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the_big_1
10-01-2014, 12:17 AM
Hi all,

Trying to find an answer to a fault on my S5 4.2 V8.

Around 2500rpm there is a flat spot / hesitation.

I have de-carboned the intake valves.
Changed to new NGK plugs
New air filter.

There are no error codes or warning lamps.

The car starts fine, idles fine etc.

So I am down to injectors or from what I can find, engine mounts?

Tried injector cleaner and not solved the issue.

Changing gear from 1st to 2nd feels sloppy on take up, but most seem too (Manual transmission)
Would the engine mount fault give me the flat spot without poor cold idle and no error codes?

JamesRS5
10-01-2014, 07:54 AM
Engine mounts don't cause flat spots in a torque curve so you can cross that one off your list.

How about putting it on the rolling road, this way you can monitor fueling, ignition, etc right through the rev range and pin point exactly what's going wrong.

the_big_1
10-02-2014, 12:20 AM
Hi,

Its looking like it is engine mounts, common fault.
Here is why I think it happens

Hi,

All I can think of is the following:

The motor mounts only have a solenoid in them, not a sensor. The faults can be open circuit, short circuit or overload.
Now if you don’t have any fault, then electrically its OK so this can not have an effect on engine running.
But if the mount has leaked, then the mount has less damping effect than it should. The engine ECU has no way of knowing this.
The reason for the variable mounts is to lower engine vibration frequencies at different RPM, these frequencies change as RPM change.
So I am guessing if the frequencies at say 2500 rpm are not dampened correctly, the engine knock sensor could read this as pre-ignition and retard the timing.
Pre-ignition is not a fault but if it isn’t actually happening but the ECU thinks it is, retarding the ignition could have the same symptoms but wouldn’t be registered as a fault.