NEED HELP Stalling coming to a stop

gmcman

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Dec 12, 2011
4,656
2002 Envoy, have had zero issues with the accelerator or (gas pedal sensor).

Just recently on 3 different occasions within 2 weeks, after reaching a stop, when the gas pedal is pressed, no engine response, not even a blip from the tach...just an idle.

There are no codes or CEL, REP light, if you shut the engine off then restart, all is good again.

This doesn't happen while driving, only after coming to a stop.

I checked the wiring, no visible rodent damage.

I'm thinking to replace the gas pedal assembly to be on the safe side. Has anyone had this happen?
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Very odd that no error came out as the throttle system has a couple of redundancies with double checks of input and output. As suggested, using a scan tool capable of live data should be used to see what the PCM is seeing for pedal input. A Bluetooth ELM327 adapter with the Torque app should be able to do this.

Another thing you could try is disconnecting the fan clutch. It is known to kill the 5V reference for the pedal system when it shorts out however it usually throws a code for it.
 
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gmcman

Original poster
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Dec 12, 2011
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So here's the updates and progress, mods, feel free to modify the thread title to add "engine stalling" as a suffix.

What was initially thought as a bad gas pedal, has turned into a stalling issue after coming to a stop. The Envoy as been used by our newest driver (yikes) and what I was told was there was no response when pressing the pedal, which still may be an issue.

I drove it a good bit this weekend and it will stall after coming to a stop after about 10 seconds or so, not every time but maybe 1 out of 3 times.

I have just replaced the plugs with about 95K miles, and the #1 coil which went bad a week ago.

I would try to read the Throttle Position Sensor voltage with a scan tool.

using a scan tool capable of live data should be used to see what the PCM is seeing for pedal input


I put the Tech 2 on it and the values reciprocate without any hesitation or delay, doesn't appear to have any dead spots, and voltage looks good.

The throttle body is clean, I even installed a spare clean TB and same result. There is no oil in the CPAS connector but I will remove it to check the screens.

No vacuum leaks and the vacuum is good at 19-20"Hg, I think that's based on MAP voltage though, and that appears to be good.

With the fresh plugs, it idles like a top, when it acts up, the RPM's will stumble enough that I can catch it with a quick blip of the throttle and this saves it from stalling, then it's good to go.

The engine only does this after coming to a stop, haven't confirmed fuel pressure but it's a fairly new Bosch pump.

Maybe the slightly worn plugs have taken a toll on the coils, I did go against my life long practice of only using OE parts for electronic items and bought a cheap 6-pack of coils 2 years ago, I have since replaced 2 of them.

Two things that raise a brow, when the engine is idling in gear, I can see the timing go down to as low as 1 deg adv to 3-4 deg. I also noticed on the Torque app that one stalling event showed the #6 coil have about 8 misfires in the process of stalling. Maybe it's random coil packs doing this, I have a spare and will change #6 but this misfire event doesn't happen all the time.

The ignition switch came to mind but this only acts up after a stop, steady cruise is completely uneventful.

When it stalls, it goes from idling perfectly smooth to a hard stumble then stall, takes about 1 second to stall....it's a fairly quick event.
 
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Mooseman

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Dec 4, 2011
25,257
Ottawa, ON
I'm betting on that misfire. Could be those cheap coils. Maybe swap the misfiring coil to one of the new ones and see if it moves with it.

I'd also try disconnecting the battery (or pulling the PCM fuses) for 30 minutes to reset it again as it is presenting the same symptoms of a bad idle after disconnecting the battery. The throttle body has to be spotless when doing this.
 

budwich

Member
Jun 16, 2013
2,027
kanata
I am interested in your "timing observation". I have seen some "funny numbers" and somewhat question the values output by torque. Not sure what in the PCM causes it to "request" little to no timing advance but "past engine design observations" would expect to see an idle of 6-10 degrees. So to me, either something is feeding "bad info" to the pcm and it is acting on it, the torque value is "out to lunch" or there is some "strange algorithm" in today's controllers for things related to emission control or otherwise.

The misfires probably aren't helping.

You might consider monitoring your fuel trims and the vacuum. The map can be checked (a technique was posted somewhere in the past). I am leaning on the MAP sensor (wiring or otherwise).
 

gmcman

Original poster
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Dec 12, 2011
4,656
Here's the current update.

