MOBSTER

My friend just bought a 2000 prelude base, and just sometimes the idle on the car jumps up and down. The car does it after it is warm and has only done it twice so far. I checked the vaccum on the Idle control valve and it seemed fine. If it was the TPS would it not do it all the time? It is also getting some puffs of smoke in between shifts when he is reving it high. I can't remember for sure, but I think the smoke was white which would indicate over fueling. He had codes ran and it was showing a pressure drop in the catalitic converter, if it was clogged wouldn't it idle erratically all the time? Car has a DC header and a apexi exhaust and a no name intake. I noticed that the hose that was hooked up from the valve cover to the intake was by passing the the two fittings where the original lines hooked into, would this cause any proplems or are the other lines just some kind of drain tube? I did a search on this topic, but didn't get enough info.

mtbprelude

Did you do a search? I'm sure this has been brought up before?? The I/H/E should have no effect on the idle. If the car has a fouled O2 sensor, and/or cat issues a CEL would reflect that, but rarely affects the idle.

In short: You need to check any and every vacuum line running to the IM, not just the IACV. Also attached to the bottom of the throttle body is the fast idle control valve: to check it, detach the intake pipe from the throttle body let the car warm up (fan clicks on at least once) then put your finger over the hole in the lower left, if your finger gets sucked onto that hole...there is your leak.

MOBSTER

As I stated in my original post I did a search, but there just wasn't enough detailed info. I thought the hole inside the throttle body was supposed to be sucking air. You put your finger over the hole while the car is at idle speed and if it causes a drop in idle everything is OK with the IACV. If i am wrong let me know, I have owned 4 different 5th gen ludes, but I have never messed with one this new. Is the part your talking about called the Fast Idle Thermo Valve if so isn't it only on A/T cars, this is a M/T. Thank again.

mtbprelude

Part #12 in this diagram:

(From slhondaparts.com, for 2000 MT Base Lude, engine, throttle body page)

Its on all 4g/5g preludes. It is supposed to be sucking air, until the car is warmed up. Then once the thermostat opens, the hot coolant heats a wax plug, and closes it off. The wax plug deforms/fails with high frequency. You have 2 choices if you don't want to replace it...do the FIV/IACV coolant bypass modification (if you live anywhere its cold for extended periods of time, or strict Air Restrictions Board don't do this...) or cut an aluminum can/sheet to insert between the FIV and the TB.

MOBSTER

Can the fast idle valve be replaced or is it rivited onto the throttle body like the tps is?

mtbprelude

No rivets. 3-4 10mm Bolts. It is replaceable, I believe it costs around 58 bucks.

You can also disassemble it and screw the wax plug closed, coolant will still circulate etc, but until warm your car will run rich, until warm.

MOBSTER

At what point should there be coolent flowing through the fast idle valve, if I understand it right it should flow in after the car has warmed up and the thermostat has opened. I have removed both lines, the one going directly into the fast idle valve and the one going into the bottom of the throttle body, which I was guessing was a return line, to try to see if the IACV was letting coolent circulate at all. I couldn't get coolent circulate, is it supposed to and at what point? If it is not circulating is this an indication that the IAVC isn't funtioning properly? Thank you for the help.

mtbprelude

Sorry been busy, grad student + exam time = sux to be me

So, the FIV has coolant flowing through it once the thermostat has opened. All ICE (internal combustion engines) run best in a window of temperature. So until its brought up to approx that temp your thermostat is closed. Once above that, coolant flows. Including to your FIV/IACV. Before that happens the FIV is open, letting more air into the TB, and marginally increasing your idle & running rich, this is REALLY obvious when its cold. Otherwise, not so much. Once the coolant reaches the FIV, it heats up a wax plug that then closes the FIV. Only the FIV is on the TB....

The IACV is on the front of your intake manifold (IM) it looks something like the FIV, and IIRC the line out of the FIV runs to the IACV or vice versa...in either case, both can be removed, and with a thin piece of aluminum can be layered between the valve and the TB/IM. I know I used a Coke can to help a friend out with this when we didn't have anything else handy.

MOBSTER

If coolent is not circulating from the IACV to the FIV is this an indication that the IAVC isn't funtioning properly?

mtbprelude

Yah, it may mean that the IACV, which, IIRC, has a similar wax plug/plunger valve and is stuck in the open position, which means your idle will be higher than expected, again.