Car: 1999 Honda Civic EX, stock motor D16Y8. Back ground, bought the car with a suspected head gasket issue, black contamination in cooling system. Replaced head gasket, had head checked for warp at a machine shop, adjusted all valves, new timing belt, water pump, belts, etc.

I have searched and searched, and come up with the same answers, all of which don't fix my problem..

My Problem: after clearing CEL and resetting ECM, cold start, car idles great, right at 850 (once its warmed up a few minutes). Once I even touch the gas peddle, the idle starts bouncing from 1,000 to 2,000 rpm's. It will do this until the CEL comes on, then idles at 1,500rpm's until I reset. Code is a P0505. Common issue, tons of threads.

Here is what I have done:
replaced intake manifold gasket (twice) /Replaced Throttle body gasket / Replaced IACV (twice, thought first one was a dud) / Replaced MAP sensor, TBS (and calibrated it, .5volts at closed, 5 WOT) all vacuum lines. New PCV and grommet. Replaced both O2 sensors and Catalytic converter. I have chased the engine compartment with propane, starter fluid, no change in RPM. I even blocked every vacuum port off (besides the on that goes to the fuel rail, but that's a new hose) Nothing is making a difference. I even took the intake off, put my hand over it, and it immediately killed the engine, and visually inspected the intake out of the car, and appears to be in great shape. I have done the throttle learn step- let car idle until fan comes on, As far as I am aware this car doesn't have a coolant bleed valve that I can find, but I did buy one of those funnels that goes on top of the radiator to bleed system, I let the fan come on at least 10 times to make sure all air was out. I still have the issue. WHAT AM I MISSING??? Thank you in advance for any assistance.

 

Something is wrong with the IACV, Code P0505 or the wiring so check the plug and its wires..

On cold start it should idle high like 1200 and slowly drop down to 850 as it warms up. It sounds like the IACV is stuck open or just not functioning properly.

 

Its the second new IACV in two weeks of trouble shooting. It has an obvious change if I unplug it. So it appears to be communicating with the ECM. I will check and clean the connector again just to be sure.

 

Did you forget to connect the FIA hose to the IM?

 

No, its hooked up. Any other thoughts?

 

Did you forget to connect the cruise control, brake booster, PCV, FPR, or EVAP system vacuum hose?

Also do this:

Remove air intake system.

While engine idle is surging, use your finger to cover hole leading to IACV inside throttle body. Does idle speed drop and surging cease?

 

Ok, Thanks for your time so far, and your patients. I did the finger in the hole, RPM's dropped to barely enough to keep the engine running, but it ran. Take finger off, back to up and down. So I unplugged the IACV, and RPM's steadied out, and seemed to be within a normal range. Once I plugged it back in, went to a high idle, fast up and down, and CEL. From everything I have read, I have a bad IACV. BUT, I bought a new one from Autozone, and it didn't fix it. Spent another week chasing all other possibilities, to come to the conclusion maybe the new IACV was bad out of the box. So I exchanged it for another new one. And here I am today, still with the same problem. Whats the possibilities that I got two bad one??? Is there a possibility it could be something else based off what you had me try?

 

If you unplug the 2-wire ECT sensor under the distributor, does it prevent the surging idle with the plugged in IACV?

Was the heater hose valve near the firewall wide open when you bled the cooling system?

 

okay, got it up to temp, started surging, unplugged the 2-wire ETC sensor and the surging stopped. RPM's went to 1900, and CEL came on. After plugging it back in, nothing changed. stayed at 1900 rpm's. I didn't let it got for more than a minute, not sure if it would have started surging or not, but it did stop the surge when I unplugged it..

 

With ECT sensor plugged back in, does surging idle return when your turn engine off and then restart it?

 

yes. Without clearing the CEL and code it goes right back to the fast surging between 1800 and 2000 rpms. I’m sure resetting the ECM would make a difference. But to answer your question it does the same thing after I plugged the ECT back in after shutting engine off and restarting.

 

Do this:

Fully warm up the engine.

Turn off the engine.

Unplug ECT sensor.

Measure resistance between two pins of the sensor. It should read 200-400 Ohms.

 

it reads 245.

 

 

Yes it was.

 

yes it was

 

and seriously purged the cooling system 4 different time thinking that had to be my problem. Got a little air the second time, but the last two, nothing. And I did it at a high rpm to make sure coolant was moving.

 

Have you tried to adjust the idle speed as shown in service manual?

Do you have an auto or manual transmission?

 

I haven’t touched any idle screw yet. It’s a Manual.

 

Following the procedure in the diagram below, are you able to set the base idle speed to 450 rpms using an external tachometer or code reader to monitor engine speed?

 

I will do this tomorrow when I get home from work. Based on everything so far, should I buy a new ECT sensor tomorrow? Or is it something else?

 

The ECT sensor tested as fine.

 

Ok, I did the procedure. Idle was high, around 870. I adjusted it to 670. With the headlights on and AC on, it didn't increase at all. actually dropped to about 650. I plugged the IACV back up, reset ECM, and initially seemed to be fixed. During this whole time I took advantage and put the funnel back in the radiator "just in case" there was still some air. After the reset and cool down enough to clear the funnel and cap the radiator, I went for a drive. When I returned I had the surging idle again for about 1 minute and it settled down. If I rev the engine it surges again, but seems to find its way back to idle. I let it idle for about 10 minutes, and no CEL. But still has to find the idle after even touching the gas.

 

Some improvement?

Were you able to set the base idle speed to 450 rpms? If so, any surging at base engine idle speed?

 

honestly forgot you mentioned the 450. I was strictly following that guide that said 670. So will dial it down to 450 and see what happens.