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FC Disease

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    Grampa Spec Cockhead

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Hands up out there how many of you guys own high powered imports with a Japanese “straight fit” APEXi Power FC ECU running the show. I bet there are a few of you, and I also bet that when I ask how many experience surge and stalling problems, that there would be a few more with their mitts pointed skyward.

A frustrating and totally bothersome experience, the syndrome known as FC Disease causes an otherwise perfectly tuned car to come back to idle with the revs surging anywhere between 500 and 1500rpm, black fuel smoke pouring out the exhaust, and if you aren’t that lucky, stalling completely.
A real pain when you are pulling up at the lights, or trying to negotiate a tricky traffic manoeuvre, the cars affected by FC Disease can be more of a chore to drive than you first might think.

What is FC Disease?
This is no official name for the problem, but rather a colloquial expression born of several different tuners’ frustration with the problem over the years.
As you are already aware, poor idle quality and stalling are the symptoms of FC Disease, but the real cause is fairly complicated and comes back to the overall design of the Nissan fuel injection system more than anything else.
Consider the humble Nissan AFM (Air Flow Meter) for a moment. That small (or in some cases big) device that sits in front of the turbocharger, measuring the incoming air stream to the engine and letting the ECU know exactly how much air and fuel to inject into the engine.
An extremely sensitive piece of equipment when it comes to measuring airflow, the AFM contains a tiny hot wire that is cooled down when air passes by it, and in turn changes resistance and therefore output voltage to signal the ECU how much air the engine is swallowing.
With a dead stock factory car, the AFM is carefully placed by the factory so that it should never suffer any interrupted airflow that could possibly cause any issues.
The real problems come with aftermarket turbochargers, low restriction inlets, and different BOV’s, which all have one thing in common, that being they alter the amount of air reversion coming out the inlet of the turbocharger.
Snap the throttle shut very quickly on a modified turbo engine, particularly one with a big power capable compressor, and a weird oscillation effect can be set off in front of the turbocharger.
Air can quite literally pulse in and out of the compressor inlet like an invisible spring, and that’s why you can sometimes hear that “ch-ch-ch-chew” when getting off the throttle.
This bouncing causes the AFM to report sudden changes in load, alternating high and low constantly, which in turn makes the ECU add and subtract fuel giving an alternating rich and lean mixture, upsetting idle stability and possibly causing a stalling condition.
After several oscillations, the air will slowly stop bouncing back and forth, and if you are lucky, the engine’s idle will return to normal.

How Do You Know If Your Import Has FC Disease?
There is a simple test procedure you can follow to determine whether your import has the dreaded FC Disease. It’s as simple as getting the engine to operating temperature, giving it a free rev to around 4000-5000rpm, stepping off the throttle and watching how well it returns to idle.
An acceptable fluctuation is maybe one oscillation of the idle speed, anything worse and your car has the dreaded FC Disease.
For those of you that have a hand controller for your Power FC, the causes of the poor idle from an FC Disease equipped car will be easy to see. Using Monitor mode and then selecting Map Tracer you will be able to watch the load signal for the engine bounce up and down anywhere between full and half load as the issue is occurring.

The Jerry Rig
There are several different cures for FC Disease. The first, simplest and most effective method is to go for the quick and dirty approach known as the jerry rig.
Using the Map Tracer function and Ghost Trace (turned on by pressing the next button when in the Map Tracer screen), you can see how far up and down the AFM signal is bouncing in the first RPM column. It sounds hard, but it’s actually quite simple.
When you have identified these key load points, all you need to do is halve the values of their load points. Just don’t modify anything below load five, or you may in fact cause driveability problems.
This approach stops the engine over fuelling when the air bounces back and forth at idle speed, and while not fixing FC Disease 100%, can in fact make the car behave in an acceptable and much more manageable fashion.

While the whole concept of FC Disease is something quite new to most Aussie tuners, it’s a well known syndrome in Japan, where HKS came up with a unique device known as the EIDS (Electronic Idle Stabiliser).
Comprising of a little black box that is wired into the ECU, the EIDS would lock the AFM voltage for a user adjustable period of time once you stepped off the throttle.
The whole concept of the EIDS is quite sound, but the reality is that getting an EIDS working correctly is a real balancing act that requires some patience and a fair bit of testing and tuning.
Coming in at just over $300.00 for an EIDS, they represent reasonable value if your situation is a bad one and can’t be fixed easily with the jerry rig approach.

The Plumber
While FC Disease is a pretty cool tag for the more technical ailment of AFM reversion, the reality is that it’s not actually very fair to the Power FC management system.
Relying on an AFM for its major input, the ECU is only doing what it’s being told by the AFM, which is getting its air signal from the turbocharger.
Another way of fixing FC Disease then is to modify the plumbing on the inlet side of your turbocharger. The concept is that the further away you move the AFM from the inlet of the turbocharger, the less pulse reversion it will see, and the lesser the problems of FC Disease become.
There is however, another option that not too many consider, and that’s moving the AFM onto the pressure side of the inlet system.
Mounting the AFM in an intercooler pipe that sees full turbocharger pressure isn’t a problem at all. The reason for this is that the AFM knows nothing of boost, it only senses movement of air, and as such can be placed anywhere in the system before the throttle body.
Moving the AFM to the pressure side removes it from the proximity of the turbocharger blades and hence gives a stable, solid running result.

It’s all quite simple then to identify and then spend some time curing FC Disease. The best part is that if you are prepared to put in some time and effort, then it’s a process you can do yourself, without affecting the important full load tune parts of the car’s setup. Something for a sunny Sunday afternoon? Why not!

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