I pasted the findings I posted on my website:

Decided to deal with the odd-ball idling so it was backed out and I went at it. First the blanking plates I'd installed were removed and sure enough I still had the surging idle. Reading the Honda Service Manual helped, plus reading up on what people had done on honda-tech.com. So I had it all hooked up and confirmed the surge was there, so now what? Well the fast idle valve is supposed to close when the engine warms up, but it didn't. I became suspicious of the thermostat because the Honda thermostat does two things at once, controls the introduction of cold coolant returning from the radiator, but also controls flow to, you guessed it, the FIV and IACV. So I took out the old one and of course, it looked perfect; I even put it in hot water and verified it opened properly. But since I had a new one I replaced it anyway, and then noticed the inside of the thermostat housing, yuck! Either the original owner of the car didn't take care of it, or more likely, it sat with coolant in it for years in my garage. So then I became suspicious of the whole cooling system.

That's about when I noticed the hoses going to both the FIV and IACV were cold, and stayed cold even after the engine was warm. So I unplugged one hose and hot coolant came out... huh? I finally figured out the hose the hot coolant was coming out was actually the return line and was under no pressure even when the engine was running. That's because it was trying to suck the coolant into the water pump. Okay, so that's the return line, so the other one must be the supply line, right? Disconnected that one and, surprise, nothing came out of it. Fine, so the hose was pulled off, and still, nothing came out of the metal nipple off the end of the cylinder head; that can't be right. So I found the appropriate sized drill and pushed it in there. HAH! It was plugged up with dried coolant goo! Whoo Hoo! So after cleaning it out the system was put back together, topped off with coolant and started up. It went into its cold warm up mode and I waited for the coolant to come up to temperature. Feeling the hoses wasn't encouraging as they remained cold. Then within just a few seconds the idle started coming down and the pipes got hot in a hurry - neat! I also verified the IACV was now correctly controlling idle speed, and I was all set!

Hope that helps some of you with this ailment.





Modified by kb58 at 8:24 AM 1/1/2006

 

haha wow talk about build up. kinda makes you think whereelse its built up like that

 

Wow, did you engine run hot before? I guess it's hard to compare to an actual Prelude since you don't have our guages.

 

sounds like you need to do a full coolant system flush and get all that buildup out of there... just to be on the safe side