I went to a boneyard and grabbed 5 delphi coil packs from an 04. Installed them with zero change....less misfires, almost none, but it idles forever, smooth, drives smooth, zero issues...but when coming to a stop, it stalls, sometimes right before I stop, or right after.

I flashed the PCM with the stock tune again, no change.

I swapped fuel pump relays as I had a spare, nothing.

I can't figure this one out, when it stalls, it's a quick stumble then the stall, almost no time to react.

Only happens when rolling to a stop or after you stop....about 1 in 4 stops is a stall.
 

gmcman

Original poster
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Dec 12, 2011
4,656
The map can be checked (a technique was posted somewhere in the past). I am leaning on the MAP sensor (wiring or otherwise)

Worth a try for sure.

The O2 sensor has about 30K miles on it, AC Delco.

I do see a random spike in STFT...goes way up to about 18-20% briefly then back down.

I will also check the cam sensor for oil, CPAS connector is clean.
 

Chickenhawk

Member
Dec 6, 2011
779
If it stalls almost immediately after coming to a stop, one problem that can cause this is the torque converter clutch not releasing. It might be at the TCC solenoid, solenoid wiring or the TCC valve inside the transmission. There are ways to test the solenoid using a multimeter before you have to drop the pan.

If you need to replace it and the pan is off, one thing many of us do is install an aftermarket pan with a drain hole.
 

budwich

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Jun 16, 2013
2,027
kanata
Worth a try for sure.

The O2 sensor has about 30K miles on it, AC Delco.

I do see a random spike in STFT...goes way up to about 18-20% briefly then back down.

I will also check the cam sensor for oil, CPAS connector is clean.
A faulty "off shore purchase" of a map caused me a lot of problems in the area. After install, the idle process (let idle for 2 minutes then shut off and restart, should be ok... re do) only worked a bit. The truck soon there after threw a code for too rich (iirc) as the trims ran towards -25. I put back the old one, problem went away. Eventually, got a map that "functioned" but also pulled a gm one from pull it yard.

Anyway, I tested the "bad" one with the technique posted here... vacuum pump / gage, key on, read the result via torque. The "bad" map would response to the vacuum as expected, values from torque and the pump gage compared from 0-20. However, cycling a couple of times, the "bad" map would stay at high vacuum reading even though the vacuum was released. Not sure why... but maybe some internal issue with the chip / membrane.

sorry for the long story.... but thought it may give you some ideas on where to look. I figure your "die after braking / stop" is somewhere in the "region of vacuum" which is "MAP domain". I am still interested in your observation about the timing which to me (I see that in my vehicle) is also strange... IF the values that torque is outputting are correct. Not sure why timing "command" would "drop" so far at an idle condition.
 
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Mooseman

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Dec 4, 2011
25,257
Ottawa, ON
Since it's an '02, there is no MAF or SAIS. What about the exhaust manifold? If it has a crack, maybe it's introducing extra oxygen and screwing with the O2 sensor since you saw a jump in the trims. Maybe check for a vacuum leak and move around hoses. Check that vacuum plug at the front of the intake manifold as it can go bad.

Along those same lines, disconnect the O2 sensor and see if it improves.
 

gmcman

Original poster
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Dec 12, 2011
4,656
If it stalls almost immediately after coming to a stop, one problem that can cause this is the torque converter clutch not releasing

I suppose to rule that out, is it will stall after stopped for a min, sometimes at an idle after starting but that's only happened once. Def plausible if only when slowing down.
Since it's an '02, there is no MAF
No, I believe only the MAP sensor was mentioned.

I do get some crazy short term trims when decelerating at times, up to 20%.

I will try a fresh AC MAP sensor, I can't remember what my old values were when it was running correctly, but I'm seeing 4 psi at idle in Torque. Obviously dependent on barometric pressure but seems slightly high....that's purely speculation though.

I was also thinking of a brake booster diaphragm leak, but would think that would happen continuously.
 
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gmcman

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Dec 12, 2011
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I went back to the pick a part, and grabbed a MAP from an 04.

I swapped it in the parking lot and for a good 30 min, drove around with multiple stops and no stalling..only 1 stumble but it recovered.

About 10 min later it did stall at a stop, and honestly I believe the MAP sensor was helping. I did notice also the upstream O2 sensor was tanking hard, and this is much different from a week ago, even 2 days ago.

