My engine starts fine warm but not cold. Where do I start?

Check the coolant sensor reads ambient on a cold engine, then cranking enrichment at the cold end. If it fires and dies, look at ASE next.

Should warm-up enrichment be 100 % at operating temperature?

Yes, on almost every engine. Anything above 100 % at the warm end is fuel that belongs in the VE table.

Can I tune warm-up enrichment from a log?

Yes, as long as the log starts cold and includes a wideband. Wait for ASE to finish before reading WUE, and only use steady idle or cruise samples.

Guide · 4 min read · updated 6 November 2026

Cold start: priming, cranking, after-start and warm-up enrichment

A cold engine needs extra fuel to start and to keep running until it warms up. The four corrections that supply it, how each ECU names them, and how to tune them in order.

Why a cold engine needs more fuel

Fuel only burns as vapour. In a cold engine much of the injected fuel condenses on the port and cylinder walls and never burns, so the mixture in the cylinder is leaner than what was injected. The ECU makes up for it with extra fuel that tapers away as coolant temperature rises.

That extra fuel comes in four stages, each with its own settings, almost always as curves against coolant temperature:

Stage	When	Typical size
Priming pulse	Once, at key-on	A few milliseconds, more when cold
Cranking enrichment	While the starter turns	Several times the running fuel
After-start enrichment (ASE)	Seconds after the engine catches	5 % hot to 50 % cold
Warm-up enrichment (WUE)	Until operating temperature	100 % hot, up to about 150–250 % very cold

How common ECUs lay it out

Speeduino

Priming Pulsewidth is a curve against coolant. Cranking Enrichment is a curve of modifiers to normal fuelling (100 % = no change) with a taper time into ASE. Afterstart Enrichment has an amount curve and a duration curve that share coolant bins. Warmup Enrichment (WUE) is a percent multiplier curve; the warm end should be 100 %.

rusEFI and epicEFI

A priming fuel pulse, cranking fuel from a base cranking fuel table with coolant and TPS multiplier curves (Cranking Coolant Temperature Multiplier, Cranking TPS Multiplier), an After start enrichment table, and the Warmup fuel manual Multiplier curve against coolant. A warm-up AFR target multiplier lets you run richer targets while cold.

MegaSquirt MS2 and MS3

Priming and cranking pulse widths are curves against coolant, with an optional cranking taper. Afterstart Enrichment (ASE) is a percent adder with a duration in seconds or engine cycles. Warmup Enrichment (WUE) is a percent multiplier; MegaSquirt's own note is that for almost every engine the final row must be 100 %.

Tuning order

Tune the engine warm first. Warm-up enrichment multiplies the VE table, so if VE is wrong when warm, every cold correction is built on a wrong base.

Tune VE and idle on a fully warm engine.
Let the engine cool completely, ideally overnight. You get one cold start a day, so log every one.
Priming: too little and the first crank does nothing; too much and the engine floods. Start from the firmware default and change it in small steps.
Cranking: the engine should fire within one to three seconds. Long cranking then a sudden catch usually means too little; a stumble with black smoke, too much.
ASE: if the engine starts then dies or sags within a few seconds, add ASE amount. If it runs rich for a few seconds then settles, remove some.
WUE: drive or idle through warm-up with a wideband and compare AFR to target at each coolant temperature. Adjust the curve point the engine was at.
Repeat at different ambient temperatures. A winter morning reaches the cold end of the curves that a summer one never does.

Common mistakes

Tuning WUE with ASE still active. Wait until ASE has finished before reading WUE from the log.
A WUE curve that rises with temperature somewhere. A cold engine never needs less fuel than a warmer one.
A warm end above 100 %. That adds fuel to every warm cell and hides a VE error.
Idle valve changes during warm-up. A fast idle that steps down changes airflow, which reads as a mixture change.
A coolant sensor that reads wrong. Check it against ambient on a cold engine first.

Using efi2000 for cold starts

Autotune's Warm-up mode records a cold start and proposes warm-up enrichment changes as coolant temperature rises. It keeps cold-engine data out of the VE corrections, and holds any change that would make the curve rise with temperature or drop below no enrichment.

With continuous logging turned on (Logs → Log settings; it is off for a new project), the one cold start you get each day is always recorded. In the log viewer, plot coolant, AFR, target AFR and RPM together to see each stage of the start.

Questions

My engine starts fine warm but not cold. Where do I start?: Check the coolant sensor reads ambient on a cold engine, then cranking enrichment at the cold end. If it fires and dies, look at ASE next.
Should warm-up enrichment be 100 % at operating temperature?: Yes, on almost every engine. Anything above 100 % at the warm end is fuel that belongs in the VE table.
Can I tune warm-up enrichment from a log?: Yes, as long as the log starts cold and includes a wideband. Wait for ASE to finish before reading WUE, and only use steady idle or cruise samples.