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Matt
Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 21378
Experience:  BEng hons Mech engineering, in auto industry 22 years
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A couple of days ago my car was idling whilst having tyres

Resolved Question:

A couple of days ago my car was idling whilst having tyres fitted and tracked I noticed it revving and going back to normal revs by itself. No-one was in it at the time. Since then, I have noticed it happening all the time, both when driving and idling. It is as though someone is "blipping" the throttle and the revs go from 600 to 1200 rpm and back. It happens several times. It is an Audi A3 1.6 petrol, from 2006. What can be causing this and how can it be fixed, please?
Submitted: 8 months ago.
Category: Car
Expert:  Matt replied 8 months 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.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

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.

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.

if the engine condition is the same then chances are the meter or the connection to it is faulty

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading

Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile

Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 21378
Experience: BEng hons Mech engineering, in auto industry 22 years
Matt and other Car Specialists are ready to help you
Customer: replied 8 months ago.
Matt, Joe here again, I was telling a neighbor how good my free trial question was, when he said my car is petrol and doesn't have an air flow meter. Is this correct?
Expert:  Matt replied 8 months ago.

Hello

on some models they used a MAP ( manifold pressure sensor) instead of a MAF

take a look near the air/ filter box if there's cylindrical pipe that exits the airbox with a connector to it then that is the MAF

and many if not most petrol engines use MAF

Customer: replied 8 months ago.
Matt, thank you, ***** ***** check that and tell my neighbor he was wrong. Joe.

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