IAC Operating Theory Question / Low Idle in gear

[kishy]

Alrighty, bit of a 2-part question that has resulted from a bunch of tinkering, a lot of failed searches of this and the other 3-letter site, and general confusion. As I write this, it will become more than 2 parts, but that's how many I'm starting with...

Issue: idle RPM drops too low when car is put in gear. '91 MGM, stock.
Have base idle set around 700-900 depending on the accuracy of my cheap tach, followed procedure as found on article on SBFtech: http://sbftech.com/index.php/topic,1031.0.html

Idle was raised due to recommendation that it will assist with emissions test, as well as the fact that it was previously low enough to stall at various times.
There are other issues at hand and I have a beast of a thread on that other site, but right now I just need to clarify how the ECU responds to a drop in RPM.

If I am stopped, holding the brake pedal, and shift from park or neutral into reverse or OD/D/1, the RPM drops as much as 400RPM.
Assuming the IAC valve is functional: is the computer going to attempt to raise the RPM via the IAC?
I guess the question is, if base idle is set at some amount, and the throttle position sensor indicates the throttle is not open at all from its base position, if the RPM drops below that set amount, will the ECU compensate?

Some answers I've found suggest that the RPM drop is normal and ECU should not compensate. That doesn't make sense though, what is the purpose of having the IAC at all or even having a "base RPM" if the ECU will not continuously attempt to idle at that RPM?

My expectation is that the RPM will drop but quickly be compensated by the ECU opening the IAC as the RPM reported by the pickup is too low.

If someone(s) knowledgeable about the way this system is designed to operate can chime in that'd be fantastic, please. Brief reading about Chevies suggests my expectation is correct, but I'm having difficulty confirming for sure that Ford did the same.

[kodiak]

I have no idea, give this a shot.
http://autorepair.about.com/cs/troub...042603a_08.htm

https://www.google.com/url?sa=t&sour...oETmeXFWGUYM5g

Sent from my '85 Lincolin Town Car

[sly]

the idle should not drop off like that as the computer will control it before it even looses 100rpm. Either the IAC is dead or going out, or the throttle body and IAC ports need to be cleaned of the carbon build up in a bad way. Carb/throttle body cleaner will work fine on a box. follow the directions on the can.

also, if you replace the IAC, you may only have to remove the motor/solenoid section (two little screws) and swap with the new part so you won't have to disturb the gasket and save a few bucks there. mind the O-ring that sits between the solenoid and ports as that provides the seal in there. A little petroleum jelly just to make sure the o-ring goes on smoothly might be a good idea too.

[kishy]

Thanks, both of you, good information to be had here.

So then, it is as I suspected and something's up. Throttle body, IAC ports etc all thoroughly cleaned and there's no build up remaining (started out as a mild film). That would mean I'm now 2 for 2 Duralast/Wells TV200 IACs that can't properly hold idle when put in gear. The first one simply died in less than an hour of drive time, wouldn't cycle anymore with key turned to run. The replacement seems to still cycle but RPM drops like a stone when put into gear. When the replacement was first put in (yesterday), the idle would "compromise", a 200ish RPM drop instead as if the solenoid was operating, but today it seems to still drop as if the solenoid wasn't connected at all. It might be temperature sensitive since they seem to at least half work until fully hot, but that's not exactly gonna fly when it has to mount to a part that has a water jacket inside...

So count that as a recommendation against Duralast (or the non-private-labeled Wells) TV200, and if someone who happens to work for Wells stumbles upon this, you really need to work on that part's engineering since it seems I am not an isolated case.

[86VickyLX]

At low rpms, and the TPS value being lower than 1.00v the idle is controlled by the computer via timing. The IAC comes into play when first starting up, slowing down, or if the a/c is on.

[mobydick]

At low rpms, and the TPS value being lower than 1.00v the idle is controlled by the computer via timing. The IAC comes into play when first starting up, slowing down, or if the a/c is on.

So then at low rpm if you have poor idle then a TFI issue is suspect?

[miamibob]

Thanks, both of you, good information to be had here.

So then, it is as I suspected and something's up. Throttle body, IAC ports etc all thoroughly cleaned and there's no build up remaining (started out as a mild film). That would mean I'm now 2 for 2 Duralast/Wells TV200 IACs that can't properly hold idle when put in gear. The first one simply died in less than an hour of drive time, wouldn't cycle anymore with key turned to run. The replacement seems to still cycle but RPM drops like a stone when put into gear. When the replacement was first put in (yesterday), the idle would "compromise", a 200ish RPM drop instead as if the solenoid was operating, but today it seems to still drop as if the solenoid wasn't connected at all. It might be temperature sensitive since they seem to at least half work until fully hot, but that's not exactly gonna fly when it has to mount to a part that has a water jacket inside...

So count that as a recommendation against Duralast (or the non-private-labeled Wells) TV200, and if someone who happens to work for Wells stumbles upon this, you really need to work on that part's engineering since it seems I am not an isolated case.

While this incident is a small sample, it is why I go OEM whenever possible...

[kishy]

At low rpms, and the TPS value being lower than 1.00v the idle is controlled by the computer via timing. The IAC comes into play when first starting up, slowing down, or if the a/c is on.

If this is the case,

• What inputs to the ECU are used in calculating timing?
• What is the expected timing of a fully warm idling 5.0 in these cars expected to be?

