I have a 90 gtu,the engine idles normal with a/c and stereo on or off,but when I turn the fog lights or head lights or both the engine surges from 1000 rpm to 1500 rpm,as soon as I turn off the lights it idles normal,I thought maybe a bad act,so I replaced it but no luck,any help would be appreciated,thanks.

 

Bad battery? Or alt?

 

Thanks for the reply,the battery is new,and had the alternator checked,and it's putting out 14.20v the weird thing is it doesn't do it all the time.

 

This may sound bad but mine has done this for years. Either surges or just idles high with the lights on. I have never bothered to do anything about it. In all honesty it hadn't occurred to me it might have a problem.

 

https://www.youtube.com/watch?featur...8S3dwZsYmw#t=7

Is it kind of bouncing like that?

 

Yes,just like that,I thought maybe the idle control,but no luck.

 

What modifications do you have?

Engine management, fuel injectors, etc...

The fuel injector dead time varies with voltage, and this is different on various kinds of injectors.


So, If the system voltage varies, this can create a situation that the voltage compensation programed into the ECU doesn't match the requirements and will commonly cause and idle surge like you describe.

 

What is your car idling at without the lights on? It should be around 750rpms fully warmed up. Whenever the ecu senses extra load(lights, ac, ps, ect) it ups the duty cycle on the idle valve to compensate for the load.

 

The mods are,racing beat road race exhaust,k&n drop in filter,at idle the car is @ 750rpm with the a/c on it goes p to about 850-900rpm, but with the lights on it goes to 1500 and surges to 2100rpm, with a/c on and stereo on it still idles normal.

 

I wonder if a bad ground could be causing it?

 

I checked all grounds and cleaned them,they seem fine,with the parking lights there's no problem as soon as I turn on the fog lights or head lights or both it starts surging.

 

Alt belt/tension?

 

I,checked that,I'm thinking of getting an alternator that's 130 amps,maybe that'll help,one thing I did find that if I push down the tps the idle goes back to normal.