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What is it that causes an EFI engine to hunt at idle once modified and yet carburetor engines don't.
Well let's start with a carbied engine. First of all in the carburetor itself the jets are fixed and if the revs were to fluctuate, the amount of fuel variation would be minimal, and secondly inside the distributor the advance weights wouldn't be spinning fast enough to throw out and advance the timing. If either the fuel or timing were to fluctuate this would cause the engine speed to fluctuate.
This is what is happening in an EFI engine when it is hunting up and down at idle. This usually happens after a change to a bigger cam and the manifold vacuum has decreased.
Note that in an EFI engine we talk in manifold (absolute) pressure (MAP) which reads opposite to vacuum ie Zero vacuum would be maximum pressure which is usually 100kpa which also is usually full noise.
Let's say with the stock cam the engine used to idle at approx 40 kpa at 700 rpm but now with the bigger cam its trying to idle at 60 kpa .The standard calibration would inject more fuel at 60 kpa than 40 kpa in the Fuel Map and this would increase engine speed (unless it was too rich) but as soon as it begins to pick up speed beyond the programmed idle speed of 700 rpm the Idle Air Control motor (IAC) tries to reduce excess speed and this then reduces load and manifold pressure to say 30 kpa as motor slows down leaning it out at same time, but then as motor slows down and begins to labour the load and manifold pressure increases to say 90kpa injecting heaps of fuel and now rpm is at say 400rpm. If the engine was lucky enough not to stall it would now rocket up to say 1000 rpm. Meanwhile the Ignition Advance map is also changing at all these load points and by this stage the engine doesn't know what to do.
Have a look at the 3D Fuel map and check out the different heights of the graph at the different load points
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