Why a fast throttle opening goes lean
There are two separate reasons an engine leans out for a moment when you open the throttle quickly.
1. The pressure signal lags behind the engine
The MAP sensor on a Haltech is quick, easily fast enough to follow real load changes. The weak link is the hose between the intake and the sensor. When the throttle snaps open, pressure at the intake ports changes almost instantly, but it takes a moment for that change to travel down the hose to the sensor. For that brief moment the ECU is fueling for the old, lower load.
You can shrink that delay by keeping the vacuum hose from the manifold to the sensor as short as practical. It won't go away completely, though. Even a very short hose leaves some lag.
2. Fuel drops out of the air
At light throttle, fuel stays suspended as a fine mist. A sudden rise in manifold pressure knocks some of it out of suspension and onto the intake walls. That fuel arrives late, or as liquid, instead of burning properly on that cycle, and the engine hesitates.
Manifold design makes a big difference here. If the injectors are placed so they spray onto the runner walls rather than into the airstream toward the valve, the wall wetting gets worse and so does throttle response.
The fix: transient throttle enrichment
Transient throttle enrichment adds a short burst of extra fuel when the throttle or load changes quickly, covering the lean gap until the MAP reading and the fuel film catch up. On an Elite, you tune it through the Transient Throttle function. Two related topics explain how that extra fuel is triggered and delivered:
- Delta Load: using the rate of change in load, rather than throttle position alone, to decide how much extra fuel is needed.
- Asynchronous vs synchronous enrichment: whether the extra fuel is squeezed in as an extra pulse right away, or added to the next scheduled injection.
How we approach it
- Fix the hardware first. Shorten the MAP hose, make sure it isn't kinked or leaking, and check the injectors are aimed properly. No amount of enrichment makes up for a badly placed injector.
- Get steady-state fueling right. Transient enrichment should cover only the transition. If the main fuel table is lean, you'll end up covering it with enrichment and making things worse elsewhere.
- Tune with a fast wideband log. Make repeatable throttle stabs from a steady cruise and look for the lean spike right after the throttle moves. Add enrichment until it goes away, then check you haven't created a rich dip instead.
- Check cold and hot. A cold engine wets the walls more, so it usually needs more help than a warm one.
A stumble that only happens at one engine temperature or one RPM range tells you a lot. Log throttle, MAP, lambda and coolant temperature together so you can see exactly when the lean spike appears.
For the bigger picture on getting a tune dialed in, see our tuning guide.
