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Matt
Matt, Engineer
Category: Car
Satisfied Customers: 20612
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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Car jumps and jerks on start seems to run ok when up to

Customer Question

car jumps and jerks on start seems to run ok when up to speed
Submitted: 1 year ago.
Category: Car
Expert:  Matt replied 1 year ago.

Hello

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by theECU 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 enginerunning.

Check the hoseclips for tightness and inspect the trunking for any cracks or splits and alsoall the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way tolocate 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 sparkto set it off and I’ve used this method for many years without incident.

Work your waythrough each possible joint one at a time and you should find it. I use aslightly flattened piece of brake pipe and some rubber hose from the can oflighter gas to provide a spraying 'wand' and allow a direct blast of gas intoeach 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 ofcorrosion 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

Air leaks are verytemperature 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 tobe quite intermittent in the case of mild leaks

its also worth cleaning the airflow meter by removing it from the car and spraying the exposed sensor wires inside the tube with a brake or switch cleaner - ensure that the cleaner is one of the old fashioned, non Eco type that does not leave a residue

On no accounttouch the sensor wires with anything physical as they are extremely fragile

Expert:  Matt replied 1 year ago.

Hi

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