Cheers, Guys...hoping you folks can provide some input on the unusual idle problem I've got...I've been trying to set the minimum idle on my car for a time, but I can't get the idle under 1050...I've checked the IAC for proper operation...good, checked the IAC passages and throttlebody for clean passages and operation...clean as a whistle, checked for vacuum leaks..found none, even took it to the top vette mechanic in my area, and they could not find any either. When trying to adjust the idle, using the min. idle procedure, by adjusting the throttle screw on the throttlebody, I get no change in idle speed, until I either back the screw out altogether ( at which point the idle falls so low my car dies), or I open it up to the point where the idle increases from the 1050 rpm that it wants to idle at. Scanning with Datamaster, the idle is rock steady at 1050, IAC position is zero, IAC steps are 40. TPS is .55, no codes. Car runs strong, rev's good, O2 counts are good, and I move from open to closed loop. Idle is always at 1050, rather in open or closed loop. I'm really scratching my head over this one, and hope you guy's can point me in the right direction.....

Cheers,
Michael

 

Which one is it ?? 0 or 40 ?? 40 would be a high idle for sure. I'm at 20 steps for my idle.
Are you disconnecting the EST wire before re-setting min idle and TPS ??
What is target idle in chip ??

 

sorry for the confusion...I believe datamaster shows two items for the IAC: one is position, which is at zero, and one for steps, which is 40....
Ther appears to be,at least to me, whether the esc should be disconnected, or not....some say yes, some say no....I try and follow the FSM, which says yes....and I do.....
Target idle in the chip is 650....chip is from PCMforLess....

Cheers,
Michael

 

Reducing the airflow either through the IAC or the the throttle blades has to bring the rpm down, unless there is another signficant airflow path.

Drawing air through the intake valve guides maybe?

IAC counts of 40 is only about 2 grams/sec flow (roughly 0.05 gm/sec/count).

1050 rpm requires about 10-11 gm/sec of flow. Your throttle or other unknown air source is contribruting the other 8 or so gram/sec of airflow needed to maintain 1050 rpm.

Your min air is too high and your IAC is also likely being held up by the Warm IAC park position table. Reducing the min air should help. If not, its getting the air from somewhere else.

Leave the est connected when setting min air since the car may have a tough time running on only 6 degrees, depending upon the mods.

 

It is getting air from somewhere.

This makes me think the throttle is sticking. Most likely the throttle blades sticking in their bores. When you first back off the throttle stop screw, try manually pushing the throttle arm further closed. Most often, throttle sticking is caused by worn throttle shaft bore wear in the pot metal TB body. If you can verify that, you may find a place that will bush (and seal) the TB body for the throttle shaft, or get a different TB. The stock TB is not bushed. The throttle shaft wears directly against the pot metal casting of the TB body, which is why this happens.

RACE ON!!!

 

CFI-EFI, I was hoping you would chime in...I have verified that the throttle blades are not sticking, using the procedure you described above...I can not move the blades any, by pushing the throttle against the idle screw...they stay tight against the screw, and I have visually verified that they move with the idle screw, when it is turned....both in and out....throttlebody is new, and has had bushings placed on the shaft...I think tequilaboy is right about getting extra air, but i'll be damed if I can find any,,,no air available, except thru the MAF and the associated ducting...and I can not find any leaks there.....fuel pressure is steady, and does not flucuate, so I'm really at a loss at the moment...

Thanx for your input....

Cheers,
Michael

 

Tequilaboy, I suspect you are right...according to Datamaster, my gm/sec flow is indeed 10.63...my problem is that even though I back off on the min idle screw, I can not get the idle below 1050, until I back the screw totally in, at which time the idle goes to about 250, and the engine dies....any thoughts as to where extra air might be coming from....I've checked the MAF ducting, and can't find anything there....also, no codes.... Any help is appreciated...BTW, you can see my confusion about the EST....the FSM says to disconnect, and you mention to leave it connected...

Cheers,
Michael

 

Tequilaboy, just looked at my bin, and the warm IAC park is only 5 steps...looking through the bin file, I noticed that the Idle RPM vs CTemp Table has an idle rpm of 1050, when the Ctemp is 60 or below....wonder if this is holding my idle high, although my Coolant temp readout on my cluster seems to read correctly, and Datamaster show a coolant temp that approximates my gauge reading....I'll do some checking on my coolant sensors.....

Cheers,
Michael

 

With the MAF showing 10.63 gm/sec, that is a good indication that all or at least most of the air is flowing through the MAF and into the throttle or IAC. That is a good sign and doesn't really point to vacuum leaks.

If coolant temp looks good in datamaster, it should be ok. Are you still running an 85 $1F bin?

Coolant temp should be in the 25 bytes available.

Now the question at hand: Why doesn't the engine respond to reducing the airflow through the throttle, and why won't it idle below 1050 rpm.

Tell us about your mods. Do you have a huge cam (something over 240 degrees duration)? Also what is going on with the ignition timing in the bin? Does the timing table drop suddenly below 1000 rpm.

Perhaps the timing below 1000 rpm is just too low for the engine to run.

Is this idle behavior in gear or in park/neutral? Is there a large park/neutral offset in the bin (> 50 rpm)?

With an auto trans car the desired idle rpm is intended for in gear, park or neutral usually adds a 50 rpm offset. I didn't see any mention of the trans or in-gear, but I'm assuming its an auto. If its a 4+3 it should run at the desired idle rpm witout an offset.

