opinions, advice, help? Diagnosing an idle surge

[72240z]

So as the title says I have a idle surge in my 89 vg30i. In the last year I swapped the motor from a z but kept the stock "electro injection", new alt, bat, dizzy, maf, aac/aic. Added headers and deleted the emissions. Out of left field I developed an idle surge, always between the same rpms, about 700-1700k. After checking all sensors one by one with the multimeter I got to the chts. I checked resistance at 3 dif temps and its fine, then with the truck surging checked voltage and it actually fluctuates in sync with the motor .1-.5. I checked a new sensor, it's the same deal. It has to be something to do with that but I'm kind of at a wall. Anyone have some advice or new perspectives?

[Fineline]

O2 sensor? If the voltage signal from it surges it will lean and richen the motor causing the surging?

[72240z]

O2 sensor? If the voltage signal from it surges it will lean and richen the motor causing the surging?

 

Ya possible but it wouldn't change the voltage coming from the chts. The ecu changes the idle according to temp, lower the temp higher the idle so the voltage is def the cause of the surge but I can't figure out if not the sensor itself what it causing the fluctuation in voltage.

 

Thanks for chiming in though.

[72240z]

Also something is over riding it too. When the engine is 100% cold, idle just hangs out high at 2k, then as it warms it begins to fall, once it gets to about 1800rpm in the fall the surge starts.

[Fineline]

True but I believe the increase in idle when cold is controlled by IAC valve. If its getting a mixed signal or even it is malfunctioning itself would also cause the surge. It also takes a O2 sensor time to warm up. Hope this helps.

[yello620]

A sticking IAC motor will also cause a surging problem like this, but i am not very familiar with the VG30 electro injection system. IAC motors are fairly common problems on 90's honda's. Not sure if your electro injection will even have one.

 

Jason