TPSdot for most engines, because it reacts first. MAPdot can be smoother if your TPS signal is noisy, and some ECUs let you blend both.

Why does it only stumble when cold?

More fuel sticks to cold walls, so a cold engine needs more AE. Raise the cold adjustment or the AE-vs-coolant curve instead of the warm AE curve.

Does autotune tune acceleration enrichment?

No. efi2000's autotune deliberately ignores transient readings, because AE is short and hard to measure automatically. Use the log viewer and tune AE by hand.

Guide · 3 min read · updated 6 November 2026

Acceleration enrichment: TPSdot, MAPdot and wall wetting

Open the throttle quickly and the engine briefly goes lean. Why that happens, the methods ECUs use to cover it, and how to tune a clean throttle response.

Why the engine goes lean on throttle

When the throttle snaps open, manifold pressure rises and the airflow jumps at once. Fuel doesn't keep up: part of each injection lands on the port walls as a film, and as manifold pressure rises less of that film evaporates. For a moment the cylinder gets less fuel than was injected, and the engine hesitates or stumbles.

Acceleration enrichment (AE) adds fuel for a short time when the ECU sees the throttle or MAP rising quickly. It is the electronic version of a carburettor's accelerator pump.

The methods

TPSdot: enrichment by how fast throttle position is rising, in %/s. Reacts earliest, because the throttle moves before the air does. The usual choice for most engines, and the only good one for individual throttle bodies.
MAPdot: enrichment by how fast manifold pressure is rising, in kPa/s. Smoother on noisy throttle sensors but a little later.
Wall wetting (X-tau): models the fuel film directly, with the fraction of each injection that sticks to the wall (X or beta) and how fast it evaporates (tau). More physical, more settings, and it also handles deceleration.

How common ECUs lay it out

Speeduino

Enrichment mode selects TPS or MAP based AE, each a curve of added fuel against TPSdot or MAPdot. Settings include a threshold and minimum change, Accel Time, taper start and end RPM, and a cold adjustment that adds more AE on a cold engine. Deceleration Fuel Cutoff (DFCO) is on the same page.

rusEFI and epicEFI

TPS acceleration enrichment and wall wetting can each be enabled. TPS AE has an engine cycle vs throttle opening multiplier table, a rate of change vs RPM curve, an instant fuel pulse with its own multipliers, and an AE vs coolant curve. Wall wetting uses tau (evaporation time) and beta (impact fraction), each with a curve against coolant.

MegaSquirt MS2 and MS3

Time-based AE uses TPSdot and MAPdot curves, which can be blended, with thresholds, accel time and taper, cold accel adder and multiplier, and RPM-based scaling. Wall-wetting AE (EAE or X-tau) is a separate option with its own coefficient curves.

Tuning out a stumble

Tune VE first, warm, at steady states. AE only covers the transient; it can't fix a lean cell that you pass through on the way.

Log at a high rate with TPS, TPSdot, MAP, AFR and RPM. AE events last a fraction of a second.
From a steady cruise, make the same throttle opening several times: a slow roll, a medium push and a snap.
Find each event in the log. A lean spike on AFR just after TPSdot rises means too little AE; a rich spike means too much.
Change the AE curve point for the rate you used. Small rates are part throttle; large rates are snaps.
Check the threshold. If small throttle wobbles trigger AE at cruise, raise it.
Adjust the cold multiplier separately, on a cold engine. Cold engines need more AE.

Remember the wideband lag. The lean spike from a stumble shows up in AFR a few hundred milliseconds after the throttle moved.
A stumble with no lean spike may be timing or a misfire, not fuel.
Too much AE also causes a bog, often with black smoke on a snap.

Using efi2000 for acceleration enrichment

The log viewer draws TPS, TPSdot, MAP and AFR on aligned lanes with a shared crosshair, so you can see where an AE event starts and whether AFR spiked lean or rich after it. With continuous logging turned on in Logs → Log settings, any stumble you felt on the road is already in a log.

AE curves and settings open in the same editors as every other curve and dialog in the INI, and save back to your MSQ.

Questions

TPSdot or MAPdot?: TPSdot for most engines, because it reacts first. MAPdot can be smoother if your TPS signal is noisy, and some ECUs let you blend both.
Why does it only stumble when cold?: More fuel sticks to cold walls, so a cold engine needs more AE. Raise the cold adjustment or the AE-vs-coolant curve instead of the warm AE curve.
Does autotune tune acceleration enrichment?: No. efi2000's autotune deliberately ignores transient readings, because AE is short and hard to measure automatically. Use the log viewer and tune AE by hand.