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Matt
Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 21982
Experience:  BEng hons Mech engineering, in auto industry 22 years
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We have an A reg Volkswagen Fox that has had a very annoying

Resolved Question:

Poor idling and frequent stalling on VW Fox

we have an A reg Volkswagen Fox that has had a very annoying intermittent fault...every so often it will stop when idling..often at the most inappropriate place (!)..then if you wait for almost exactly 10 minutes it will always then start again...follows this regular pattern every time..have had two RAC breakdown mechanics who have looked at it an just shrugged their shoulders without a clue! help!!

Have changed all 4 ignition coils...have changed air filter...fault follows such a regular pattern..is waiting for almost exactly ten minutes before the car will restart

Submitted: 3 years ago.
Category: Car
Expert:  Matt replied 3 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.

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
Customer: replied 3 years ago.

Hi Matt

Thanks for your reply...still think that the "cooling off" period of almost exactly 10 minutes before the car will start again, is significant...is something overheating then getting the chance to cool again to start?..why would a short ten minute wait solve an air leak problem?

Expert:  Matt replied 3 years ago.
Hello

I agree that the same time period repeating might well mean its more electrical

but I've known mechanical issues to be remakably repeatable too as things cool down and expand as a set rate

so it may always take 10 minutes for the effected parts to cool down and contract
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