But, this should tell a story, when idling it looks like it's nearly flatlined.lol. I will replace this on Monday.

The crazy STFT is not a regular value, likely from the flatline.
Screenshot_20221019-232502_Gallery.jpg
 

budwich

Member
Jun 16, 2013
2,027
kanata
hmmm.... quite a "dump". :smile: As usual, I find some suspicious looking numbers... beit your vehicle itself or torque... I don't know which.

I find it kind of weird that your timing advance at an "idle" is almost 20 degree. To me that is very unusual for a gas / air engine.... historically, engines would be around 6-10 advanced at idle.

The trim values are "bordering on" triggering "too rich" conditions (>25% negative). The "throttle number" (not sure which one is being displayed) also looks "different" as from my monitoring of "absolute throttle" is about 16-20% at normal "hot" idle. Intake air seems reasonable.

Not sure what to say... maybe the seating of the MAP, either the vacuum itself or the electrical connector is not good. The "flatness" of the O2's is not good... if it is true, they have either failed or you have significant leakage of gas if I understand things (leaning of trims) or a significant vacuum leak wreaking havoc on the computer trying to figure out what the engine / operator is wanting to do.
 

NJTB

Member
Aug 27, 2012
612
Flemington, NJ
Same thing happened to mine this summer. Good hot day, about a 30 mile ride, came to a stop sign and it stalled (once).
Hooked up the scanner and noticed the same things, STFT, LTFT out of wack, but not as badly as yours, about 7 or 8 on both, no codes. For the last 10 years they've both been in the 2 to 3 range.
Also noticed the O2 sensor being a bit lazy, so yes it's going to be replaced shortly.
 

gmcman

Original poster
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Dec 12, 2011
4,656
My upstream O2 sensor is pretty much dead. This is a normal upstream sensor when driving.

Screenshot_20221020-211443_Gallery.jpg



This is mine.

Screenshot_20221020-211756_Gallery.jpg
 
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gmcman

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Dec 12, 2011
4,656
Now it's getting interesting, new upstream O2 sensor, same data.....the PCM is not getting the data or there's a 5V reference issue.

I need to trace the wires to the connector from the PCM, not sure what each one does so if someone already has a reference I'd be happy to view it.
 

Mooseman

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Dec 4, 2011
25,257
Ottawa, ON
The schematics would be in the manual (link in my signature)
 
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gmcman

Original poster
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Dec 12, 2011
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Thanks.

I have a Tech 2 so this shouldn't be too hard to track down. Sucks that my kid is now driving it and I can't have it stalling out. I'll tackle this on Saturday, I don't like to be a replacement tech, I want to figure this out.

I did have plans to change the O2 sensor anyway as I do with every plug change.

Grrr.
 

budwich

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Jun 16, 2013
2,027
kanata
Now it's getting interesting, new upstream O2 sensor, same data.....the PCM is not getting the data or there's a 5V reference issue.

I need to trace the wires to the connector from the PCM, not sure what each one does so if someone already has a reference I'd be happy to view it.
You might be barkin' up the wrong tree.... by assuming that the o2 is the issue. IF I understand your posts earlier, they appear to indicate you have a v8... right? (ie. s1 s2).

IF there was no 5v, it is likely that the system would throw a code. When you say that the "replacement" is acting the same... what does that mean? what does the trace look like. My thinking is that if it is "acting the same", that probably means its reacting to the same system conditions as the other one.
 

gmcman

Original poster
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Dec 12, 2011
4,656
2002 Envoy 4.2.

This is my trace at idle, cat hasn't lit off yet.

This shows a small change, was a flat line at one point.

No foul odors from tailpipe, runs and idles fine, just stalls.
Screenshot_20221021-164948_Torque.jpg
 

budwich

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Jun 16, 2013
2,027
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I guess I should have figured the "MF counts" for 6 cylinders... :smile: Anyway, if your replacement is acting similar to the previous one, that would be suspicious in terms meaning that the change did nothing.
This latest data dump shows 174F... how long was it run before this was taken. Maybe you have a temperature sensor issue???? causing the engine to run rich... maybe.
In its current operation, at normal idle, your upstream 02 "never" oscillates up and down like a sync wave????
 

gmcman

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Dec 12, 2011
4,656
Sorry @budwich , I should have posted a screenshot that was up to temp.

