I own a 1999 Honda Civic EX. D16Y8 Engine. The engine light has been going on and off with the same code for the Idle Air Control. The car had high idle when I started it up. The Idle would also go between 1500 to 1700 rpm while Idling. Then the car started dropping rpm while driving. For example, driving ~35mph @ 3000 rpm, the rpm drops down to 1500, back up to 2500, down to 1500, back to normal driving. It happens after the engine is sufficiently warmed up and I am basically coasting at one speed. The car feels as if it is about to stall out then comes back to life. I suppose it might not be exactly an idle issue because its happening under load. Engine light code is P0505: Idle air control system. This is what has been done so far:

- Checked made sure there is no air in the cooling system
- Smoke tested the engine, no vacuum leaks.
- Cleaned and checked throttle body
- Replaced gaskets on IACV and TB
- Replaced IACV
- Voltage tested all the sensors, all good.
- Checked PCV valve, working properly and cleaned.
- Timing Belts and Water Pump working properly
- Tried Resetting base idle, didn't help
- No problems with fuel lines

It doesn't seem to be a mechanical issue, I could be wrong. Probably going to buy a used ECM and install that because that is what multiple mechanic friends of mine have recommended. If that doesn't help, which i'm hoping it does, where should I go next? Clean the EGR?

Thanks for your help!

 

Can you clarify a couple of bullet points?

• Tested all sensors <- Can you list all the sensors you tested?
  
  
• Tried Resetting Base Idle <- Can you describe how you went about doing this?

 

Voltage tested the tps, map, and replaced the iacv. Tried resetting Honda base idle by clearing all codes. Turn on car with all electrical assecories turned off. Hold rpm at 3000 until radiator fan came on. Let idle for 20 min. Check engine light and problems started right back up.

 

Losing power while driving is a different issue. Perhaps misfiring because of weak sparks? Does it do it mostly at heavy throttle or light?

When you unplug the IACV, the idle speed should drop well below normal, to like 400 or 500 rpm. If it does not, extra air is getting into the intake somehow. (FITV, throttle plate not closing all the way, brake booster leak, etc). This test works only for 2-wire IACV's which are on the manual EX. If your car is automatic, forget what I said above. Also check the wire colors and make sure all the 2 wire plugs around the intake are plugged into the right things. They are not mechanically keyed to prevent mis-plugging like most cars are.

 

Spark plugs are new along with cap and rotar for the distributer. Car is automatic. Does not have a FITV. Plugs are properly connected. They are different sizes. They only fit in one way.

 

The rpm drop while driving happens under light throttle pressure while I'm coasting at speed. I took apart and cleaned the TB. Plate is closing fully.

i appreciate the help! Thank you!

 

You have not tested your ECT sensor in the list you provided.

I have been told a bad ECT sensor can cause erratic idle problems once warm.

Unfortunately, you cannot reset base idle when there are CEL issues. And your CEL indicates a problem with the IACV circuit. It does not exclusively mean the IACV itself is bad, it could be a bad ground or a short in the wirings to and from the IACV.

You say you have a 99. I think there is a TSB about the engine wire harness on the 96-99 civics. You may not be to the point of popping the fuse yet but may have intermittent shorting issues with the engine wire harness causing your IACV CEL code.

I have attached the TSB for your reference.

TSB-x99-029e.pdf

 

What is the proper way to test the ECT sensor? I checked the wiring, ground is ok.

 

Did you read and check the harness concern in the TSB?

The ECT sensor troubleshooting starts at 11-128 of the 96-00 Factory Service Manual.

I also would verify the parts mk378 mentioned concerning your IACV.

You can ignore the portion about the FITV as you already know the 96-00 does not have the mechanical FITV valve of the previous generation.

The rest of his post is spot on.

 

Finally solved the issue. Ended getting a used throttle body from a junkyard with the IACV attached. cleaned them up and installed them. idle is back to normal and no more problems while driving. perfect fix.

 

Doesn't the throttle body have the FICV attached? Or is it different on the '99 Civic?

 

not on the automatics. there is no fast idle valve. idle air control valve is mounted seperatly on the bottom of the throttle body.

 

FITV was only up to 1995 on the civics. 96+ did away with the FITV and purely relied on the IACV to do the job.