Former User

Thought I'd post this information as it might help others with a similar idle problem.

Yesterday, my stock 97 Civic DX (D16Y7) 5-speed developed the idle problem stated in the title.

With the engine warmed up, the idle speed was at about 1800 rpm and after prolonged idling at a stop signal, the idle speed began to fluctuate between 1100 and 1800 rpm. When the A/C was running, however, the idle speed steadied at 1100 rpm.

Although the CEL never turned on, when I got home from work, I hooked up my Actron scan tool and found that one pending code was retrieved - P0505.

P0505 has many causes, the most common of which is an IACV with a mechanical problem or a dirty IACV.

However, a bad TPS or ECT sensor can also throw this code and cause idle problems, as can a misadjusted idle adjust screw. Given that the scan tool showed no abnormalities in the TPS and ECT sensor readings, I quickly focused on the IACV.

After removing the air intake system, I started the engine and while it was high idling, I covered the lower throttle body port (red highlighted) leading to the IACV with my finger and found that the idle speed dropped substantially. I also unplugged the IACV and found that the idle speed did not change. These collective observations suggested that the IACV air controller window might be stuck partially open.



To test this idea, I turned off the engine, unplugged the MAP, TPS, and IACV, disconnected the throttle cable, removed the throttle cable bracket, disconnected the two coolant hoses attached to the IACV, and removed the throttle body (TB). Luckily the TB gasket did not break or tear during the removal, though a replacement only costs about $2.

The problem was immediately apparent after I removed the IACV from the TB -- the metal window that the IACV uses to control the amount of air bypassed to the intake manifold was indeed stuck partially open and could not move. This problem was quickly solved by spraying brake cleaner over the window a few times and wiping clean with a shop towel.

For good measure, I similarly cleaned the TB with brake cleaner, and then reattached the cleaned IACV along with its rubber gasket to the cleaned TB.

I next reinstalled the TB and throttle cable bracket onto the intake manifold, plugged in the sensors, and reinstalled the air intake system.

As the disconnected IACV coolant hoses spilled some coolant, I topped off the radiator with coolant and then bled the cooling system.

Today I again have a Civic with a perfect idle, and this fix basically cost me $0.