This weekend, I went about checking/resetting "minimum air" on my car, but was a little rusty on the procedure. So I pulled out my GM shop manual and read the instructions (what a concept ). It said to disconnect the ESC bypass! It even said Important! I had never done that when setting minimum air before.

The tech article here doesn't mention it.

Vader's "article" (the one he's posted several times in response to similar questions) doesn't mention it.

The article in Car Craft (or was that Hot Rod?) didn't mention it.

Other responses did mention it, however.

Since the ECM will advance the timing at idle, which increases RPM, this should have a significant impact on the minimum air setting. Should I or shouldn't I disconnect the ESC bypass before setting minimum air?

------------------
'87 IROC 5.7l TPI - original owner!

 

I tried sending this msg three times; hope all of them don't post.


I know exactly what you mean. My 86 Vette GM Service Manual says the same thing; Important to disconnect the EST wire before setting minimum idle speed.

I always do it that way and then disconnect the battery for at least 30 seconds to clear the error code that gets set when the ECM detects the engine running with the EST wire disconnected.

Jake

------------------
1986 Corvette Coupe, 415 CID, Edelbrock 6073s, ZZ9

 

I adjusted mine last week with the instructions given at TGO and did it without disconnecting the ESC bypass. I couldn't get the idle that I was looking for so maybe it is required to do it right.

------------------
89 IROC-Z 5.7L
2.77 GEARS
K&Ns FILTERS
ALL FREE MODS
FLOWMASTER MUFFLER
PERFORMANCE RESOURCE CHIP
Best 1/4 E.T. 14.37 @ 95 MPH
Best 1/8 E.T. 9.22 @ 76 MPH
Best 60ft. 2.07

[This message has been edited by whiteroc (edited June 11, 2001).]

 

If the computer advances timing, it also advances idle speed. My 89 GM Service Manual states to disconnect the EST connector when setting the minimum air.

You could probably set the timing either way. Since there is little to no effort in disconnecting the EST, I do it anyways.

------------------
1989 IROC-Z 5.7L
NOS 150HP kit
ProBuilt 700R4, PI Vigilante 2800 stall lockup
Baer Brakes 12" Sport System

 

If you are just setting curb idle to be sure the engine won't die if you stop suddenly then you don't neccissarily need to unplug it.
However the reason you are instucted in the manual to do it is so the engine will run in case of a system failure.
If the computer goes in limp mode or something just fails and the timing drops back to baseline and the IAC can't react quick enough the engine can then die at idle.
Unpluging it before setting curb idle just allows you to set it for a worse case type scenerio.

------------------

Tracy /AKA IROCKZ4me
'86 IROC-Z Camaro
"Cogito ergo zoom"

• 355 cid
• AFR heads
• Arizona Speed & Marine hydraulic roller cam w/ AFR hydra-rev kit
• modified SLP runners
• TRW forged pistons/ceramic coated
• fully balanced
• Edelbrock headers/ceramic coated
• SLP cat-back
• Paxton supercharger
• Nitrous Express nitrous oxide

EFI Performance Club on Yahoo

 

By the way - anyone know if it should be done with warm or cold engine???

------------------
-Greg

'85 Trans Am - WS6,TPI:
Original owner, original paint/engine/interior, plus the usual bolt-ons

 

That makes sense... I'll disconnect it. In my case, this will result in a significantly greater opening than from the factory, though.



------------------
'87 IROC 5.7l TPI - original owner!

 

Warm engine. I do know that much, for sure.



------------------
'87 IROC 5.7l TPI - original owner!

 

PJM,

Good question. I'll have to check the Helm's more closely. I don't recall reading that, and have always been able to attain the desired RPM with a clean IAC and the timing controlled by the ESC. I'll post what I find, and amend the instructions if necessary.

Thanks for the heads-up!

------------------
Later,
Vader
------------------
"Let the bodies hit the floor!"
Adobe Acrobat Reader

 

Hate to jump in on someone else's topic, but... When i try to set my idle and unplug the iac the idle drops to about 500 rpms. If i set it at the factory recommended 400 rpms and plug the iac back in it idles at about 700 rpms. Why?? It never idles at what i set it at. And at 400 rpms, with the iac unplugged, it sounds as if it's going to stall. Thanks for any help.

 

PJM,

K. I read through the '86 Helm's and I didn't see any mention of the EST bypass connector for the minimum air position setting procedure. However, that is only one of many years, and several fuel control systems, and a different year might well require disconnection of the EST bypass.

While timing is a factor in idle speed, the EST should theoretically only operate as a function of engine RPM, temperature, and detonation sensor inputs. To remove all doubt, disconnect the EST bypass connector is your car is so equipped. Some TBI and V-6 engines do not have this bypass connector, and therefore must be set with no regard to the EST system. Also, the EST can be bypassed on some cars by grounding the diagnostic terminal at the ALDL and continuing with the procedure, but the fuel mixture will be skewed to the rich side, affecting idle speed as well. In any event, the minimum air position idle speed range is wide enough to allow for some variations. As always, it is best to consult your service manual for the exact procedure for your system.

I'm trying to decide how to add this kind of disclaimer to the text of the procedure I have.

The ThirdGen years also encompass the CFI system, but I'm not even going to try to explain the manometer procedure required to set base idle on the CFI system. I know - I'm a wimp...