So then at low rpm if you have poor idle then a TFI issue is suspect?

If that is how it operates, then I draw the same conclusion, that the TFI module can cause timing discrepancy-related issues in the idle...

While this incident is a small sample, it is why I go OEM whenever possible...

It was an option, but much more expensive with a much shorter warranty. My thinking was that I don't mind changing them once a year if the Wells unit keeps failing, but at this rate I'll be doing it once a week and that just isn't gonna work for me. I'm curious to know who the actual OE is for Motorcraft...Advance Auto would have me believe it's Delphi: http://shop.advanceautoparts.com/p/d...140/25987782-P

Someone on the other site said Delphi is what worked for them too...tempted to see if AutoZone will refund the IAC instead of swapping this time and maybe go that route. I'll be back in the states to try to collect front end parts in the near future so it won't be a dedicated trip anyway.

[kodiak]

IIRC with the plug removed on the dist it is 10* BTDC. With the plug in its computer controlled.

The following may or not have what info your looking for.
https://www.google.com/url?sa=t&sour...f-3gyIWHR8PMTw

Sent from my '85 Lincolin Town Car

[kishy]

IIRC with the plug removed on the dist it is 10* BTDC. With the plug in its computer controlled.

The following may or not have what info your looking for.
https://www.google.com/url?sa=t&sour...f-3gyIWHR8PMTw

Sent from my '85 Lincolin Town Car

Yes, 10BTDC is correct with it disconnected. I want to know which inputs to the ECU are used in calculating timing, in other words, what is the ECU using to decide what timing should be.

That document appears to be more technical on the actual operation of the ECU (appears to talk about its base programming) which is unfortunately too low level to address this question unless I go learn Assembly or whatever language it's coded with.

Edit: but this does all depend on the fact that idle speed is controlled by timing...I'm not a master technician by any stretch here but I've certainly never heard of timing being used as a means of idle speed control before. I can see how it would work, though.

[kodiak]

Be aware I'm running out of stuff to throw at you!

Sent from my '85 Lincolin Town Car

[kodiak]

Pin out http://www.auto-diagnostics.info/ford_eec_iv

Sent from my '85 Lincolin Town Car

[gadget73]

timing is determined with temperature sensors, the MAP and the TPS mostly, though considering thats basically everything that doesn't narrow it down much. No load at idle, timing is often in the 20-30 btdc range but it varies somewhat. It will be more than 10 btdc though.

The IAC is in use at least somewhat at idle. If its working normally, the engine should idle around 500 rpm with it unplugged. Plugged in, target idle is something like 675 rpm no matter what gear its in, and its somewhat higher with AC. I think about 750, though the exact number escapes me. I have seen the cheapo IACs not respond properly. usually with an engine running self test, you'll get codes for not being able to control the idle, sometimes high, sometimes low, sometimes both. Every time I've seen that its been with a non-OEM IAC, and replacing it with a used old OEM one has fixed it.

[kishy]

timing is determined with temperature sensors, the MAP and the TPS mostly, though considering thats basically everything that doesn't narrow it down much. No load at idle, timing is often in the 20-30 btdc range but it varies somewhat. It will be more than 10 btdc though.

The IAC is in use at least somewhat at idle. If its working normally, the engine should idle around 500 rpm with it unplugged. Plugged in, target idle is something like 675 rpm no matter what gear its in, and its somewhat higher with AC. I think about 750, though the exact number escapes me. I have seen the cheapo IACs not respond properly. usually with an engine running self test, you'll get codes for not being able to control the idle, sometimes high, sometimes low, sometimes both. Every time I've seen that its been with a non-OEM IAC, and replacing it with a used old OEM one has fixed it.

See, that's what really kinda makes me wonder what's going on. The car will self-test including cylinder balance without difficulty and idle does not waver around while it's being held high. Mind you, last time I did those, I was using the factory IAC which would not cycle with key turned to run - how can an IAC not cycle, but still hold idle up for a self test?

Curious to see what happens if I run the tests now with the Wells IAC...

[kishy]

See, that's what really kinda makes me wonder what's going on. The car will self-test including cylinder balance without difficulty and idle does not waver around while it's being held high. Mind you, last time I did those, I was using the factory IAC which would not cycle with key turned to run - how can an IAC not cycle, but still hold idle up for a self test?

Curious to see what happens if I run the tests now with the Wells IAC...

Well (pun? maybe?), it self tested fine with the Wells IAC...the second one, since the first one was actually flat dead.

For whatever reason I was able to justify buying a Delphi IAC, which claims to be the OE (impossible, Delphi didn't exist yet in 1990) but which was recommended by someone on the other site. Had a coupon code, it worked out alright.

Observations...the Delphi seems to work just the same as the second Wells did. That is, start engine, revs high smoothly for maybe 10 seconds, idle drops to the same point as if I had the IAC unplugged. Put in gear and RPM drops like a rock, bit under 500. Due to my recent emissions test I know my tach reads around 100RPM high so it's most likely in the 400s.

Can someone please confirm these these instructions are valid and work for our cars? http://sbftech.com/index.php/topic,1031.0.html
That is the process I used to attempt to raise my idle. Of course, the unloaded idle RPM went up (duh, open throttle plate more = more air = it doesn't have a choice), but the loaded in-gear idle is still low (computer does not seem to realize that it needs to lift it back up again).