hungerford / Newbury who has shunting and wants to be a test car, please drop me a mail.

[penno]

I tend to drive around in third much better at low speeds it sounds better and doesnt shunt at all. I think really some people are driving these cars as a modern turdo diesel putting it in 5th at 30mph and wondering why it shunts. I'm sure there was an article in an old sprint pointing out the best way to use the gears.

[Pupp]

Must admit I do wonder at the logic of throwing thousands at a car to 'tune' it and then drive around completely outside the sweet spot that your hard-earned has resulted in. Suppose some of these guys are also lugging boilers and copper pipe in 'em. Makes as much sense

[Taxi King]

I have an update on this topic for you all.

My mechanic suggested the first thing to do was to change the fuel. I have been filling the car with super unleaded, usually from Esso, he suggested trying Shell V Power so I ran the tank low and refilled with the Shell fuel. I never would have thought that fuel producers would vary the quality of fuel, I can understand there being a difference between Rugular and Super but not between Esso Super and Shell V Power. Obviously, my daily driver runs happily on any fuel.

Hey presto, the shunting has all but disappeared. He is going to clean the stepper motor and run some injector cleaner through the fuel system which i hope may kill the shunting completely and I am going to ask him to check the diff bolts just in case.

Thanks for the history on what happened to Rovercraft. I will let you know the results and we are going to try the same fuel swap on the Rovercraft engine to see what difference it makes in that beastie.