BISS screwed itself?

[Chad Giffen]

Weird shit happening with my car lately. Gave me a scare. Thought a blew a head gasket....


Anyways, alternator crapped out. I cooked it from the stainless manifold....     swapped it out with Ryan Coft's and all was well. I had oulled the rad to do the job.


Didn't tighten the top rad hose quite enough and had a temp spike on the eay to work a few days later. No big deal, just tightened it and added water.

On the way home the other night, my rpm's starting dropping at idle. The car would stall out at times and I had to hold the throttle down while starting to get it to move. Made it home and thought I blew a head gasket....    Or was on the verge.


Did a compression check...   GOOD
Did a ignition check.....         GOOD
checked my timing belt marks.....    GOOD

I was like wtf! Is it as simple as turning up my idle?

AND IT WAS!

I've owned the car going on for 6 years and the BISS has never moved on me once...     but the car drives totally fine now...   


Thoughts?

Maybe a sprung a vacuum leak and had to turn the idle up to compensate?

Chad

[Brett Haviland]

i have had the oring in the BISS screw get hard and it will screw itself in.... causing a low idle/stalling problem.  This was on a 4g61 hyundai elantra i had.  I ended up taking some chewing gum, removing the BISS screw and wrapping the gum around the threads like teflon tape sort of.  I then put the BISS screw back in, set the idle to 750 ish and away i went!!  never threaded itself back in or out after that! AHAH!


On my 89 Colt GT with 4g61t, i also had a problem with the biss screwing OUTwards...  That time i ended up just pulling the biss screw out again, but this time i replaced the old hard, worn out with a brand new one.  re-installed it, and set my idle again and wala!!  no more high idle problems.


So the jist of it is that YES, your biss screw can screw u up! ahahah

[John Hartman]

and if it was set correctly before, but now something has changed and the idle is not where its supposed to be so the ECU tries to put it there, but if the BISSS is set wrong, the IAC will be too far one way and cannot move any more that way.

Reset the BISSS and the IAC is now centered, or at least close to it, so it now has authority to change the idle speed.

However, it doesn't take much of an overheat to warp these heads a little and/or damage the head gasket.  Car will still run, compression will still be good, but it can have damage.

Keep your eye on that.

[Chad Giffen]

Thx for the info John.

For my own curiosity, I run compression tests every six months and my values haven't changed even 1 psi.

I didn't overheat the car either...  the temp never went critical..   just about 3/4 before I stopped it.

It seems to be ok now, except when I start it, the rpms don't jump very high and come back down. It used to jump to 2500rpm, then quickly drop to idle. Now it fires upto about 1200 rpm, and drops to idle.

I think im fighting a vacuum leak.

[John Hartman]

could be.

As for overheating, the needle isn't really linear.  Even a needle thickness above normal is quite a bit hotter.  If its 3/4 the way up, its way too hot.

Not trying to scare you or anything, its just that these needles are a bit wierd on these cars. Like the "boost" guage for example.

As for the idle speed, do you still have your fast idle air valve in play?

[CamWeiss]

I'm paying attention to this one as my idle sucks too.

Does the FIAV compensate for load on the motor at idle, or is that the ISC?

[Chad Giffen]

I have my FIAV in place and I have the upgraded black ISC (the beige ones fail easily)

My compression was solid though. I do have a slow coolant leak somewhere though....     it's very slow and been happening for over a year. I think my freeze plugs are leaking a bit as my block has been frozen solid a couple times over the years.

[Jason Harwood]

I was driving my car home from the track one time and it kept stauling everytime I stopped and was running a little sluggish (felt like a boost leak) . I shot my BISS screw OUT at the track.

Thank god I had a spare throttle body around the house as the time.

[John Hartman]

Cam, the FAIV is only based on coolant temp/flow.  It melts a wax pellet that allows a spring to open a valve.  Its usually shit on older cars tho, the spring breaks/rusts in half or something similar.

Chad, lost coolant on a car that has an intact FAIV with coolant flowing like stock can acutally be lost into the engine.  If it fails internally, coolant can(and will) go out the air port and into the intake manifold.

I've seen it twice on my own cars.  I posted a few years back on this because of crappy ass Delco coolant. Soon after I changed coolant to this shit, I lost both my Talons and my Galants FIAV seals.  Smelled coolant, it was getting lower, finally noticed a wee bit at the intake pipe on the Galant.

[Jordan Kruger]

FIAV delete.

[CamWeiss]

Okay, this one should go in the noob section...

But what exactly does the ISC do?
What exactly does the FIAV do?
What other idle controls are there?

[Jordan Kruger]

The Fiav allows your car to get a little more air at idle on a cold start. Once warm the spring changes the mount of air that can bypass. The ISC has a plunger that allows you to release the throttle from any position, and the ISC plunger will open accordingly to allow enough air to keep the rpms from dropping to erratically. There is also an Idle stop switch. Its the one wire stopper that makes contact with the throttle plate as it closes. When the stop switch is set correctly it will bump the throttle open when you release the gas pedal, to keep the rpm up enough for the idle to settle correctly.

FIAV = Good for cold start but they dont last and could end up leaking coolant into your throttle body

ISC = Good for helping your rpms settle but the plunger can wear off the end and leak unregistered air into your throttle body

Idle Stop Switch = Good for assisting your tb to find idle when you close the throttle quick.

The FIAV and ISC both are located on the lower half of the throttle body and share the same gasket. That gasket gets brittle after years of heat cycles and it can crack (mine did)  If you suspect issues with the throttle body I would take it apart to ensure that this gasket is good. If the gasket isnt good, then your FIAV and ISC could be sharing air and that will only make diagnosis harder. Its also good to check on the condition of the plunger tip on the ISC   they can get brittle and sometimes even jam in their bore.
 

[John Hartman]

ISCs more commonly fail electrically internally in the windings, either from corrosion due to liquid intrusion.

The ISC is also there to control idle speed normally, not just for when the throttle is closed quickly.

Power steering, A/C and other things that make the idle drop will be picked up by the ECU and compensated for with the ISC.  It can bump the idle up appropriately when needed.

The FAIV is just dumb in my opinion.  Even in the coldest weather, the ISC will allow the engine to idle high until it warms up.  Most correctly working FAIV cars I have seen will make the engine idle at 2000rpm when cold!  Yea thats great...rev the piss out of it before oil is even circulated...

[CamWeiss]

Sounds like the ISC is knackered. On a cold start it immediately goes to a regular idle. PS/AC/aux fan load isn't compensated for and the idle will drop. Sticky throttle doesn't help, but it also very easily seems to die if you put in a very small throttle input off idle.

[Jordan Kruger]

Your TPS might be out.

Oh and I meant to add that the ISC does adjust throttle due to various factors like John mentioned. Not just when throttle is closed.