Sunday, July 22, 2012

Fuel Injection Problem And RPM suddenly increases

Starting Problems on VW.

VEHICLE: Volkswagen Passat 1999 Year model.

Engine: 1.6L.

PROBLEM: The Rpm suddenly increases to 3500 to 4000.
The issue noticed when engine gets warm.Also fuel injection issue.

QUESTION: Why engine runs lean,then runs rough also why there are no error codes stored in ECU or no check engine light.And how to troubleshoot this issue?


There are few basic possibilities that can cause such problem.All needs to be inspected.I will explain the problem and troubleshooting possibilities in details.

The check engine light comes on when problem is electrical.but if the issue is mechanical then many a times no check engine light will come up and no error codes will be stored in ECU.

The issue can be related to air leak after the airflow meter/.
If any air dragged in here then it will not noticed  by the ECU and so not compensated for and this 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.
I suggest you to inspect and properly check the hose clips for tightness and inspect the hoses for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.
Many local garage mechanics try one simple procedure to locate the leak.I will mention the procedure below.

Its one of the best way to locate a leak.For that start the engine,let it  run and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Work your way through each possible joint one at a time and you should find it.Don't spray on all hoses and vacuum lines together,spray one at a time.If the vacuum line and hose area is difficult to reach then Some mechanics use 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,.

check the wiring and connector to the airflow meter.If there is any signs of corrosion or damage. Then replace it.
Also try these to confirm problem with airflow meter.for that 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.

Air leaks problem which is mentioned above are very temperature dependent as gaps can open or close up as things expand with heat, so the weather and engine temperature can effect them.

This leads them to be quite intermittent in the case of mild leaks.
Because the hoses and vacuum lines expands when engine gets warm and contracts when its cold.If the vacuum line or hoses are worn out.Then a slight crack gets created in them.And when engine gets warm the rubber lines expands and the leak issue is noticed.

One more thing you can try is clean the airflow meter by removing it from the car and spraying the exposed sensor wires inside the tube with electronic parts cleaner - ensure that the cleaner is one of the old fashioned, non Eco type that does not leave a residue.

On some cases the problem can be sensors or the issue with wiring to the sensors causing this.
Most possible can be :---

  • Airflow meter
  • Crank position sensor
  • Cam position sensor
  • O2 sensor
  • Throttle position sensor

Check this sensors and its wiring one by one.If its worn out or damaged then it has to be replaced.

Other then this sensors also check the resistance of the spark plug leads ( 13-15Kohm / metre) and replace if any doubt or high resistance.

These are the possibilities that can cause the problem you noticed.
All the parts cannot be faulty at once.But proper inspection and testing will let you make confirmation about which is faulty and so you can continue parts replacement in that direction.

This details will help.

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