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Buick Lucerne random stall - Thoughts?

posted May 14, 2016 15:02:09 by Tyler
Had a challenging one at work this week, an '08 Buick Lucerne 3.8L with a complaint of random stalling. One time in a parking lot, couple times while coming off the highway, all over the span of a month. Always restarts. No pattern to the symptom that the customer has noticed.

A symptom this infrequent is tough to diagnose, but it's worth trying. No codes, fuel trims are excellent at all load ranges. The throttle body was somewhat dirty, so I cleaned and relearned it.

Put 30-some miles on it yesterday, watching scan data, when I came to a stop and felt the engine try to stall. Finally! But just as quickly as it happened, the engine smoothed out and acted like nothing was wrong. Kept driving well for the rest of the day. *sigh* Figures.

Here's the data capture during the 'event'. As always, hit the Open PID List button on ScanShare to view the PIDs.

https://www.scanshare.io/share/hs335cMGpkmDXy0vlzVBcQ



A few things stood out to me immediately, around frame 755. One was the ignition mode switching to 'bypass'. Another was the drop in Ignition 1 voltage. Another was the jump in engine load.

I wanted to bring this to the community for any thoughts or theories you may have, or any suggestions for diagnostic direction. To be honest, I may never be able to post a fix for this, as I may not have the chance to pursue this further. But, I'm very interested in what you might see here, and what test you might do next if this was in your bay/garage.

There are no stupid questions/comments on this one, so post 'em up!
[Last edited May 14, 2016 15:04:50]
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9 replies
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AndyMacFadyen said May 14, 2016 20:06:11
This probably won't be relavent on 3.8 litre engine but I have seen apparently random stalling on smaller cars caused by faulty connection on the PAS pressure sensor, the car would stall at slow speed in parking lot when the steering was turned.
[Last edited May 14, 2016 20:06:59]
"Rust never sleeps"
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cheryl hartkorn said May 14, 2016 20:33:38
any reflashes available for this concern?
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Tyler said May 15, 2016 00:50:29
This probably won't be relavent on 3.8 litre engine but I have seen apparently random stalling on smaller cars caused by faulty connection on the PAS pressure sensor, the car would stall at slow speed in parking lot when the steering was turned.


Ah, because the PCM couldn't raise the idle to compensate for the additional load, right? This doesn't have a power steering pressure sensor, and the steering wheel was straight when this happened, but I like the idea here.

any reflashes available for this concern?


Not that I've found so far. Maybe I'll pull the PCM cal ID next time and see if there's an update anyway. Unfortunately, I don't have access to J2534 tooling any more, otherwise I'd be tempted to try it.
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Noah said May 15, 2016 12:53:24
Is it going into bypass mode because the RPM drops down into an RPM range close to what the PCM would consider cranking speed?
I'm sure you've seen tons of 3.8l engines with bad crank sensor and junk pulleys, although I don't see that from these captures. The dual ckp trace never drops right out or looks glitchy.
Must be fly by wire right? No IAC to blame for this one?
It would be cool to see what the o2s see when this is happening.
Its weird that engine load increased at that moment, could it be fielding the alternator to compensate for the drop in voltage? Just thinking out loud... Looks like you got a good one there!
Massachusetts, USA
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Tyler said May 15, 2016 13:40:33
Is it going into bypass mode because the RPM drops down into an RPM range close to what the PCM would consider cranking speed?


Not sure, but I think that would go along with something like Andy mentioned - an extra load applied to the engine that wasn't anticipated by the PCM.

Good thinking, sir. I'll consult the GM SI and see when the switchover is supposed to occur.

I'm sure you've seen tons of 3.8l engines with bad crank sensor and junk pulleys, although I don't see that from these captures. The dual ckp trace never drops right out or looks glitchy.


Surprisingly, I haven't! But yeah, I like to think that CKP A or B would have glitched if there were an issue. Those PIDs are actually why I chose to watch this data list during the test drive, looking for a CKP issue.

Must be fly by wire right? No IAC to blame for this one?


Yep, fly-by-wire. I do think it's interesting that all of the reported stalls seem to happen near closed throttle (parking lot, decel off highway). That seems to be the only common theme here.

It would be cool to see what the o2s see when this is happening.


Aye, I feel that this is a shortcoming with Snap-On equipment. There are preset data lists, but no one list has everything you want :-( Ah well.

Its weird that engine load increased at that moment, could it be fielding the alternator to compensate for the drop in voltage? Just thinking out loud... Looks like you got a good one there!


Agreed, this and the IC bypass are the two things that stick out the most to me. During the event, the throttle didn't open, but the MAP went higher. Does that account for the load PID increase?

I'd considered a sticking open EGR valve, but couldn't explain why it'd just open out of nowhere. Also thought about a glitching MAF. But, the scan data doesn't show any change. I'm now thinking about a 5V ref or PCM issue, but that's pure speculation.

I appreciate everyone's comments thus far, thanks!
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Noah said May 15, 2016 13:46:59
Aye, I feel that this is a shortcoming with Snap-On equipment. There are preset data lists, but no one list has everything you want :-( Ah well.

HATE THAT!
Massachusetts, USA
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Noah said May 15, 2016 13:49:52
Only one tps pid? That must be the one on the throttle? Wonder what the app sensors look like during the near stall...
Massachusetts, USA
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Tyler said May 15, 2016 17:17:52
HATE THAT!


I know! I don't know jack about scanner programming, but I feel like this is a problem Snappy could fix if they wanted.

The other extreme is the Autel I've tried a few times. It piles all data PIDs from a given module into one master list, no presets. The problem is scrolling through (what feels like) a million PIDs to pick out the handful you actually want. Just the act of scrolling through the data list is clunky.

You CAN create the list you want/need, but it's time consuming to do so.

Only one tps pid? That must be the one on the throttle? Wonder what the app sensors look like during the near stall...


Aye, I think that's a computed value. I think next time I get this vehicle I'll watch another data list. Preferably one with MAP, MAF, TPS/APP voltages and that IC data PID. I feel (pretty) sure this isn't a CKP thing, and that was the only reason I was watching this particular list.
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Noah said May 15, 2016 17:55:01
Right-O, I would've been watching that list myself.(the one with the ckp data that is)
[Last edited May 15, 2016 17:56:24]
Massachusetts, USA
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