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Old 11-26-2022, 02:02 PM   #1
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Default Intermittent Stall at Idle

Hello all,

A2Saturn's 1994 Sl1 (TBI) has been having some intermittent stalling at idle, and I cannot seem to find the source. Usually it will idle fine, then have a quick misfire/stumble, then stall. It seems as if the misfire occurs and the idle cannot be corrected quick enough.

I have some details below.

Details from live data stream:
-Idle RPM: 750 +/- 50
-TPS %: 0.39% at idle
-MAP Load: 29 kPa
-Ignition Advance: about 5-8*
-Knock Retard: 0*
-Injector PW: 0.8-1.0 mS
-Coolant Temp: 170-180F
-Air temp: 75 F
-02 Sensor Voltage: 0.30-0.80, swings rapidly, indicating a non-lazy sensor
-EGR: Closed

From what I can see, everything is functioning normally.

The bottom end of the engine has 495,000 miles, but the cylinder head has around 120,000. Plugs are a nice brownish/white color.

Perhaps it is just getting tired?

Any suggestions are appreciated.
...
1994 Sl2 5spd: First Car, Stock: 56,576 (Totaled 1/10/2018)
1994 Sl1 5spd: Inherited, Stock: 494,500
1995 Sl2 5spd: Retired Daily, Not Stock: 196,000
1995 Sw2 5spd: Daily, Stock: 105,000
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Old 11-26-2022, 02:46 PM   #2
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Default Re: Intermittent Stall at Idle

When I had a similar problem with my '94 SL2, it turned out to be the 5V supply in the PCM was flakey. My symptoms started with the "idle stutter", then progressed to include sometimes hard starting, and finally often intermittent "crank-no-start". Once it got down to the often no-start, I was able to track it down to the PCM. The first thing I noticed in that process was that the FP drive signal from the PCM was not normal.

The 5V supply would sometimes come up fine (instantly), but sometimes take a second or two; and others it would only get up to about 3.5V It is easy to check that 5V supply action at the MAP or TP sensors.
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Old 11-26-2022, 04:43 PM   #3
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Default Re: Intermittent Stall at Idle

Examine spark plugs; dry black indicates rich fuel, oily black indicates ring wear, blistered white ceramic indicates lean mixtures. Tan ceramic is normal.

If the throttle body injector was never overhauled, consider it because the fuel pressure regulator is rubber and wears out along with the injector. Evidence of a leaking injector; look down the throttle body while someone turns on ignition (no start). The pcm operates the fuel pump for a few seconds then turns off if the engine isn't started. At ignition on, engine off time, the tbi should not spray fuel since the engine isn't started. Fuel under pressure will try to go thru any opening. A worn injector not closing fully, possibly worn or has hard buildup preventing pintle valve closure will leak fuel immediately and richen fuel mixtures all the time with engine running. The fuel pressure regulator diaphragm lasts only so long before it wears out, usually lasting almost forever until mystery problems appear.

Do not assume an ignition issue until its tested visually with a spark test, especially when erratic problems appear. The tbi is designed to least thru new car warranty but that ship sailed long ago so age related issues should be considered relative to ignition and fuel systems. And don't leave out the possibility of a worn out fuel pump. Testing for fuel pressure is needed to determine where a problem lies.
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Old 11-26-2022, 06:47 PM   #4
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Default Re: Intermittent Stall at Idle

Quote:
Originally Posted by billr View Post
When I had a similar problem with my '94 SL2, it turned out to be the 5V supply in the PCM was flakey. My symptoms started with the "idle stutter", then progressed to include sometimes hard starting, and finally often intermittent "crank-no-start". Once it got down to the often no-start, I was able to track it down to the PCM. The first thing I noticed in that process was that the FP drive signal from the PCM was not normal.

The 5V supply would sometimes come up fine (instantly), but sometimes take a second or two; and others it would only get up to about 3.5V It is easy to check that 5V supply action at the MAP or TP sensors.
The PCM was replaced in 2014 according to the paperwork I received with the car, but I would not rule that out.

Apparently it is also the wrong PCM, as the EVO signal is nonexistent, even though the window sticker clearly lists variable power steering.

I will have to check the 5v reference next time I have it out, it is easy enough to check.
...
1994 Sl2 5spd: First Car, Stock: 56,576 (Totaled 1/10/2018)
1994 Sl1 5spd: Inherited, Stock: 494,500
1995 Sl2 5spd: Retired Daily, Not Stock: 196,000
1995 Sw2 5spd: Daily, Stock: 105,000
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Old 11-26-2022, 06:52 PM   #5
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Default Re: Intermittent Stall at Idle

Quote:
Originally Posted by fdryer View Post
Examine spark plugs; dry black indicates rich fuel, oily black indicates ring wear, blistered white ceramic indicates lean mixtures. Tan ceramic is normal.

