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The Diagnostic Port

Question for the week of September 29th, 1997

What is "fuel cut"?

When the ECU detects a condition that the factory didn't intend the engine to experience, it will try to save the engine using something commonly known as "fuel cut". All of the injectors are shut off as well as all the spark plugs, effectively shutting down the engine. When the condition has passed, the ECU will go back to running spark and fuel as normal. You might experience this as a violent bucking sensation.

What conditions will cause fuel cut?

There are three conditions under which the DSM ECU will chop fuel.

  1. Engine speed higher than 7500 RPM. This is overrev protection. If your engine rotates faster than the valve springs can pull the valves back up into the head, the pistons could smack into them, causing major damage. Note that simply chopping fuel will not stop the overrev condition if you misshift and the transmission rotates the engine past the revlimit. (The author did this to his friend's M3.)
  2. Engine air flow greater than a certain amount. If the airflow into the becomes too great, the phenomenon by which the MAF counts the air entering the engine starts to break down, missing counts. The factory ECU sets a very conservative limit here. It doesn't need to be as conservative as it is to prevent missing counts. After we removed this limit, we were only able to hit the MAF counting breakdown problem on a first generation DSM with a 20G set to 19 psi on a 3" exhaust.
  3. Engine under high vacuum for more than a couple seconds. This is not a matter of engine safety, but of smog control. Some of the worst smog can occur during engine deceleration. If you are coasting down a steep hill using the engine to help slow you, the ECU will turn off all fuel to the engine to both aid in engine braking and smog production.

What does fuel cut feel like?

Like you hit a brick wall, if it happens when you have the throttle to the floor. Typically, the ECU thinks too much air is entering the engine, the engine shuts completely off, the air amount decreases to under the limit, and the engine goes back on again. This can be quite jerky.

Some days I get fuel cut, some days I don't, even running a consistent boost pressure.

Fuel cut does not depend on boost pressure - it depends on air flow. Remember the gas laws from high-school chemistry? If you keep the pressure constant, you can pack more of a gas when it is cold into a given space than when it is hot. So even if you keep your boost pressure the same, more air will be flowing into your engine when it is cold at night or in winter than when it is hot during the day or in summer. Thus, you might start experiencing fuel cut at night or during winter that you don't experience at other times.

How can I prevent fuel cut from happening?

Of course, we recommend the Stage II ECU modification, which removes the air flow limit imposed by the ECU. There are other methods that use MAF modifications to "fool" the ECU into thinking less air is entering the engine ("Dime Store Fuelcut Remover", MAF gutting). There is a danger with these "fool-the-computer" mods, however. The ECU might not provide as much fuel as the engine requires - possibly running lean. With our modification, the ECU still knows exactly how much air is entering the engine.

I am getting something that feels like fuel cut, but the car only has a slight hesitation, not a dramatic on/off.

Here are three possible reasons for fuel-cut like hesitation. Keep in mind that none of these involve the ECU directly.

  1. Lack of spark. Under high boost conditions, if the spark is weak due to dirty plugs, it can be completely blown out. If the spark plug wires are old, the insulation might be breaking down, letting the spark arc to the engine instead of firing in the cylinder.
  2. Lack of fuel. Fuel filter or fuel injectors could be clogged. The fuel pump might not have enough output to handle your fuel load. Sometimes, simply running an extra heavy gauge wire to the fuel pump can overcome voltage drops that occur when the pump is drawing a lot of current.
  3. Modified MAF. If you have removed the honeycombs (they produce a laminar airflow) from a DSM MAF, you can disturb the phenomenon by which air flow is counted. If the MAF is missing air counts, the ECU will not put out enough fuel to properly match the air the engine is getting. This can cause a lean condition.

A possible fourth reason for this sort of fuel cut might occur if you have our Stage II fuel cut removal and you are running a modified MAF on a 1st generation DSM. The vortice field that the MAF uses to count the air might be breaking down if there is air disturbance due to honeycomb removal. At this point, you will want to step up to the large MAF used in the Stealth and the 2nd generation DSM. Technomotive will has been running two daily driver cars with a 95 MAF with great results. Look for the 2G MAF upgrade soon.

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