Terrible Idle problem. - Honda Prelude Forum

burnout01134 · 07-03-2011, 12:36 AM

I have a '97 Honda Prelude Vtec Type SH and she's breathing real bad. When I start the car the idle goes to around 2.3k and stays there for about 2 seconds and slowly starts going down and then starts bouncing from 900 to 1700 rpms over and over and over. It will do this until I turn on the a/c all the way. The a/c has to be on feet and face and full blast and Cold on or it will breathe wrong. Everytime it goes into neutral it starts breathing this way. I've tried cleaning the IACV and it didn't help. She started to sound different, kind of like an exhaust leak almost and her I'm pretty sure she's had a power loss. I'm also having terrible gas mileage and burning rich in oil. I took it to get the codes read and this is what I got
P1508
P0108
P0505
P0420
If anyone knows any suggestions to fix this problem PLEASE help me.

Nighthawk01 · 07-10-2011, 11:08 AM

P1508 Idle air control Valve Circuit Failure - possible bad IAC
P0108 MAP sensor - possible bad map sensor or vacuum leak
P0505 Idle air control System Malfunction
p0420 Catalyst System Efficiency Below Threshold - possible bad 02 sensor or cat

first things first - check for any vacuum leaks. How many miles do you have on the car? move onto the MAP sensor and try replacing that. Then onto the O2 sensors, you may want to try replacing both. Do you have a factory cat? do you smell anything odd from behind the car like rotten eggs or something to give a sign of a bad cat?

burnout01134 · 07-13-2011, 12:34 PM

Yes, there is a bad smell definitely when there is little gas. I also hear a sound coming from under the car that is probably the cat making a noise. Could a bad cat cause all this? i dont wanna pay $200 for a IACV if thats not the problem. i've looked around for vacuum leaks but couldnt find any. but also, there is a hose connected to the plug on top of the engine that makes a crackling noise when the car is running. not sure what it is but it doesn't sound good. 140xxx miles

burnout01134 · 07-13-2011, 06:09 PM

Also, the engine bay gets REALLY hot after starting. I can't touch any pieces under the hood without getting burnt. My temp. sensor reads in the middle. Should i bleed the cooling system? I heard this could be a problem for the IACV and would also explain the heat. I plugged the hole in my intake valve and it did nothing so i'm guessing it's not my FITV. I've taken off my IACV and cleaned it but still no fix. I unplugged the MAP sensor and the idle calms down.

minilogoguy18 · 07-13-2011, 08:48 PM

^Clogged cat is causing the heat, if you let it cool, remove the heat shield from the exhaust manifold you can watch it get red hot and glow. Start with IACV and catalytic converter, reset ECU and see what codes come back, P0108 could be getting tripped by the IACV and not the MAP sensor or vac leak.

Nighthawk01 · 07-13-2011, 10:18 PM

If you smell a rotten-egg or similar smell, than it's sounding more and more like a bad cat converter.

Which would also explain the o2 sensor code and excessive heat.

If you replace the cat - be sure to get a new 02 sensor as well. for the time being you can at least remove the cat and have a mechanic look at it - they are expensive to replace.