If the throttle body injector was never overhauled, consider it because the fuel pressure regulator is rubber and wears out along with the injector. Evidence of a leaking injector; look down the throttle body while someone turns on ignition (no start). The pcm operates the fuel pump for a few seconds then turns off if the engine isn't started. At ignition on, engine off time, the tbi should not spray fuel since the engine isn't started. Fuel under pressure will try to go thru any opening. A worn injector not closing fully, possibly worn or has hard buildup preventing pintle valve closure will leak fuel immediately and richen fuel mixtures all the time with engine running. The fuel pressure regulator diaphragm lasts only so long before it wears out, usually lasting almost forever until mystery problems appear.

Do not assume an ignition issue until its tested visually with a spark test, especially when erratic problems appear. The tbi is designed to least thru new car warranty but that ship sailed long ago so age related issues should be considered relative to ignition and fuel systems. And don't leave out the possibility of a worn out fuel pump. Testing for fuel pressure is needed to determine where a problem lies.
Tan electrode and ceramic, probably the nicest plugs I have personally seen.

TBI unit was overhauled twice, last time being about two years ago, along with pressure regulator. It does not drip while the car is off after priming the pump and from what I can visually see the amount of fuel coming out while running is not excessive.

I will have to check the fuel pressure next time I have the car out, but from one of Scott's old posts, it read fine in 2021.
...
1994 Sl2 5spd: First Car, Stock: 56,576 (Totaled 1/10/2018)
1994 Sl1 5spd: Inherited, Stock: 494,500
1995 Sl2 5spd: Retired Daily, Not Stock: 196,000
1995 Sw2 5spd: Daily, Stock: 105,000
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Old 11-26-2022, 07:34 PM   #6
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Default Re: Intermittent Stall at Idle

If the fuel pump is original, it will fail after so many years and mileage as nothing lasts forever.
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Old 11-26-2022, 10:37 PM   #7
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Default Re: Intermittent Stall at Idle

Quote:
Originally Posted by fdryer View Post
If the fuel pump is original, it will fail after so many years and mileage as nothing lasts forever.
I had to do some digging on here, looks like it was replaced in 2014, at 325k miles, so the pump is 8 years old with 170k miles under it's belt.

I will check the fuel pressure and then the PCM 5v reference line.
...
1994 Sl2 5spd: First Car, Stock: 56,576 (Totaled 1/10/2018)
1994 Sl1 5spd: Inherited, Stock: 494,500
1995 Sl2 5spd: Retired Daily, Not Stock: 196,000
1995 Sw2 5spd: Daily, Stock: 105,000
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Old 12-10-2022, 03:55 PM   #8
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Default Re: Intermittent Stall at Idle

Checked the fuel pressure, around 26 psi idle, which seems to be correct for the year and engine configuration.

Will check the 5v ref line, but I really do not think that is causing it.

It happened a few times today on my drive, so it is still occurring.

It seems as if the PCM is commanding the idle to be lowered, as it drops incrementally, then it just flat out stalls.
...
1994 Sl2 5spd: First Car, Stock: 56,576 (Totaled 1/10/2018)
1994 Sl1 5spd: Inherited, Stock: 494,500
1995 Sl2 5spd: Retired Daily, Not Stock: 196,000
1995 Sw2 5spd: Daily, Stock: 105,000
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Old 12-10-2022, 04:25 PM   #9
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Default Re: Intermittent Stall at Idle

The engine dropping out at random may be difficult to diagnose with several issues that can become intermittent at the most inconvenient time - driving.

Spark is relatively easy to check when the engine dies; Two people may be needed - one in the car to startup while you remove plug wires from coils to observe for spark across both towers.

Fuel pressure may be observed if a fuel pressure gauge is left connected while driving. Under normal conditions; engine idling will show fuel pressure, shutting engine off should show pressure (several minutes before bleeding down). If the fuel pump drops out while driving, injectors will bleed off remaining fuel with pressure dropping quickly until the engine dies. This would be seen on the f/p gauge. However, the engine dying with fuel pressure seen on the f/p gauge implies something is killing the engine.

The pcm controls the entire EFI system; spark, fuel pump and injectors. The crank position sensor outputs timing signals directly to the pcm, allowing the pcm to run. The crank sensor is the electronic heartbeat of the pcm. No crank sensor signals, no pcm operation.
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Old 12-13-2022, 09:44 PM   #10
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Default Re: Intermittent Stall at Idle

Quote:
Originally Posted by fdryer View Post
The engine dropping out at random may be difficult to diagnose with several issues that can become intermittent at the most inconvenient time - driving.

