TwinTurbo.NET: Nissan 300ZX forum - Does it happen only on warm-up?
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Subject Does it happen only on warm-up?
     
Posted by zromtech on April 18, 2014 at 11:51 AM
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In Reply To 1990 TT stumble below 3,000rpm posted by Fly-Surf on April 18, 2014 at 08:51 AM
     
Message If so, it could be the ECU retarding the ignition timing to rapidly heat up the catalytic converters. This is normal behaviour and is a common source of intermittent hesitation and stumbling below 3000rpm.

Under certain cold-start conditions, the ECU switches to using a secondary ignition map instead of the regular maps. This secondary map retards the timing significantly, raising the exhaust gas temperature, which in turn rapidly heat the catalytic converters up to their operating temperature. This is actually a common technique used in OEM engine management systems for reduced emissions on cold-starts, known as running at the Retard Limit (aka full retard).

The exact conditions under which our ECU uses the catalyst heating map are as follows:
- Coolant temperature measured at engine start was between 15C and 35C
- Current operating coolant temperature is between 30C and 55C
- Engine RPM is less than 2600rpm
- Engine Load (TP) is less than 44 (for 370cc injectors)


The hesitation/stumbling will go away abruptly once coolant temp hits 55C or when RPM goes above 2600rpm, so it is fairly obvious when this is happening. In practice I find this issue only occurs (or is most noticeable) for a few weeks per year when the start-up temperature is closest to 15C.

     
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