Heard about it. Almost did it but worked it out in my tune. Before you go drilling make sure the IAC calve and everything associated with it is clean and working properly

 

I did it. It didn't help. If your car is mostly stock do the diag and figure out what's wrong with it. There;s a giant checklist floating around of all the potential causes, makes the process foolproof. If it's surging due to cam profile, tune it or take it somewhere to get tuned.

very shortly after hooking up my quarter horse my car went from a surge so bad it was undrivable, to idling almost as smooth as stock. Better yet, the car is fun to drive again!

 

Wouldn't opening the throttle blade a little more with the throttle set screw do the same thing??

 

Exactly the same thing with EFI. Won't help a bit.

That idea might have come from throttle body injection, or from what people did with carbs and big cams. With a carb and with low idle vacuum, the hole helps, if properly located and sized because it keeps the throttle plate from moving down on the idle transfer slot.

 

Don't drill, tune or fix other issues

 

drilling is a one way street so be very sure
The reason to drill a hole is to have the same TPS reading with adifferent airflow
Another wayto check this is make a small vacuum leak somewhere

 

The vacuum leak lets unmetered air in -- different than drilling a hole or adjusting the throttle stop.

Edit -- oops; assumed mass air conversion. Is the 87 speed density?

 

Yea it has maf convert ion, I unplugged a vac line and it perfected my idle. I have stock heads edlebrock cam TFS intake all the other goods except matching maf for 24 lb injectors.

 

Your MAF probably isn't compensating the 24 lb injectors at an idle. Your probably too rich. Can you pull the fuel pressure down more at idle??

 

Do I need and adjustable fpr to dial back the fuel or is there another way?

 

Adjusting the EEC with a Tweecer or something works, or getting a chip made. An adjustable regulator is another way.

Your fuel system should have a vacuum line to a regulator. When the engine has vacuum the regulator pulls the rail pressure down. This helps the idle.

You could have something else wrong, check the system, but it could just be you have confused the idle (which does not depend on the MAF so much, because it is closed loop) with the 24 lb injectors.

It also could be you have the new throttle plate system closed too far at idle, and the IAC is out of range.

These recalibrated MAF meters are not what they are cracked up to be, because the engine load and other things like closed loop (low load, light throttle) get thrown off. They work OK open loop where the MAF is telling the injectors how much fuel to spray and the O2 system is out of the picture, except the EEC can go to the wrong lines in some fuel and spark tables.

If a controlled vacuum leak fixes it and you don't want to mess around at the moment, use it.

Tom

 

"These recalibrated MAF meters are not what they are cracked up to be, because the engine load and other things like closed loop (low load, light throttle) get thrown off. They work OK open loop where the MAF is telling the injectors how much fuel to spray and the O2 system is out of the picture, except the EEC can go to the wrong lines in some fuel and spark tables. "

Lots of generalizations here that simply aren't always true. LOTS of folks out there with properly calibrated mafs that have stock idle and drivability quality. Mine is one of them - PMAS meter with 24 lb/hr injectors. Having said that -- an improperly calibrated meter can wreak havoc with all sorts of things. And as Tom alluded to, if a vacuum leak seems to solve the problem, to me that seems to point to the maf and ecu not playing together nicely. However, I'd solve the problem rather than treating the symptom. And creating a vacuum leak is definitely treating the symptom.

 

The throttle body scalar is subtracted from the Air Mass entering the engine. The MAF and throttle body scalar work in unison to calculate IAC duty cycle, and desired rpm at idle. The .5kg/hr wouldn't even let an engine idle.

So again, the MAF is significant at idle. Closed loop and open loop doesn't matter either. You still have to know the Air Mass entering the engine.

 

The MAF IS NOT used to control idle strategy but it is used at idle to control fuel. Even though the MAF doesn't control idle strategy it's still helpful to dial idle air in. It doesn't matter whether it's in CL or OL it uses the same idle strategy.



The ECU looks up the idle air function (FN875n) to calculate how much airflow (AM) is needed to reach a DSDRPM (desired RPM), by knowing what the actual AM (known by completing step 1 below) is and also knowing the amount of airflow sneaking past the TB (ITHBMA) it can calcualte the duty cycle needed of the ISC valve to get the proper amount of air to reach DSDRPM (desired RPM).

Keep in mind I'm a CBAZA guy working with BE12 but this should also be spot on for you GUFx guys too...(maybe with a little more math needed tho)




To dial in idle you...

1. Once fuel is dialed in and on a warm stabilized engine with no RPM adders, start by entering the AM (airmass) and NUBASE (base neutral idle speed) into the neutral idle air function.

2. Now you enter the proper throttle body airflow scalar (ITHBMA). To determine what the value for the ITHBMA scalar should be you subtract AM (airmass) from DEBYMA (the lb/min contribution from the ISC) and update the scalar accordingly.

3. Repeat step 1 again for higher rpms (because a cold engine needs higher rpm's to stabilize).

 

Ford makes a part to help with idle air control. It has adjustable air bleed screws and a provision for 5.0 displacement versus 5.8 and larger. I would recommend going with this rather than a hole in the butterfly.

Here's a link to one source for purchasing the part.

1986-93 Mustang 5.0L/5.8L Idle Air Control (Iac) Adjuster at LRS - Free Shipping!