I have a 98 Civic EX with D16Y8 engine.

For a while now, it's had a rich condition that is far worse under low manifold pressure/high engine vacuum. My last few emissions tests have just passed CO and HC. I own a ton of Scan Tools and can monitor everything and command various items as well.

Here's what I have replaced:

1) Front O2 Sensor
2) NGK Wires
3) NGK Plugs
4) Cap & Rotor
5) Removed and cleaned IAC and Throttle Body
6) Air Filter
7) Timing is bang on 12 deg BTDC
8) Idle is set to exact spec of 750 for Canadian M/T Civics with no load

At idle, the Short Term Fuel Trim always stays negative, anywhere from negative 2-10%. During cruise with light throttle, it can go as low as negative 30%. I basically know how to force it to have high engine vacuum by just barely opening the throttle at cruise, which results in engine vacuum of about 22-25 inHG and causes the ECM to see a high rich condition and reduce injector pulse.

Now, from what I understand, the only things that could really cause this sort of richness would be:

1) Purge stuck or sticking/leaking or EVAP canister saturated
2) Fuel injector leaking or stuck open
3) Spring inside Fuel Pressure Regulator sticking open causing pressure to be too high during high vacuum which should be lowering the pressure

For the Purge Valve, I have tested it multiple times with Vacuum Pump and it doesn't leak at all. So unless it's leaking intermittently or only under certain conditions, I would doubt that's the problem.

What does everyone else think??

Thanks

JEFF

 

Any CEL codes? What's your gas mileage?

Have you tested the MAP sensor?

Have you measured the fuel pressure and tested the FPR?