FWIW, my upstream waveform has pretty much always mimicked a sine wave at idle....just a slower update, and when at speed or under load, much faster response.

The Envoy drives perfect, just shuts down after a stop 1 out of maybe 4 times.

Here's a screenshot at temp at 60 MPH.

Screenshot_20221024-173740_Gallery.jpg

That's all over the place.


This was during the warmup, a few min into the cold start.

Screenshot_20221024-183022_Video Player.jpg
 
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budwich

Member
Jun 16, 2013
2,027
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ok... thanks for the clare. There appears to be some "funny" with throttle position values across what you posted. These are quite different from what I see when monitoring my vehicle (2008). The small number would seem to suggest an almost closed throttle at times... maybe a potential for a stall if that occurs at the right time.
 

TollKeeper

Supporting Donor
Member
Dec 3, 2011
8,047
Brighton, CO
I wonder if his throttle might be getting stuck on some carbon on deceleration?

Maybe a throttle cleaning, disconnect battery, and try again?
 
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gmcman

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Dec 12, 2011
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I swapped to a spare TB, both were clean, both had multiple relearn steps taken.

Same result.

This only happens after coming to a stop, or the last few MPH when coming to a stop.

Never while driving at a steady cruise.
 

TollKeeper

Supporting Donor
Member
Dec 3, 2011
8,047
Brighton, CO
Check your intake, and intake tube. Unmetered air would also cause this.
 
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Mooseman

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Dec 4, 2011
25,257
Ottawa, ON
I was thinking the same thing. Maybe go around with brake cleaner or carb cleaner to look for a vacuum leak.
 

gmcman

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Dec 12, 2011
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Long story so bear with me, I think we might be able to (hopefully) bring this to a near close.

Since this only happens after I begin to stop, or while stopped, I wanted to be sure the brake booster doesn't have a ruptured diaphragm....as this stalling points to either a TB issue or a vacuum leak as we have mentioned.


I checked the booster last night and all was good.

The back story of my fuel pump sealing issue I believe plays a part, I still get random P0440 evap codes as I have been off and on for a year or so, shows it's face a couple times a year. I didn't discover this until about 4 months ago when I replaced my fuel pump.

Right now, I don't have a code when it stalls, but it will show up then go away, I've replaced the gas cap also but still get the codes.

When it began stalling, I noticed misfires then a faulty coil, also considered my new driver using the "cheapest gas" and thought maybe the tank was filled with E85, so my thoughts were elsewhere.

So after I checked the brake booster, I remembered back in June, when my son and I were checking the evap system for leaks. All lines were good, but it was the evap test that perked my interest.

So here's where it gets interesting...

I played around with the Tech 2 on this screen....

20221028_092252.jpg

The highlighted line for the vent solenoid, you can toggle ON/OFF to close or open the vent in the rear of the tank.

Then go down one line to the purge solenoid command and start changing the duty cycle in 10% increments. You can manually increase or decrease the vacuum placed on the fuel tank and observe the fuel tank pressure sensor data.

The PCM performs a test to ensure the system is "sealed" to the vehicle standards and I believe it has to maintain 12 or 13 mmHg, it won't be the full value of the total engine vacuum as the gas cap has to vent a little. The PCM will also only perform this test with the fuel tank between 15 & 85% filled.

So the EVAP line from the TB, goes into the solenoid, then the charcoal canister, then the tank, then the vent valve.

When the PCM runs the test, the solenoid allows a small amount of vacuum per each 10% step of duty cycle...should reach the target tank vacuum around 30-50% duty cycle I assume, then release.

We. ran it to 80-100% to check for leaks, only heard a slight vent from the gas cap but came up about .2-.5 mmHg short of the mark at times, not all the time.

Instead of backing down the duty cycle of the solenoid, I hit EXIT and the motor stalled....I thought nothing of it as the next time I backed it down slowly and it didn't stall.

I believe since June, the fuel pump locking tabs have not been sealing as well despite the fuel-resistant sealant and now the random EVAP tests by the PCM are commanding a higher duty cycle to reach the target tank pressure, thus ultimately not reaching that target then opening the vent valve to end the test, resulting in a huge void of vacuum that the motor can't keep up with....thus stalling.

That was my thought process, so I pulled the EVAP line from the TB, then capped of the port on the TB.

