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Matt, Engineer
Category: VW
Satisfied Customers: 20361
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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My grandsons 1998 VW Jetta 2.0 litre will start, idle and

Customer Question

My grandsons 1998 VW Jetta 2.0 litre will start, idle and accelerate briefly then cut out. To keep the engine running I must lift the accelerator pedal and then depress again for the engine to run again briefly. I am a retired heavy equipment mechanic and do posses a code scanner but don't have the codes and haven't tried to read them. I assume the port is under the steering column. I suspect a defective sensorsomewhere since the car sat a great deal of time before we attempted to resurect it and it ran ok when I put it in storage. Do you have any suggestions as to where to start. Fresh fuel and no mouse nests found.
JA: Has the vehicle turned into a gas hog? And how does it start -- same as usual?
Customer: Thanks.
JA: Are you fixing your Jetta yourself? What have you tried so far?
Customer: I tried cursing. Actually I found intake hoses off so no MAP info I assume. Installed hose and it at least would keep running at idle.
JA: Anything else you want the mechanic to know before I connect you?
Customer: I can get codes but it will be later this morning. It would only fire when starter was spinning initially until I found intake hose disconnected. Then it would run and accelerate before cutting out. I think fuel is being cut off by computer due to failed sensor but don't know which one. Thank you.
Submitted: 15 days ago.
Category: VW
Expert:  Matt replied 15 days ago.


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 idle speed control motor / valve 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

Expert:  Matt replied 13 days ago.


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