I have a 98 Civic DX that has a surging idle. The CEL code is P0505. I cleaned the throttle body and IACV several times and bought a used IACV at the junkyard; still didnt work. I bought a new IACV, the problem was fixed for a few days and now has gone back to surging. I have checked for vacuum leaks and bled the coolant system. I replaced the ECT because someone told me that could be a problem. The idle problem only starts when the car warms up. If I remove the TPS and MAP sensors, the problem stops. Obviously I get two new codes once I pull the sensors. If I clear the codes while the car is idling, it starts surging again.

I also have another problem that I'm not sure if its related or not.
While I'm driving, if I let off the gas while in gear, the car jerks. When I push the gas pedal again it jerks as well. It doesn't happen all the time and only happens when I let off the gas and initially push the pedal. I have replaced the engine and trans mounts.

I would appreciate any help, Thanks.

 

Unplug your IAC valve and start the car, if it idles very high, you have a vacuum leak. The IAC is trying to compensate. If it continues to fluctuate with the IAC unplugged, you have some other major issue.

 

Thank you, I will try that and see if it works. I actually found a vacuum leak in the gasket of the IACV. I fixed it and the CEL clears. I don't have the surging idle anymore, but my CEL keeps comin on after and hour or so. It's still the same code (p0505)

 

I don't think the unplugging IACV test works with 3 wire IACV's.

Make sure the ECU has backup power. It should be able to remember codes with the key off. If it is resetting every time you start, it will almost constantly be in recalibrate mode which makes the idle vary.

Try closing down the idle alr screw on the throttle body. Make sure throttle cable goes slack when not pressing the gas. If your model has a plastic FITV on the thermostat housing, it is easy for those to break and become a major vacuum leak.

 

What do you mean by backup power? I'm using a OBD2 reader to pull and clear the codes. The code doesn't come up right when I start the car. Usually it takes 30 minuets of driving for it to go off. I don't have the FITV on my model. I've tried adjusting my idle screw. I get a surging idle if I open the throttle too much, but if I adjust it right, I don't get the surging idle or the the CEL code when idling. At low rpm when the car is in gear my car jumps sometimes. (When I barely push the gas it jumps, and when I let off the gas in gear it jumps). I've heard it could be the TPS. Wouldn't that set off a code though? I've measured the voltage from the TPS, and am getting about 4.5v at idle. I don't remember what the voltage was when the throttle was open, but I remember that it increased as I opened the throttle. The throttle cable has slack when idling as well. Hope this info gives you a better idea. Thanks!

 

Is P0505 still thrown?

The TPS should receive ~5V from the ECU whenever the key is in ON(II). However, the TPS should output only 0.5V back to the ECU at closed throttle. Calibrate the TPS accordingly.

The TPS can also be Ohm tested to see whether it may be bad.

 

Yes it's still the same code (p0505). I haven't touched the TPS. Do they need to be calibrated every so often? I will check the voltage again. Wouldn't that throw a code if it were not responding right, or improper voltage?

 

A miscalibrated TPS won't throw a TPS CEL code.

 

Do they need to be calibrated every so often? I've never touched it.

 

Usually not.

Go ahead and check output voltage just to verify. If it's fine, then Ohm test it.