Jerky travel - why?

[duplicity]

I am new to this TVR thing, got a '97 4.2 Cerbera 2 months ago now. One thing I have noticed is that when traveling at a constant speed, say 40mph, the car doesn't travel along smoothly. It jerks slightly, almost as if it was going to stall or run out of fuel. Is this a fault or just one of those things, any ideas?

[joospeed]

they all do that, sir.

[mike_e]

Only jerks slightly? Sounds about right to me. Try running on Shell Optimax for a while, it seems make a slight improvement.

[anonymous-user]

Or, if its bad, try looking at the throttle pots / potentiometers.

Cerbera's go through a lot of these !
(Don't worry, its not expensive)

[nick_is_home]

I get this aswell!!! Only when going at constant speed aswell. If i accelerate or decelerate just a little it goes away.

Seems to be a bit of noise from the diff area when the jerking occurs could it be the diff that is causing the jerking?

Do I need a new diff!!

Thanks guys
Nick

[joospeed]

it's all down to those pesky green lobbyists. you've got catalysts in your exhausts to bring down those harmful gas thingamebobs, but they get angry if they aren't fed a proper diet of burnt gases so you have a couple of lambda sensors which monitor the situation and tell the ecu to alter the fuel to keep those cats happy. unfortunately the mixture required to keep the cats happy isn't rich enough to make the engine smooth, so in order to satisfy the green people we have to put up with cerberas that jerk around. Pah! If you could run the mixture a bit richer all would be sweet with the world again.

[duplicity]

So if I get my car Decatted will the problem cease? Would the lamda sensors still work with Cat bypass pipes? I have an MOT coming up, but once its got through Decatting would be very tempting......

[p7ulg]

duplicity said: So if I get my car Decatted will the problem cease? ....

Probably not but it will sound great!

[nick_is_home]

What do you guys recon about my diff question below??

Joospeed what do you think?

Any advice appreciated....

L8rz
Nick

[gadget9]

Hi just to add to what bandit has already said i have just had my 1000 mile service on my 4.5 and one of the warrenty jobs i gave them was the car was behaving as yours is and i use optimax fuel and as bandit said it was a throttle pot, they replaced it and it is as smooth you could wish for now.
I was told that leads breaking down can cause the same effect though?

[joospeed]

leads are a bugger on them, that's why for a few pounds extra over the tvr stuff you can have magnecor ones fitted which are great.
decat pipes .. if anything these tend to exagerrate the lumpiness at 1800 / 2200 revs. you still have the lambdas in there so no chance to up the fuelling unless you tell the ecu to target a higher output voltage or set the ecu to disregard them altogether.