Spark is relatively easy to check when the engine dies; Two people may be needed - one in the car to startup while you remove plug wires from coils to observe for spark across both towers.

Fuel pressure may be observed if a fuel pressure gauge is left connected while driving. Under normal conditions; engine idling will show fuel pressure, shutting engine off should show pressure (several minutes before bleeding down). If the fuel pump drops out while driving, injectors will bleed off remaining fuel with pressure dropping quickly until the engine dies. This would be seen on the f/p gauge. However, the engine dying with fuel pressure seen on the f/p gauge implies something is killing the engine.

The pcm controls the entire EFI system; spark, fuel pump and injectors. The crank position sensor outputs timing signals directly to the pcm, allowing the pcm to run. The crank sensor is the electronic heartbeat of the pcm. No crank sensor signals, no pcm operation.
Spark is good, it is not a hard cut either, more of a stumble and fumble (engine does not recover).

Immediately starts, or is fine if I blip the gas.

Crank sensor is good, tach still functions properly during the idle stumble/stall, indicating that the crank sensor is still providing a signal.

Going to pull apart the TB assembly and inspect the stepper motor.
...
1994 Sl2 5spd: First Car, Stock: 56,576 (Totaled 1/10/2018)
1994 Sl1 5spd: Inherited, Stock: 494,500
1995 Sl2 5spd: Retired Daily, Not Stock: 196,000
1995 Sw2 5spd: Daily, Stock: 105,000
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Old 12-14-2022, 02:54 PM   #11
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Default Re: Intermittent Stall at Idle

Before removing the TB, I checked the TPS with a known good one, no difference made.

Checked the 5v reference signal, showing 5.02-5.04v with the engine running.

I allowed it to idle, then unplugged the IAC. Plugged in an IAC outside of the TB (spare one I had around) to observe the actions of it.

- The pintle steps back and forth in small increments while the engine idles normally, which is what I assume the response to minor fluctuations in rpm

-A couple times, it extended fully and rapidly, until the pintle shot out, which I assume is not a normal response

Going to do some digging and see what condition would cause it to attempt to completely shut off the air flow at idle, as that behavior is not regular, and would certainly stall the engine in the manner that it does.
...
1994 Sl2 5spd: First Car, Stock: 56,576 (Totaled 1/10/2018)
1994 Sl1 5spd: Inherited, Stock: 494,500
1995 Sl2 5spd: Retired Daily, Not Stock: 196,000
1995 Sw2 5spd: Daily, Stock: 105,000
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Old 12-14-2022, 03:29 PM   #12
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Default Re: Intermittent Stall at Idle

Now that you know how easy it is to check the 5V, do it again about 10 times or more. You don't need to start the engine, just go key-on/off to see how the 5V comes up. It should always come up immediately to 5V. When mine was bad, sometimes it would come up fine, sometimes it would come up to 5V but take a second or so, and sometimes it would only come up to about 4V
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Old 12-14-2022, 04:40 PM   #13
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Default Re: Intermittent Stall at Idle

-MAP Load: 29 kPa

-Coolant Temp: 170-180F

29 kpa converts to 8.6 in Hg (vacuum). Vacuum should be between 15-20 in Hg.

Your coolant temps seems lower than ideal - 185F-195F.

You may have two issues; a worn thermostat/damaged seals and/or faulty map sensor. T-stats should regulate towards 195F, not 170F-180F. Map sensor tests should display ambient pressure with ignition on, engine off to reflect local barometric pressure (30.26 in Hg or 102.47 kpa). Measuring ambient barometric pressure with ign on is one way to determine if a map sensor is operating correctly while values should drop immediately when the engine's running, indicating intake manifold vacuum varies as a good map sensor detects changing vacuum, continually sending signals to the pcm to search fuel maps needed to change injector pulses (rich/lean mixtures).

