[Fletuitus]

OOOk, where to start. I suppose it is best to start with what I started with. I started with a stock 1999 Honda civic HX coupe. For those who do not know the major differences in this model Civic are as follows, it has an egr valve, it has roller rocker arms, it has a 5 wire primary or upstream O2 sensor, only 1 intake valve opens until vtech kicks in.

On my particular engine I have a zex light weight fly wheel with an act pressure plate and a stock clutch disk, Dc sport 4 to 1 headers, max speed exaust, Aem cold air intake, Aem adjustable fuel pressure regulator, skunk2 silica valve springs and titanium retainers, jackson racing underdrive pully's including the crankshaft pully, Aem highflow fuel filter, and finally the stock pistons have been replaced with PM6 pistons for an approximate compression ratio of 10.34 to 1.

Now for the problem. After doing a recent engine dress up including the piston swap the car developed an odd surge appearing mostly around 2500 rpms.

Keynote.. the primary oxygen sensor was replaced as part of the dress up.

An idle surge will be your Idle Air Contol Valve, or a Vacuum leak. No vac leak so that wasnt it. All the fuel injectors were ticking the same so most likely was not those. Compression was good and even across cylinders, not it. Ecu and distributor was swapped, not it. All sensors checked and no codes pending or otherwise, not it. No exhaust leak, not it. Timing all perfect, cam and ignition, not it. All sensors were throwing proper voltages with one anomaly, both the primary and secondary o2 sensors pulsed. The primary one is supposed to but the secondary should read flat.

It turned out that the primary o2 sensor had a bad resistor. As of now I am not sure if it is the one inside the connector (you have to remove the white insert inside the plug removed with needle nose pliers) or if it is the resistor inside the o2 sensor itself.

Best solution is to replace the o2 sensor.

Added note.. the iacv idle surge could be a clogged screen or an air pocket or clog in the coolant line that runs through it.

Added note on O2 sensors. I have run L1h1 o2 sensors and they have worked fine but I have read that L2h2 sensors come stock and both are listed as discontinued and dated. The newer o2 sensors are all LZA-** sensors. Though I believe mine were LZA-09 sensors. The next post will contain more info on this. Apparently the resistance should be around 6 OHMS, the new sensor I had installed read near infinite ohms. Not enough to cause a CEL apparently.

All info courtesy of "Kevin and Conners Shop" located in San ramon, 3211-A Fostoria way, 925-837-0554