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QUATTR0

Jerking under full throttle

Hi. I am new to the forum, but I hope that you may be able to help. We have a few of the old school and would appreciate any suggestions regarding our '94 100 S4.

We get fairly violent jerking at full throttle at the upper end of the rev range that has been pretty resistent to our attempts to fix it. She has had: both voltage amplifiers, N71, Engine temp sens, Hall sens, wastegate solenoid, fuel pump, and both knock sensors. All were brand new VAG. We have had her many years and she gets full regular servicing with all plugs, filters (inc.fuel) etc. The car itself is pretty original, the main changes being a new VAG RS2 exhaust manifold (when the original cracked) and stainless pipes. Replacement ECU made no difference, nor did changing the bank of coil packs with our Ur S6.

This part-throttle existence is not good.....

Greg
QUATTR0

When she is good...

Glossglockner Pass, Austria

Click to see full size image
5potmad

If it holds back when going full throttle it could be a boost leak putting it into limp mode
dest

Violent holding back will definitly be a boost leak
5potmad

More photos aswell    
Contigo

It's not the first time I've jerked under full throttle! Ooo Er!
QUATTR0

Where would be the most likely source? The pipework has been checked out.
dest

Have you just visually checked the hoses, or done a leak test?
The tiniest of leaks will cause boost cut!
Remove and reseat all boost hoses, use new good quality clips too
QUATTR0

Col de la Bonnette, France
5potmad

Any of the lines that carry boost can be the culprit. best thing is a boost pressure test to find out the source of the leak
QUATTR0

Thanks guys. I will investigate the pipework again. Now where was that magnifing glass...
5potmad

^^^^^^^^^^^

if you need a hand give me a shout

............post pics off all the cars in the "members rides" section
Ben H

MAF?
quattrokid

shit fuel ?
QUATTR0

New MAF and runs almost exclusively on V-Power. Occassionally BP 97.

Greg
chapel

hmm.
How is the car modified?
I saw the RS2 manifold, but do you have any ECU upgrades or fueling upgrades?

What kind of spark plugs are you running?

I would suggest looking for a boost leak first though.
Have you also tried replacing the N75 Wastegate Frequency Valve?

Do you have a boost gauge? do you know how much boost the turbo is making?
QUATTR0

Hi Chapel, thanks for your thoughts. Good to see another UrS6 joining the forum. The only modifications are a new VAG RS2 manifold and a bespoke stainless exhaust without cats. Ohh.. and the vinyl. Standard ECU, plugs, and V-Power fuel.

Just spoke to the garage today. They couldn't find any boost leaks and the wastegate was doing its job properly. They think that it is the turbo. Oil consumption is well within normal limits, but I do think that the turbo is making a bit of noise.

We only have the standard boost guage, but it does register up to 2.1 bar under full enjoyment.

G
chapel

when you say 'standard boost gauge' do you mean the digital gauge in the MultiFunction Display?
if so, you're running 1.1bar (the gauge runs in absolute numbers)

so, that's about 16psi of boost.
That's about right depending on your altitude.

when's the last time you swapped your spark plugs.
martinjs

QUATTR0 wrote:
Just spoke to the garage today. They couldn't find any boost leaks


Did they take all the hoses off and inpsect them properly? especially the accordian hose from the crossmember to the bottom of the i/c, they split under the wire reinforcing rings so can only be found when the hose is of the car. BTDT
Have you checked for fault codes?
QUATTR0

Looks like it may be cured! We have just had the turbo refurbished (by Universal Turbos) and replaced the pipe running from the MAF and we are no longer getting the jerking! Red-line... how I have missed you  
DamianS4

could be a bit left field but could it be the injectors maxing out?
LunarWolf

Check the hoses and their fitments first.
That happened to mine. Luckily, it wasn't a 400 turbo problem but a 17 hosing deterioration!
Surprising how sluggish it was without a turbo though, huh?
noodles

Without the turbo working, makes a huge difference. Glad it was just a hose, and you didn't get any other damage.
ANDY S2

The throttle body hose has a tendency to split and it's quite difficult to see unless you pressurise the system or remove he hose ,it splits were the jubilee/ hose clip seats .my Audi S2 suffered this I'm not sure if its a similar set up though .
ANDY S2

Wtf this was last year lol.
noodles

I know the TBH splits, the standard ones generally do. Mine on the S4 looked perfect until i took it off, and looked at it, and then you could see two splits in it by the clips.
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