High raising and falling idle speed, but normal when in gear

The surging of the engine is not generally electrical. It can vary from sensor, vacuum leak, coolant level, air in coolant system, and intake manifold gasket or intake leak. You need to look the engine over real well to see if the hoses etc are properly connected. Check coolant, bleed the air out of it. Check dash for check engine light.

Check engine is now off after the battery is replaced. Yet this problem (surging and lowering idle speed) persists . And the vehicle had none of these problems, when it was taken to the body shop. My frank question is: if the mechanic played a nasty role in tweaking the system to create this problem, what he could have possibly done. I am sorry to be frank in my question. As I was positive this issue came up only in body shop, it was non-existent earlier. I suspect the mechanic might have tweaked little here and there that is creating the problems, like lossening the hose to cause vaccum leak etc. What can I suspect that could have happened at body shop that caused this issue. your reply would definitely help me to discuss the solution with body shop. Pardon me if I am too rude in expressing my views. As said earlier, the check engine light is off with the new battery in place. Appreciate your expert views in this matter.

Replacing the battery should only be a temporary turning off of the CEL. The codes should still be in the computer. The extent of the damage to the vehicle. An accident? Did the car get a new front end clip? Did they have to remove the radiator? Not sure what they had to do. They could have made an honest mistake and failed to hook up a hose, not bleed the cooling system. They may have accidently sat something on the engine that knocked off a hose or a wire. Look around very closely and even have a second pair of eyes help you.

Additional information I noted for the above issue is that the idle speed fluctuates in a rhythmic fashion between 1500 to 2000 rom only when the radiator colling fans (2 fans) switch off. As soon as the fans switch on the idle speed stabilizes (1500 rpm) at well above the normal idle speed. As informed earlier my car is Honda Accord EX 6 cylinder 70000 miles. I see no leakges around. The code reader gives code of P0505, which is related to Idle Idle Air Control Valve. Please suggest what needs to be done. The issue looks interesting particularly since the raise and fall of idle speed is only happenng when the radiator cooling fans get switched off. Thanks for your help.

Here is what I would try. I would take the IAC valve off the car and carefully take it apart. Clean it completely. I believe there are screens in there and put the part back on. Bleed the system of air and see what happens. If the part looks bad replace it.

You said, bleed the air out, can you please explain what it is, if I can do that on my own and what are the steps involved. Thanks

Look at the hoses for the cooling system. Where the hoses connect you will find one or two bleeders that you open with probably a 10 wrench to let air out of the cooling system. Do this with the engine running and warm. Open slowly and be careful of hot coolant. You need to let out the air and it could look like steam. It needs to bleed until just coolant flows out. Open and close it a couple of times. You might even drive it and then check it again.