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FIAV, ISC.... Idle Surging Horribly!

So my idle surges very badly.... and it's QUITE annoying.. Anyway, When i first start my car the idle jumps to about 2300 rpms. Then a few seconds later (if warmed up) it drops and surges like crazy! Sometimes it will idle at 900 and stay steady. But not very often. I heard i could unplug the FIAV or ISC or something of the sort. But i was wondering if anyone had a link that shows how to do this? (not the block off plate...) I heard there is a way to unplug something and see if the idle goes to normal. What do i unplug and where is it? Any help would be much appreciated! thanks! /ubbthreads/images/graemlins/banghead.gif
 
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spoulson

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If there's one thing that's for certain, it's that the FIAV has a limited life. It's job is to allow additional air to bypass the throttle when the engine is cold, then slowly shut as the engine reaches operating temp (180*). It's a mechanical ball of wax, quite literally, and it doesn't last forever. It loses its ability to close the air passage and you get a high idle.

When too much air gets by, the idle wants to go too high. While the throttle plate is closed, the ECU stops the high idle by cutting timing to cause a drop in idle. But, the air passage is still open and the idle jumps back up. The cycle then repeats.

The ISC can cause a similar effect if its motor is dieing and leaves the air passage open.

Technically, the FIAC and ISC are non-essential and you can get by fine without either by using a blockoff plate like this one from JM Fabrications. The downside is you lose the benefit of the ISC. This keeps your idle smooth in varying weather conditions and during driving. For example, if you free rev the engine then let off the gas, the ISC will 'catch' the idle at ~1200rpm, then slowly bring it down to normal idle, thus preventing an accidental engine stall.

I solved the FIAV problem by installing a blockoff plate that allows me to keep my ISC. After doing this, I found that once warmed up, I stay at a normal, steady idle.

However, it seems the ISC does also have a function for increased cold idle. My car idles around 1200rpm when cold and drops to normal idle when at operating temp.
 
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JSchleim18

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Also, Spoulson means the FIAV BYPASS plate is the way to go if you wanna keep the ISC...i actually just ordered one from JM Fab!
 

spoulson

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It is also important to note that when trying to fix the idle surge problem, you MUST set the BISS properly afterwards. You cannot do this by simply turning the BISS in and out while running. You have to put the car in diagnostic mode and fixed timing. This is detailed in the 1G BISS VFAQ. The difference to note is that the timing plug is actually on the firewall next to the brake fluid reservoir for GVR4's.

If the BISS is set wrong, you can also get idle surge.
 

belize1334

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If you wanna make a quick check to see if the ISC is CAUSING the issue you can unplug it. This will typically cause your idle to jump up by a bit but it should also stabilize it if the isc is the culprit. The ISC plug is under the throttle body elbow on the intake manifold on the passenger side of the bay. I tried this and mine still surged. I was on the verge of replacing my o2 sensor and wiring when I check my ECU error codes and found an Air Flow Meter error (number 12 I believe). I changed to my maf and the problem was cured. Of course, mine wasn't a surge so much as a bog (it would drop to like 300 and then try to restabilize) so your problem could well be something else.
 

hmm... kind of weird... But when i mess with the MAF plug in. It will randomly idle normally for a few minutes. Then go back to surging. Bad MAF?
 

belize1334

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I never wiggled my Maf plug. I already had a 3g maf, chip and some evo 560s to go in so I just did the whole swap and everything got better. But the ecu code has never been back so I'm convinced that mine was the MAF.
 

spoulson

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The MAF contains the intake air temp sensor, used to determine air volume. I've heard of the IAT failing.

Anyway, unplugging the ISC will only freeze the plunger in place. If the plunger is in the far open position at the time, it will stay that way. The best way to check the ISC is check two points of operation: both coil resistances and that the plunger moves in and out smoothly. I don't have the howto references handy, but I'm sure you can find info on VFAQ or elsewhere.
 
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Well i read up on some how to's and that. And i finally had some spare time today to fidle with it. Turns out i do have a bad ISC. I took out my whole TB made sure it was clean and closing properly and so on... Then i took out my ISC plugged it back in, and turned the key on and off. All it did was vibrate /ubbthreads/images/graemlins/rolleyes.gif So after that i realized it's gotta be bad. As well as it was stuck on the in position. So all i did was turn it all the way out, put it back in and left it unplugged. Car runs fine now and the mixture seems to be normal. My gas millage was horrible, im guessing that was the colporate. Anyway thank you for all the of help i appreciate it. But one more thing, will it be fine without the ISC? Seems to run just fine without it i heard you don't even need it? Thanks.
 

ok cool thanks. Maybe a block off plate is something i should consider. Hoping it will not affect my a/c.
 

boostedinaz

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All the ISC should do is vibrate when you turn the key on.

now for the FIAV block off plate.

If you get a plate that only eliminates the FIAV then the ISC should still control idle. If you remove or block the entire lower portion of the TB the idle will drop with things like turning on headlights, fans kicking on, and the A/C compressor engaging.
 

the isc just vibrates when you turn the key? and thats normal?
hmmm i was told that if it vibrates something is wrong. if it moves in and out smoothly then its good
 

spoulson

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The ISC is a stepper motor, containing two coils that energize on and off in succession to rotate the plunger in and out. When you do a key-on, the ISC wants to do a self-check and stop at 50%, or so I understand.

Have you removed the ISC and watched for the plunger to move in and out on key-on?

As an aside, when repairing my faulty wiring on the coolant temp sensor, I heard the ISC jumping around as the coolant temp signal spiked while shaking the harness wiring. There must be some logic to ISC position vs. coolant temp. I'm guessing this to have a higher idle on a cold engine.
 
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yeah i had it out and checked it. i did every diagnoses i could find for it and it failed all of them. it couldnt of been working
 
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