Resetting the IAC on FreeScan…..

[GordonMasson]

So… Ages ago I had a play about with FreeScan and I noticed a strange quirk with resetting the IAC.

My GT3 has always been lumpy and sluggish at low revs and in particular in 1st and 2nd gears when you are trying to pull away gently, the engine stumbles and you have to give it fairly high revs to make sure you dont stall.

All in all it can be a pain in the arse driving around town.

What I noticed when I used FreeScan before (and also at the weekend when i managed to get the connector under the glove box working ....there are two... make sure you are in the rightone!!) was that if I reset the IAC the characteristics of the car change completely and its much smoother to drive.

After a while, as the ECU “learns” the car, it goes back to where it was again…lumpy, stumbling and sluggish at low revs.

I did replace the Throttle Position Sensor and the IAC valve but it didn’t make any difference.

Any ideas why the car is so much better after an IAC reset and then it gets progressively worse?

[Vulcan Grey]

probably an air leak in the manifold. Check all hoses and gaskets.

You can clean the IAC pintle. It my have crud on it.

Also, reply back here and tell me what your IAC number is in Freescan after the car is fully warm. Start the car and let it idle, do not touch the throttle, and look at the freescan IAC # when the coolant temp is above 80C.

[GordonMasson]

Hi Travis

I have had this problem since I got the car and it didn’t improve when I changed the IAC unit other than by resetting it on Freescan

Just had a new manifold put on it so I would be surprised if its an issue on that front since the same fault has carried over…. Could be wrong though?

I did a run in the car at lunch time and checked the IAC 3 times as you suggested. The readings came out at 58, 60 and 63.

I did a log while I was out on the run if that’s any good?

Oh I reset the IAC about 12 miles back so it may still be “learning” the engine but its so much smoother at the moment…night and day!

Thanks for your help

[Vulcan Grey]

Those IAC counts are high for a warm engine at idle.

There is an adjustment that usually isn't necessary, though it seems like many shade tree mechanics will play with it as an attempt to change the idle speed. However the IAC actually controls idle, so most attempts to adjust the minimum air rate bleed screw will not have any noticeable effect... at first.

The problem is that when someone adjusts the minimum air rate bleed screw, without the use of freescan, the IAC compensates, and the idle doesn't change. The actual effect is poor driveability later.

Assuming you do not have any air leaks in the manifold, you should really make sure, you can adjust the minimum air rate bleed screw using freescan.

The minimum air rate bleed screw is on the throttle butterflies, where the throttle cable attaches. It is a flat head screw protected by an anti-tamper cap. Do not play with the other adjustment on the butterflies!! That is the balance adjuster between butterflies and should never be touched!

Using freescan, warm the car up by idling until it gets to 80-82C water temperature. Keep the car running with the AC off.

Watching the IAC counts in freescan, SLOWLY and in TINY increments, adjust the minimum air rate bleed screw until you see the IAC counts change in freescan. You should only turn the scre ~1/16th of a turn each time and then wait more than 10s before adjusting again.

Adjust slowly until the IAC is stable between 20-40 in freescan.

If you have an air leak though this won't help much.

I had to replace my idle bypass hose recently, it was so old and hard it shattered like a brittle plastic... Any of those old hoses and one way valves connected to the intake manifold are suspect.