- - - - - - - - - - - - - - - - - - - - - - - -

AIZ,

The symptoms you describe are normal. The Idea of adjusting the minimum air position is to set a base point for the ECM to take control. The ECM uses the IAC to control the engine to the target idle speed. The hot target idle programmed in most V-8 ECM PROMs is 600-650 RPM in DRIVE on an automatic. The free idle speed in neutral is somewhat irrelevant. This target idle speed changes as engine conditions change. The engine coolant temperature and time from start have influence over the target idle speed.

Because of this, your ECM needs to have the minimum air position set corectly so that the IAC is stepped to a relatively neutral position to control the hot target idle. It must still have enough pintle travel to idle the engine higher when cold, and idle the engine lower when fully warmed. There must also be enough travel to compensate for air conditioning and/or electrical load from the compresor and alternator. This is why we set the minimum air position - not to change the curb idle speed. The only way to correctly change the curb (target) idle speed is to reporgram the PROM, not by "cheating" the throttle plates.

Thsi is also why it is very important to maintain clean IAC air passages and a clean pintle.

------------------
Later,
Vader
------------------
"Let the bodies hit the floor!"
Adobe Acrobat Reader

 

Next question...

Is this set with the tranny in or out of gear? I've heard both ways...

KAM

 

My tech article includes disconnecting the ESC. It is done by leaving the jumper in the A-B pins of the ALDL. That's why I state clearly to leave the jumper in.

Two ways to bypass ESC: disconnect it at the wiring harness, or jumper the ALDL. It doesn't throw the engine into enrichen mode like Vader said.

 

yeah i always jump the aldl - otherwise good luck getting the car to idle low the computer is gona keep raising it

------------------
91 z28 lb9 5 speed - hooker 1 5/8 headers - high flow cat- full 3 inch exhaust - home made ram air - tpis airfoil - dual chamber flowmaster - Stage 2 Jet Chip and 160 stat - 8 mm Msd plug wires - 55,000 v hypertech coil/rotor/distrib. cap - Mac. White face gauges - lots of sound system crap...

 

For the record, here's what my '87 GM (Helms) shop manual says:[list=1][*]Ground diagnostic terminal (jumper A & B).[*]Turn ignition on, wait 30 sec.[*]Disconnect IAC.[*]On 5.0 &5.7 disconnect EST bypass.[*]Start engine and allow to go closed loop.[*]Remove ground from diagnostic terminal.[*]Adjust screw to get 400rpm on 5.0, 450 on 5.7 in neutral.[*]Turn off ignition and reconnect IAC.[/list=a]

Note that it says to remove ground from diagnostic terminal. Note that it also says to set the minimum idle while in neutral. Note that it also fails to say, "reconnect ESC bypass".

Tom says that jumpering A&B is has the same effect as disconnecting the ESC bypass. The manual specifically says to "un-jumper" them just before actually turning the screw. Therefore, the bypass is still disconnected, but the ECM is no longer in diagnostic mode.

These might be trivial points, but I would like to get to the bottom of this.


------------------
'87 IROC 5.7l TPI - original owner!

[This message has been edited by P J Moran (edited June 15, 2001).]

[This message has been edited by P J Moran (edited June 15, 2001).]

 

I thought u weren't supposed to start the engine w/ the jumper in?

------------------
-Jason M. 1991 Camaro Z28

 

Exaclty.. I thought that starting the car with the jumper in would fry the computer? Atleast it did on my old 91RS 305TBI.

Brendan

 

You know, I thought I had researched this pretty thoroughly before I made the recommendation, but I had to double check to be certain.

In the case of the ALDL jumper forcing Backup Fuel Mode, which engine? The jumper in most cars won't force backup fuel, but a 2.8L may enter backup with a regular jumper (not a 3900 ohm resistor) and a 2.5L almost certainly will (for those rare few poor bastards that might happen to have the "base" engine). I realize this is a TPI board, But I tried to make the article as generic as possible to cover the bases for TPI, TBI, and MPFI, whatever, since I post it everywhere.

As for the question of DRIVE or NEUTRAL, the shop manual clearly states 400 RPM in "DRIVE" on an automatic transmission car, 450 in neutral on a manual transmission car.

And regarding the question of the EST or timing bypass, whoever recommended that is only confusing the issue. While it certainly won't cause any harm to disconnect the EST bypass connector, it isn't listed in my "article" because it isn't necessary as best I can tell. The ECM and EST will not advance timing to change engine RPM since it is not even active programming until the engine is at or above 550 RPM. The ECM wiil not adjust timing until then, so if you are even close to the recommended RPM setting, the EST is essentially inactive at that RPM. Look at the binary file addresses LC025-027, and see that the stall-saver timing cutoff is at 550 RPM. There should be NO EST action below that RPM, so I'll stand by my original statement.

And for the more obvious, of course the adjustment should be made with the engine at full operating temperature, like almost every other engine adjustment (with rare exceptions).

The Helm's GM service manual apparently directly contradicts itself, stating that the engine should be started with the ALDL diagnostic terminal grounded, but warning specifically against that procedure in other chapters. DUH! I know that it doesn't necessarily ALWAYS damage the ECM, but CAN damage the ECM, so why risk it?

And when it comes right down to it, unless you are using a calibrated handheld digital tachometer and are really **** retentive, you are probably only going to get within 50 RPM of the correct setting anyway. That is close enough, especially for those who are trying to correct the position due to someone attempting to adjust curb idle with the stop screw. It probably started so far off that anything would be an improvement, and in reality will change within 200 miles as the dirty begins to accumulate around the throttle plates again (you DID clean the TB before starting any of this procedure, DIDN'T YOU?)

------------------
Later,
Vader
------------------
"Let the bodies hit the floor!"
Adobe Acrobat Reader