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My daughter has a 1992 2.0litre Nissan Primera. When the car is slowing or comes to a halt the engine revs drop very quickly (to around +/- 500rpm) to the point that the engine is close to stalling. The engine revs then fluctuate up and down a couple of times before settling at normal idle speed. The problem is accentuated when the headlights/heater are on.

The problem doesn't appear to manifest itself when the car is stationery or if the engine is revved up or when idling and the lights/heater fan are switched on.

She has had an auto electrician do a diagnostic on the electronics but everything checked out OK. Other than this, the car goes very well with no sign of a hesitation or flat spot in acceleration etc.

The auto electrician said that it could be the air flow meter but surely this would affect the overall performance. Another person suggested the Idle Speed Control Valve may be sticking.

John

From the "Ask Jack" archives - 12 September 2009

These symptoms do resemble either an Air Flow Metre fault or a Idle Control Valve fault, so it sounds like the advise you've been given is on the right track.

I suggest the first place to look is the Idle Speed Control Valve, which predominatly helps smooth engine speed under slight load and when slowing down at lights.

The valve in question often gets clogged with carbon, and while it is controlled by the engine management computer the build up of carbon can effect its ability to operate correctly. It's usually a fairly easy process to locate remove and clean the valve. It's the cheapest place to start and there's a good chance it will fix the problem.

Failing that:

The air flow metre (or sensor) I would have thought would induce a diagnostic fault code in the engine's computer if it were to have a problem. This may not always be the case however which makes it that much trickier to diagnose. There should be test procedures to conduct on the air flow metre to confirm a fault, so a more definite diagnosis (rather than a could be this, could be that scenario) is needed here.

I would recommend taking the car to a Nissan agent, as they have probably dealt with this issue before. Expect a higher hourly charge, but they should have the background and specialised equipment to quickly pinpoint the fault.

Idle control on the road is different to idle control at standstill. With no load, the butterfly plate in the throttle allows sufficient air to bypass when it's closed to maintain a constant idle speed.

This is a setting that should not be touched once set from the factory. Generally an idle fault is just that, a fault, no adjustment will solve the problem.

Under (driving) load, additional finer control of the air bypass system is needed (idle control valve).

Hope it helps