I purchased a 2005 Accord in April for a reliable teenager driving car. 162k miles and runs well. Initially I performed all the maintenance and some minor fixes as the previous owner had few records. This forum really helped me with the above. Hopefully the below helps someone.
When the weather got cold the engine almost stalled twice while driving slowly in a parking lot and the engine was just starting to warm up. No codes were set. I did some we research and decided to look at the Idle Control Valve. I pulled the IACV and it was very dirty. I used carb cleaner to get out a lot of the gunk but noticed the rotating part of the valve did not move smoothly. It would stick in spots. I decided to put it back on the car to see if cleaning helped anything. I ran the idle learn procedure and the result was a higher than normal idle and sometimes the idle would surge up to 1500 then drop to 600 then repeat this cycle about every ½ second. MIL P0507 was also set.
I pulled the IACV and really cleaned this time. I removed all the gunk and found that the rotating valve uses two sealed bearings, one on each end, as the pivot points. Unfortunately, carb cleaner and sealed bearings are not friends so the valve rotated, but still was not smooth. In my mind, if the valve does not operate smoothly then the ECM will have a hard time controlling its position so the idle will not be happy. I put the IACV back on the throttle body, ran the idle relearn procedure, and the idle was still too high and surging. P0507 was set again.
I decided to replace the IACV. I purchased from Honda since the car will eventually go off to college and long-distance calls about car problems are not pleasant.
I added the new IACV and re-ran the idle learn procedure. The idle no longer surged but was still too high = about 1050 RPM.
Doing more web research I found the Honda idle posts from Eric The Car Guy (ETCG). Very helpful to explain symptoms and fixes to try. I ran the coolant bleed procedure. Coolant was mostly full. I also used carb cleaner to look for vacuum leaks. I found no evidence of a vacuum leak. I also used a hand vacuum pump to test the brake booster. It held vacuum.
The third thing ETCG said to check was the PCV valve. I pinched the PCV hose at idle and RPM dropped. ETCG says that a big RPM drop could indicate a faulty PCV valve. I removed the PCV, cleaned it with carb cleaner. Idle did not change. I purchased a non-Honda PCV from AZ. With the new PCV idle RPM increased about 200 RPM so it was now idling at ~1200RPM. I read more in ETCG and he said non-Honda PCVs can be problems because they are not calibrated for the engine. “Calibrated” = a calibrated vacuum leak. I replaced the PCV with a new Honda PCV. Idle dropped back to about 1050RPM. After a few days P0507 was set again.
I looked again for vacuum leaks but found nothing. I then stumbled onto a comment on a post at ETCG that recommended checking the Intake Air Bypass Thermal Control Valve, sometimes called the Intake Assist Valve. This valve is supposed to be open when cold and closed when coolant reaches 130F. Its purpose is to allow more air into the intake manifold during cold start. This causes the ECM to add more fuel so RPM increases and the engine warms up faster. Previous gen Honda engines seemed to call this the Fast Idle Thermal Valve. I checked this valve at idle and sure enough, it was leaking when warm, so much that pinching the hose dropped RPM from 1050RPM to about 750RPM, right where it should be. Doing more research I found aftermarket kits that remove this valve entirely.
I decided to bypass this valve by capping the inlets on both the intake manifold and the air cleaner pipe. My hope was that the cold idle speed would not suffer too much. So far so good. While cold the engine still idles higher to warm up quickly and when warm, the idle is in the correct range.
Lessons learned for a 2.4L Honda K series engine with idle problems:
1) Don’t bother cleaning the IACV. The solvent will just eat at the lubrication of the internal sealed bearings. Replace it if it is not behaving.
2) Use a Honda PCV valve
3) ETCG has great info about troubleshooting and fixing Honda idle problems
4) Vacuum leaks from PCV or Intake Air Bypass Thermal Control Valve are not found using carb spray
5) Check the PCV and the Intake Air Bypass Thermal Control Valve for correct operation
When the weather got cold the engine almost stalled twice while driving slowly in a parking lot and the engine was just starting to warm up. No codes were set. I did some we research and decided to look at the Idle Control Valve. I pulled the IACV and it was very dirty. I used carb cleaner to get out a lot of the gunk but noticed the rotating part of the valve did not move smoothly. It would stick in spots. I decided to put it back on the car to see if cleaning helped anything. I ran the idle learn procedure and the result was a higher than normal idle and sometimes the idle would surge up to 1500 then drop to 600 then repeat this cycle about every ½ second. MIL P0507 was also set.
I pulled the IACV and really cleaned this time. I removed all the gunk and found that the rotating valve uses two sealed bearings, one on each end, as the pivot points. Unfortunately, carb cleaner and sealed bearings are not friends so the valve rotated, but still was not smooth. In my mind, if the valve does not operate smoothly then the ECM will have a hard time controlling its position so the idle will not be happy. I put the IACV back on the throttle body, ran the idle relearn procedure, and the idle was still too high and surging. P0507 was set again.
I decided to replace the IACV. I purchased from Honda since the car will eventually go off to college and long-distance calls about car problems are not pleasant.
I added the new IACV and re-ran the idle learn procedure. The idle no longer surged but was still too high = about 1050 RPM.
Doing more web research I found the Honda idle posts from Eric The Car Guy (ETCG). Very helpful to explain symptoms and fixes to try. I ran the coolant bleed procedure. Coolant was mostly full. I also used carb cleaner to look for vacuum leaks. I found no evidence of a vacuum leak. I also used a hand vacuum pump to test the brake booster. It held vacuum.
The third thing ETCG said to check was the PCV valve. I pinched the PCV hose at idle and RPM dropped. ETCG says that a big RPM drop could indicate a faulty PCV valve. I removed the PCV, cleaned it with carb cleaner. Idle did not change. I purchased a non-Honda PCV from AZ. With the new PCV idle RPM increased about 200 RPM so it was now idling at ~1200RPM. I read more in ETCG and he said non-Honda PCVs can be problems because they are not calibrated for the engine. “Calibrated” = a calibrated vacuum leak. I replaced the PCV with a new Honda PCV. Idle dropped back to about 1050RPM. After a few days P0507 was set again.
I looked again for vacuum leaks but found nothing. I then stumbled onto a comment on a post at ETCG that recommended checking the Intake Air Bypass Thermal Control Valve, sometimes called the Intake Assist Valve. This valve is supposed to be open when cold and closed when coolant reaches 130F. Its purpose is to allow more air into the intake manifold during cold start. This causes the ECM to add more fuel so RPM increases and the engine warms up faster. Previous gen Honda engines seemed to call this the Fast Idle Thermal Valve. I checked this valve at idle and sure enough, it was leaking when warm, so much that pinching the hose dropped RPM from 1050RPM to about 750RPM, right where it should be. Doing more research I found aftermarket kits that remove this valve entirely.
I decided to bypass this valve by capping the inlets on both the intake manifold and the air cleaner pipe. My hope was that the cold idle speed would not suffer too much. So far so good. While cold the engine still idles higher to warm up quickly and when warm, the idle is in the correct range.
Lessons learned for a 2.4L Honda K series engine with idle problems:
1) Don’t bother cleaning the IACV. The solvent will just eat at the lubrication of the internal sealed bearings. Replace it if it is not behaving.
2) Use a Honda PCV valve
3) ETCG has great info about troubleshooting and fixing Honda idle problems
4) Vacuum leaks from PCV or Intake Air Bypass Thermal Control Valve are not found using carb spray
5) Check the PCV and the Intake Air Bypass Thermal Control Valve for correct operation
