91L98Z28

I have a P4 application that is more or less little known..never hacked and rarely discussed.

it's a GM 4cyl DIS SD "P" body application.

My question is.. generally speaking, in the P4 architecture, what conditions would disable both idle control AND stall saver?

on occasion, you can start the car, (and it properly goes to higher startup idle rpm, 1200-1500 or so for this application) drive to the first stoplight on my street (well under 20mph the whole way) and the car will just stall as soon as you take your foot off the gas- the IAC is too far closed and the ECM makes no effort to retract it. sometimes, instead of stalling, it will hang on, barely...at say 400-450rpm. it will STAY in that condition - no check engine light for low idle, no retraction of the IAC to increase rpm, no stall saver. It will continue on like that for however long you leave it alone.

in most cases, a simple key off, restart sequence immediately fixes the issue.

It seems very much related to engine startup temps. If the engine is stone cold, never a problem. If she's fully up to operating temp, never a problem. It seems there is a small window/zone in the middle that are a problem.

any thoughts?

I have thought about this from several angles:

If the ecm was in "idle" mode the whole time, it might step in the IAC trying to bring the RPM back down... but then why the failure to try to bring it back up??

if the ECM was in non-idle mode for some reason, it might make no effort to control the idle at all; but i can't imagine why it would think it was not idling. the TPS checks out, and i've tried idle values anywhere between 0.55 and about 0.8 with no effect.

there is nothing wrong with the alternator (and aldl stream reports normal voltage) so it's not shutting down the IAC for lack of power / low volts mode

If the IAC was physically stuck, it would throw a check engine light.

if the ECM was making effort at all to control idle rpm, it would throw a check engine light.

I have the TB set as open as it can go. When fully heated under no electrical load (all accessories off) it idles with only 2-3 IAC steps. If I open the TB anymore, it will start throwing a check engine light because the idle rpm will be above desired speed and it won't be able to close the iAC to control it.

data log clearly shows a desired rpm of say 1000 and actual rpm in the 400-500 range, with no IAC movement and a TPS reading in the 0.55 - 0.8 range (depending on how it's adjusted).

don't get it.

any ideas/thoughts on what conditions lead to a non-controlled idle speed?

Other than this one condition, the IAC system and idle speed control all seem to work well. The ECM brings the idle up a few hundred RPM when the A/C is turned off; properly steps it back down then the A/C is turned off, has proper higher idle rpm on cold startup, ... etc.

pandin

If you go from drive to park does anything change, when it is chugging at 500 rpm.

Do not assume a stuck IAC will give a check engine light. Anytime you dismiss something that could very well be your trouble.

The passage could be partially blocked or "a dead frog" could be in it (something wrong that is out side the normal).

The IAC motor could just be bad (sticking) in that one area.

Spend some time checking before just buying parts.

RBob

iTrader: (1)

Is this a TBI or MPFI setup? Do you know which MaskId the calibration is?

Not all masks retract the IAC in stall saver mode. Some just revert the SA back to the module. Basically they open the EST/BYPASS line.

With no idle control it may be that the throttle is hanging open a hair, which makes the ECM think the driver still has his foot on the go-pedal. A bad TPS will do the same. Many code bases continuously read the TPS and save the lowest seen value. That becomes the idle setting.

See if the data stream has a bit for 'in-idle', if so see what state it is in.

RBob.

91L98Z28

TBI. $48.

I've replaced the TPS once; i will check for any kind of intermittents. Didn't realize that it will continuously look for a low value and re-set.. interesting.

will check the datalog and see if it has that field.

91L98Z28

It's a stick, so no park/drive to test.

I can watch the data stream and see that the IAC is not being commanded to move. it's not stuck or clogged (TB rebuilt, IAC swapped). it's just flat out not being commanded to move (presumably) because some condition is inhibiting idle control altogether. Whatever that condition is, it also inhibits code 35 (idle speed error) from being thrown.

91L98Z28

Unfortunately, no status bit for idle.

Six_Shooter

iTrader: (2)

P-body = Fiero, no?

If it's the stock ECM it's a C3 type, not P4.

91L98Z28

87-88 4cyl is a DIS P4.

RBob

iTrader: (1)

1227748?

RBob.

91L98Z28

Thats the one!

I should mention, I have swapped the ECM at least once in the ten years I've had the car, and it had no effect. I know that in some of these years, they had problems with the epoxy shrinking and making them thermal sensitive.

91L98Z28

Interesting update.

In the past when this happened, I tried to work around it by setting the TB as open as possible (i.e. reducing the IAC counts at idle as much as possible) under the presumption that the less I depended on the IAC for air, the better.

I did some reading on a fiero forum and found this:

Now, my experience with $8D _directly_ contradicts this. The $8D in my '91 Z28 absolutely works best with the IAC also < 10 counts at idle.

But I wondered, (theory here!) is it possible to have the TB too far open such that, a minimal angular change (not enough angular change to register to the code as "driver throttle input") results in a significant air flow change, because the TB blade was already open so far; thus resulting in a significant delta IAC trying to "control" the rpm gain in "presumed" idle mode.

So, I did some datalogging this afternoon.

Background: car properly idles at 1000 rpm (stock GM rpm at operating temp); and I discovered that I was idling at ~10 IAC steps.

I first tested to see if I could increase the throttle at all, with the ECM still acting as if it was idling, and I found that I could. In fact, I could apply enough throtle to get about +300rpm over idle, and the ECM would still retract IAC steps (presumably, idle control routine here). Taking my foot off the gas resulted in a significant low RPM sag, followed by a recovery.

Then, I reset the TB position to about 30 IAC steps at idle and repeated the same test. I could barely get to 1100rpm now before the IAC quit retracting. And when I foot off the gas, the IAC recovered quickly and there was almost no RPM sag at all. Almost a non-event.

I wonder if perhaps this early P4 code just isn't very refined, and it is not very tolerant of variations such as these?

I assume that the difference between these two tests is primarily the difference in airflow between the two angular positions. If we assume that the ECM needs to see a certain angular change to get out of idle mode, then with the TB further closed, the particular angular change to get out of idle mode will result in a smaller airflow delta both numerically and by % delta; thus less delta IAC to "counteract" it.

This doesn't solve the problem of the idle simply being uncontrolled at times; but perhaps is a contributing factor that (hopefully!!) when fixed, will mask the other problem. I assume the real problem is that the "idle mode" detection routines in this code are less than ideal.

I will be driving and testing over the next day to see if I can uncover anything else, but any input in the meantime is great!!

RBob

iTrader: (1)

It is a possiblility. The MAF TPI code is the same way ('7165 ECM, also a P4). On these engines it is critical to set the minimum air and then the TPS setting.

RBob.