I just got a chipped p28 for my 99ex--the new ecu has a higher rev limit and a slightly different VTEC engagement point. When I first installed the ECU, everything was fine.

However, I went back to the stock P2P for a while because I had to get my car inspected, and NC is one of the states that now does OBDII testing. When I went to re-install the P28, my idle started fluctuating (when engine was cold, mostly) from about 900-1500 rpm.

When I switch back to the P2P, no problem. So my question is, what difference between OBDII and OBDI would cause an idle surge in one and nothing in the other?

I will be cleaning my IACV later next week when I do some other engine work, but wanted to see if it might be something else, esp. since one ECU works fine.

Also, I am 95% sure the p28 ecu has no problems itself.
Thanks!

 

bump for the morning!

 

Unplug your EVAP purge solenoid.

 

And that won't cause any other problems?

Thanks for the help!

 

OBDI doesn't need it. It's just an emissions control.