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Matt
Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 21876
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I have problem with a 09 VWFS polo revving between gear changes

Customer Question

I have problem with a 09 VWFS polo revving between gear changes
When the car is jacked up and run up the gears it does not over Rev
When revving at 4000rpm touching no pedals ,hit the brakes and it will reduce
It only over revs when over 20 mph
I free wheeled down a hill and when it got over 20 mph it started revving unaided
Any information would be appreciated
Submitted: 2 years ago.
Category: Car
Expert:  Matt replied 2 years ago.
Hello

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Might also be worth cleaning out the throttle body as these get clogged up with carbon and some brake cleaner washed through helps free things off.

Customer: replied 2 years ago.
Why would the car start over revving while freewheeling only when the speed reaches 20 mph, I tried this more than once.
Why would the revs drop when jabbing the brake also while freewheeling
It would also be drawing air when I ran the car in gear and it did not over rev then.
Expert:  Matt replied 2 years ago.
Hello

the effect of an airleak are very load dependent so when you're actually running the car on the road in gear the engine is drawing a lot more air thatn when its freewheeling

so when its freewheeling the revs are more likley to hang up due to a air / vacuum leak

its a cheap and easy thing to test and repair so well worth a try
Customer: replied 2 years ago.
Thanks I will give it a go tomorrow
Expert:  Matt replied 2 years ago.
no problem

just get back to me on this post
Expert:  Matt replied 2 years ago.

hi

do you still need help with this?