Very few, if any, are willing to go in-depth to see how the idle air control valve operates and you're one willing to observe it. By seeing the iacv operate physically, you're verifying that it's operating correctly, extending and retracting on commands from the pcm to adjust idle rpm. When an iacv fails, some fail with jammed debris while others lose one of two stepper motors. One to retract, the other to extend the pintle valve. One failing shows as idle unable to return from open throttle or remains at higher than normal idle. The pcm always adjust idle rpm to control cold engine high idle, gradually lowering as coolant temps rise until reaching operating temps where factory idle occurs, raise idle when ac is used, etc. I think your iacv is operating correctly but something else is influencing the pcm into making adverse iacv commands.
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Old 12-14-2022, 05:39 PM   #14
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Default Re: Intermittent Stall at Idle

Quote:
Originally Posted by billr View Post
Now that you know how easy it is to check the 5V, do it again about 10 times or more. You don't need to start the engine, just go key-on/off to see how the 5V comes up. It should always come up immediately to 5V. When mine was bad, sometimes it would come up fine, sometimes it would come up to 5V but take a second or so, and sometimes it would only come up to about 4V
Noted, will check it multiple times next time I am around the car.
...
1994 Sl2 5spd: First Car, Stock: 56,576 (Totaled 1/10/2018)
1994 Sl1 5spd: Inherited, Stock: 494,500
1995 Sl2 5spd: Retired Daily, Not Stock: 196,000
1995 Sw2 5spd: Daily, Stock: 105,000
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Old 12-14-2022, 06:22 PM   #15
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Default Re: Intermittent Stall at Idle

"29 kpa converts to 8.6 in Hg (vacuum). Vacuum should be between 15-20 in Hg."

Nope. You are confusing MAP with vacuum. 29kPa MAP = 21.3" Hg vacuum.
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Old 12-19-2022, 08:01 PM   #16
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Default Re: Intermittent Stall at Idle

From '97 DOHC experience.

I think a little dirt (or degradation) of the EGR can cause problems the
IACV struggles to fix.

The Fuel Pressure Regulator (spring-diaphragm) component that plugged
into the fuel rail would stick occasionally. After 150kMi it became an issue.
The pressure measured good with a gauge at idle, responding well to
vacuum changes. So I replaced it on account of suspicion ... which
made the car run nicely. The rubber diaphragm made a worn cylindrical ring inside its bore.

My guess (nothing more) is that hesitation is more likely fuel or mechanical
than ignition (electrons respond very quickly) ... good that BillR discovered
slowly changing power supply!
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Old 12-31-2022, 12:52 AM   #17
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Default Re: Intermittent Stall at Idle

Little update,

IAC is still intermittently closing, killing the engine. I confirmed this again by plugging in a spare IAC outside of the TB and watching the pintle movement.

Cleaned the battery terminals and tightened the cables up, no improvement (just peace of mind).

Checked the 5v reference line numerous times, no issues.

Took it for a ride Thursday, had a high idle/ idle hang while shifting until the temperature warmed up fully. After that, it stalled a few times sitting at a light.

Might just parts cannon it: new TPS, IAC, ECTS, CKP, and perhaps a used PCM.

From the service manual, the TPS, ECTS, and RPM are the only three conditions used in determining IAC position...
...
1994 Sl2 5spd: First Car, Stock: 56,576 (Totaled 1/10/2018)
1994 Sl1 5spd: Inherited, Stock: 494,500
1995 Sl2 5spd: Retired Daily, Not Stock: 196,000
1995 Sw2 5spd: Daily, Stock: 105,000
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Old 12-31-2022, 09:21 AM   #18
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Default Re: Intermittent Stall at Idle

If you fire the parts cannon at it, may I suggest you replace those sensors one at a time and test before replacing the next part? That way if one if those parts does fix it, maybe we can determine how the part failed having Isolated the issue.
...
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Old 12-31-2022, 02:08 PM   #19
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Default Re: Intermittent Stall at Idle

"From the service manual, the TPS, ECTS, and RPM are the only three conditions used in determining IAC position..."

That seems wrong. MAP and VSS are also valuable (necessary?) to establish the engine is at "idle" and needs IAC. Yeah, some FSM info is incorrect; maybe this is one instance.
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Old 12-31-2022, 03:23 PM   #20
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Default Re: Intermittent Stall at Idle

Quote:
Originally Posted by billr View Post
"From the service manual, the TPS, ECTS, and RPM are the only three conditions used in determining IAC position..."

That seems wrong. MAP and VSS are also valuable (necessary?) to establish the engine is at "idle" and needs IAC. Yeah, some FSM info is incorrect; maybe this is one instance.
This could be.

For the megasquirt ECU in my turbo 95, the requirements are TPS and MAP to engage closed loop idle, the ECTS for the requested idle speed at certain temps, then RPM as the feedback. I would assume the OEM computer would use similar parameters.
...
1994 Sl2 5spd: First Car, Stock: 56,576 (Totaled 1/10/2018)
1994 Sl1 5spd: Inherited, Stock: 494,500
1995 Sl2 5spd: Retired Daily, Not Stock: 196,000
1995 Sw2 5spd: Daily, Stock: 105,000
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