Hi, I own a 99 Civic DX 5 speed and recently swapped the motor for one with less miles (still a D16y7). Since the swap, it idles like it has a vacuum leak, once its driving it goes away. However, I cannot keep the car at a constant speed. Slight pressure on the throttle accelerates it slowly, any less pressure on it and the throttle closes and slows down. Ive checked/sprayed all my hoses to try to find a vacuum leak with no success. With my air box off, if I place my hand over the throttle body while its running and create the right amount of suction, it idles normally. Throttle position sensor has been replaced and did not solve the issue. Any tips/suggestions/guidence is greatly appreciated!

 

Sounds like an iacv issue. I had the same issue, jerky trottle on the highway anywhere between 0 and 1/4 throttle, idle surging and all kinds of crap. I had an air pocket in the coolant passage of my iacv. I aslo thought it was something with the TPS, but because the iacv acts as a choke and closes when the engine reached operating temperature, if it isn't functioning properly you will have all sorts of issues with other sensors as well. I'd check out that iacv first thing.

 

Cover the IACV hole in the TB with your finger. Does the idle speed settle down?

 

Agreed, but do this when engine has reached operating temperature. If you try stis while the car is cold it will most likely completely stall the engine.

 

I did exactly that, and yes, the idle corrects itself once the IACV hole is covered. Assuming I'll need to replace that now? I've got another on a spare manifold, I'll swap them and see if that resolves the issue.

 

You can check the function of you iacv by simply removing it, and checking to see if it spins freely. If it spins freely then chances are it's working properly. It can also not function properly if there is any air in the coolant passage of the iacv, or you may not have the proper voltage running to it.. Do you have a coolant leak anywhere?

 

Start by cleaning the IACV.

 

No coolant leaks that I can spot, is there a specific way I can make sure there are no air pockets in the passage? Going to be removing it within the hour and I will check to see if it spins freely.

 

Im not sure of a way to really check for air pockets, but you can burp/bleed your coolant system to assure that there is no air in the system. There is a video on YouTube on how to do that, it's basically just a matter of starting the car, letting it cool, topping off the coolant, repeat. You will definitely create an air pocket when you remove the iacv from the manifold, so this is a procedure that you'll want to preform anyway. When you remove the iacv you should have coolant pour out of it and the manifold, if you don't see any coolant then chances are there was air in the coolant passage. If it doesn't spin back and forth freely as it should, try cleaning it. Sometimes debri and goup will build up on it over time restricting the function of the iacv. If it seems to be working properly, and you burp the coolant system and the issue isn't resolved, next I would research and see what voltage should be running through which wire that goes to the iacv, and then check the voltage when the engine is both cold and at operating temperature, as the voltage changes when the engine reaches operating temp to close the iacv. I'm just not sure of voltage that you should expect to see, or which wire to check.

 

The only purpose for coolant circulation through the IACV is to prevent it from freezing up during the winter.

 

I took off the IACV, noticed it wasnt spinning with ease so I let some carb cleaner soak in it which helped free it up a bit. I reinstalled it and also adjusted the throttle cable because I noticed it had slightly too much tension, to where it wasn't allowing my throttle body to close 100%. The idle has been corrected, no more hunting (woo-hoo!), only issue now is upon startup, it idles very low, maybe I adjusted the throttle cable too far?

 

 

Thanks for the tip, running 100% now! All issues resolved, thanks everyone.

 

Glad we could help, dude!