I have since put 80 miles on it, multiple stop and go sessions.....zero stalling, not even a hiccup.

So it appears that the rusted locking tabs of the fuel pump, have created a leak large enough to keep the EVAP sealing test from reaching it's target gas tank vacuum of 12 or 13 mmHg (not sure) thus resulting in a sudden surge of a lack of vacuum for the engine to overcome.

The likely fix will be a new gas tank.
 
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Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Good detective work. Sucks that you might need a new gas tank. Maybe it's just the locking ring itself or the seal. Maybe a good used one would work as new tanks are quite expensive.
 
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gmcman

Original poster
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Dec 12, 2011
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Yeah, about $700 delivered....yikes.


Well, still no stalling which is great.

However, may be something for another thread but my upstream O2 sensor is still all over the place, but the engine runs great.

Screenshot_20221102-191121_Gallery.jpg
 

budwich

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Jun 16, 2013
2,027
kanata
I think that you are going to find that the "refresh" of the data coming from the o2 sensor in torque is not quick enough to always follow the trace especially at "speed".
 
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TJBaker57

Member
Aug 16, 2015
2,897
Colorado
my upstream O2 sensor is still all over the place


Remember now, Torque sequentially asks for each PID you have on the dashboard. It waits for each reply before asking for the next PID. The more PIDs you display on a dashboard page the longer the delay between data points on your graph.

I have a dashboard with about 16 PIDs and my testing reveals that it takes on average 2.6 seconds between data points returned for a given PID. That's a looong time. The resulting graph is understandably a crude representation of what is really happening at the sensor. My Tech 2 by comparison takes a data point for each PID every 0.19 seconds on average when reading or plotting my O2 sensors. Surprisingly, using the graphing function of Torque Pro is even faster than this when reading just the 2 PIDs of my upstream and downstream O2 sensors. Graphing my O2 sensors in Torque Pro yields a data point each 0.08 seconds.

Screenshot_20221026-142810.png
 

Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
Remember now, Torque sequentially asks for each PID you have on the dashboard. It waits for each reply before asking for the next PID. The more PIDs you display on a dashboard page the longer the delay between data points on your graph.

:goodpost: I never knew this.

I had similar concerns with my O2 sensor readings a while back, and ended up replacing the upstream one because mine was looking similar to the OP's, and didn't change afterwards either. I also had a full dashboard with a lot of stuff on it, so this explains it.
 

budwich

Member
Jun 16, 2013
2,027
kanata
Remember now, Torque sequentially asks for each PID you have on the dashboard. It waits for each reply before asking for the next PID. The more PIDs you display on a dashboard page the longer the delay between data points on your graph.

I have a dashboard with about 16 PIDs and my testing reveals that it takes on average 2.6 seconds between data points returned for a given PID. That's a looong time. The resulting graph is understandably a crude representation of what is really happening at the sensor. My Tech 2 by comparison takes a data point for each PID every 0.19 seconds on average when reading or plotting my O2 sensors. Surprisingly, using the graphing function of Torque Pro is even faster than this when reading just the 2 PIDs of my upstream and downstream O2 sensors. Graphing my O2 sensors in Torque Pro yields a data point each 0.08 seconds.

View attachment 105747
One thing I like is the downstream level which seems to be around 6.5-7 ish. I have always wondered about this level for a warmed engine. What rpm was this engine and capture done at?
 

TJBaker57

Member
Aug 16, 2015
2,897
Colorado
One thing I like is the downstream level which seems to be around 6.5-7 ish. I have always wondered about this level for a warmed engine. What rpm was this engine and capture done at?


Approximately 1250 rpm, coolant temp about 208°F, speed about 40 mph, just shy of 289,000 miles on what I presume is the original catalytic converter. And possibly even the original O2 sensors!! I bought the vehicle with 149000 miles on it and I cannot remember ever changing those sensors.
 

TJBaker57

Member
Aug 16, 2015
2,897
Colorado
In documentation literature with regards to P0440 and a few other related codes there are these "Secondary Parameters and Enable Conditions".

Screenshot_20221111-103328.png

Can anyone point to documentation that spells out what "SUC" and "SUM" are??
 
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budwich

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Jun 16, 2013
2,027
kanata
my guess based on the SM..... SUC... system unit cooling (ie. coolant), SUM ... system unit metered (ie. incoming air). But heck, I hate all "acronyms". :smile:
 
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