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Matt
Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 21707
Experience:  BEng hons Mech engineering, in auto industry 22 years
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the car is a Caterham seven and is brand new the problem

Customer Question

the car is a Caterham seven and is brand new
the problem I have is not easily described.
the car will not run smoothly at low revs in any gear
in first second and third it is particulely bad the car jerkes
and lurches quite violently and is difficult to control
can you help.regards.
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 the airlfow meter signal is the main load input in to the ECU issues with this can give driveability issues as you describe
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.

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