If you increase the throttle opening via the throttle stop screw (and re adjust the tps back down), does the IAC respond and close off towards zero as it should, or does it remain at 40 counts).

Conversely, if you decrease the throttle opening, does the IAC respond and increase counts?

With a fully closed throttle (no airflow), the IAC alone should be able to support up to about 800 rpm @ 145 counts.

I would like to understand what is going on with the ignition timing.

How much vacuum will the car pull when it does idle at 1050? I suspect late ignition timing below 1000 rpm.

 

TTS datamaster only shows what the ECM is telling the IAC to do. The only way to tell if the IAC is good, is by taking it out and checking resistance. There are no codes for the IAC, ie check engine light will not come on.
http://forums.corvetteforum.com/show....php?t=1749693

 

Good questions, Tequilaboy...here are some answers:
1. Bin is $6E Arap, modified for my 85...upgraded to a 165 ECM.
2. Motor has stock cam.
3. Datamaster shows the spark advance at the 1050 rpm to be 39.5
4. Bin uses the ARAP Spark table. I've been able to use that table with no knock, so it hasn't been modified.
5. these readings are with an Auto car, in Park....
6. The IAC counts do move as you describe
7. The Park/Neutral offset is 50 counts.
8. Engine shows 18" HG at this idle, and doesn't fluxuate.

Hope this answers some of your questions...

Cheers,
Michael

 

Yep, this was the first thing I checked....all values were as shown. Also checked correct operation of the IAC, by removing the IAC, and then turning on the key, to observe the action of the Pintle. Worked as described in the FSM. Also checked for proper length of the pintle, and it was right on the money. Thanx for helping...I'll keep digging.

Cheers,
Michael

 

Thanks for the answers. Nothing radical there.

Looking at the ARAP bin at 1000 rpm and LV8 of 64, the spark table in this region drops dramatically from 40 degress to 29 at 800 and then to 20 at 600.

Is the timing correct at 6 degrees with the EST disconnected?

You've got to get the airflow and loads down to put you into a more stable part of the spark table. It should run smoothly down to 500 rpm.

Reducing the air should work. I'm at a loss to why it isn't responding as expected and why the car won't idle down???

Any mixture issues we should be aware of? Fuel pressure and blms in the ballpark?

 

Tequilaboy:

1. Timing set at 6 degrees with est disconnected...verified again this afternoon
2. BLM's are rich at idle...around 116, with int at 130, but in all other cells at load, they are in the 126-128 range with INT's in the same 126-128.
FYI...I just reset the min idle with the EST disconnected, and was able to get the idle to 650, with the IAC unplugged. Reset the TPS, and reconnected the IAC. Started it back up with the EST connected, and it went right back to the 1050 idle....took it for a little drive to get the speed over 40 mph, hoping to reset the IAC, but no change.....

A little more detective work to do, I guess.....Thanx for all your Input!!!

Cheers,
Michael

 

I agree. That is why I opened my post with "It is getting air from somewhere."

I wasn't aware that you had a non-stock chip. The timing you've observed looks to about double what I would expect. In any case, I think the key to the problem lies in the statements, "I get no change in idle speed, until I either back the screw out altogether ( at which point the idle falls so low my car dies)" and "I can not get the idle below 1050, until I back the screw totally in, at which time the idle goes to about 250, and the engine dies....". That is what prompted the throttle sticking idea. Somehow, the engine is getting constant air (1050 RPM idle) through a rather wide range of throttle openings. When you get into modified chips and things like "CTemp Table" you've passed me by. Points to keep in mind, constant air through a range of throttle openings and a sudden, drastic change in that air supply at a certain point. Good luck, and...

RACE ON!!!

 

Thanx, CFI....I'm still lookin, but as Tequilaboy said, the flow thru the MAF is dead on for that RPM, so I think all my air is coming as it should, thru the MAF and Ducting...if I can ever get the idle down, my spark advance should drop also...just never had this problem before, when trying to set min. air....I'll keep lookin....

Cheers,
Michael

 

I have a similar problem. Turned out to be the injectors. I replaced the IAC and checked everything out.

The injectors should have 16 ohms resistance, anything less, you're screwed.

Luckily, just picked up a new set of 8 here on the forum cheap.

I am on vacation so I have a shop doing the work.

 

If the IAC counts are ever 5 to 0 at idle,you have false air or a vacuum leak coming in.An incorrectly adjusted minimim air screw can cause this sometimes too.Make sure youre doing the proper steps.Set the minimum air speed,then the iac and TPS.

 

Something is still missing here. I'm not sure what it is. If you want to send me a *.uni file from datamaster, that may help. I'd be happy to review it and look for any anomalies. My email address:

mr_tequilaboy@sbcglobal.net

It sounds as if all the bases are covered regarding the basics, which makes it even more puzzling.

With software changes only, I can make my car idle anywhere between 800 and 1600 rpm, so I know the hardware and existing PID control logic is more than capable of controlling the idle rpm vs coolant temperature to almost any rpm within reason. Something is wrong that is eluding us. Hopefully we will find the answer.

 

I think you may have missed my point. You have said that the throttle is not sticking. Therefore, I ***-U-ME that the throttle blades move as you adjust the throttle stop (minimum air adjustment) screw. I think there is a major clue in the fact that you are able to, presumably, reduce the air getting into the engine without lowering the idle speed. As I said in the previous post, "Somehow, the engine is getting constant air (1050 RPM idle) through a rather wide range of throttle openings.". That just doesn't add up, and I think your cure lies in further investigating how that can happen or exactly what IS happening.

RACE ON!!!