hi, im on the hondata s300 in my 1999 civic si. the engine idles nicely at 900rpm, but whenever the radiator fan turns on, or AC is on, the idle drops fast to 500rpm... or dies. Is there something i can change in the parameters to help?

 

Sounds like the ECU may not be compensating (A/C circuit or possible ELD). Did this happen before you installed the S300 or are you using another ECU? If you are using another ECU, if you swap in the original does the same thing happen?

 

It sounds like the ELD may be disabled within the S300 software. Try enabling it and see if it fixes it..

 

most likely you have a bad ground or a failing ignition system. coil, cap/rotor, plugs, wires.

 

If you are running large injectors, make sure you are using the correct injector data.

Monitor the condition with a wideband & see if the air fuel ratios at idle are drastically
changing.

Also monitor the battery voltage and make sure its not dropping too much due to an issue with the charging system etc.