Smallport FXGT fluctuating idle problem.

[HachirokuGTV]

A mates ae92 fxgt has an odd idle for some reason it goes up and down randomly and sounds like your revving the motor at the lights. He is now too embarrassed to drive it and looks like a wanker when he does.
Do you know what the problem is?

[rollaholic]

does it do it all the time? or only when cold?

might be idle speed control valve under throttle body

[HachirokuGTV]

all the time I think.

[gasman]

how fast is it bouncing up and down? like a rotor or more like a big moan

[crazed_silvertop]

check for air bubbles in the coolant.. I used to have a similar problem in one of my old AW11 MR2's

[HachirokuGTV]

It sounds like your pumping the accelerator pedal and revving the engine.

[Tuscan]

My ae92 did this when it had no coolant in the radiator

[Mr Revhead]

yeah air lock
or.... the really common dead idle control valve

[HachirokuGTV]

ok ill try these things.
Thanks guys i'll let you know how i get on.

[rollaholic]

4age relies heavily on water temp sensor to control idle, the one at the back of the head. (green plug from memory) which is why it does it when you have no coolant

you can check if its the idle speed valve by taking off the inlet piping and sticking your thumb over the bypass valve in front of the throttle butterfly. if it stops bouncing you found your problem.

[mr30%jr]

also theres a bigger vaccume pipe under the throtle body make sure thats blanked off its the aircon idle up

[d1 mule]

my old 85 did that when i thought i was cool by taking the water pipes off the throttle body. not sure what thats called tho...

[rollaholic]

my old 85 did that when i thought i was cool by taking the water pipes off the throttle body. not sure what thats called tho...

its the air bypass system for cold idle

[andyztouring]

Clean the throttle body where the butterfly is mounted(beside rubber/plastic intake hose),they get very dirty in there,clean it out until spotlessly clean,this has caused bouncing idle on 3 of my cars,fixed each time by cleaning the intake/butterfly.

[frost]

ok first off, what kind of jumping idle,

**im thinking its this one from the description you gave**
if its a high revving and jumping to about 2.5k and back down every second or so. then its a air leak from one of the small pipes under the throttle body, block them up with a bolt or see if there is a stray vacuum pipe thats meant to be plugged to it,

if its a small jumping idle like just a few hundred rpm and is random not timed, then it could be a few things, crap dizzy cap, bad spark plugs, bad engine earth,

if its constantly high idle, like it stays at 2k then its the cold start air bypass valve under the throttle, i can show you how to recon it so you dont have to pay $180 for a new one if you like, its a bit tricky and you'll need a few tools but i can give a photo guide to how to,

OR because you have a cut n shut small port manifold, you forget to block the hole between the manifold and throttle body, rotate the gaskit so it blocks up the hole,

[Mike-]

i had this problem, it was one of the breathers on the throttle body i think,

some goon thought it had a rota

[pc]

if its constantly high idle, like it stays at 2k then its the cold start air bypass valve under the throttle, i can show you how to recon it so you dont have to pay $180 for a new one if you like, its a bit tricky and you'll need a few tools but i can give a photo guide to how to,

This sounds useful, can you post it anyway? We probably need a faq on 16v idling problems with all the standard causes listed in order of probability with associated fix.

[ChaosAD]

air bubbles in the cooling system also make the revs increase and drop

[frost]

This sounds useful, can you post it anyway? We probably need a faq on 16v idling problems with all the standard causes listed in order of probability with associated fix.

i think thats a good idea, too many people with bad idle and dont know whats causing it, if we could make a list of most common to least, that way people can start at the top and work there way through the process,

ill start taking pictures and make a how to fix thread, i think most of the people on here should be able to do it, all thats needed is a few tools, a vice, and some good sealant, will update soon