Weird-ass Jeep Cherokee stall issue - resolved

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5chn3ll
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Weird-ass Jeep Cherokee stall issue - resolved

Post by 5chn3ll » Tue Jan 29, 2019 9:08 am

Mrs. Schnell's '17 Cherokee started exhibiting an unpleasant issue about a week ago. The car would start cold and run normally, but was consistently stalling when pulling away from the first full stop. It was reluctant to restart - the starter would crank MOSTLY normally, although it sounded a little strained. Eventually - after a LONG crank cycle (it's push start, so you don't get any input over how long it cranks) it would catch, fire up, and be fine for the rest of the day.

On Saturday morning, it got much worse - driving it for my morning caffeine infusion, it stalled at EVERY stop sign, and it damn near didn't restart. I was REALLY grateful this happened to me and not the wife.

The Google, of course, provided loads of useless and terrifying information about serial crank position sensor failure, incurable vacuum leaks, etc., so I tossed the iPad aside and started poking at shit.

There were no codes at all, nothing pending, no emissions issues, nothing suggesting any problems with fuel, air, or spark. Dammit.

I don't have a handy way to check fuel rail pressure, and thus no way to test a hunch that it might be a fuel pump issue (either a bad pump or debris from recent off-roading and/or bad gas in Big Bear), so I figure the easiest place to start is tracing grounds to the ECU, engine, and transmission. Since I'm lazy, I start with the battery end of the harness. The 10mm nut on the negative terminal is as tight as it will get...yet when I pushed back from the engine bay in disgust, the negative clamp rotated about an eighth of a turn around the battery post.

Image

This is so damn typical of "modern" vehicle design. The "old style" battery terminal clamp - a semicircle of soft metal (probably lead) with a bolt through it - could just have been tightened. Jeep switched to using a cam-lock terminal clamp that uses a tapered bolt and a nylock nut - but there is no slop in the Jeep clamp; it' can't just be tightened MORE. This wouldn't have been a problem with an old-style clamp, but the new cam-lock style clamp CANNOT just be snugged up. It's ridiculous:

Image
(Random Google image, not my car)

In the photo above, the nut closest to the battery post is what locks the clamp to the post. As you can see, there is no way to adjust the clamping force. What a stupid "upgrade."

For the time being, half of an Autozone terminal post shim is working perfectly to make the post thick enough for the clamp to work effectively. Next weekend, I'll remove that abomination entirely and replace it with a $5 lead clamp that will never fail.

The stall problem appears to be resolved, which is nice.

Understeer: You will hit the wall with the front end.
Oversteer: You will hit the wall with the rear end.
Horsepower: How hard you will hit the wall.
Torque: How far you will move the wall.

"If, after coming to an abrupt stop, your wife is not whimpering in terror, you're no longer a Porsche driver."
-Mrs. 5chn3ll


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OceanBlue2000
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Re: Weird-ass Jeep Cherokee stall issue - resolved

Post by OceanBlue2000 » Tue Jan 29, 2019 12:27 pm

I'm hoping you got it sorted, but before you identified the battery this sounds like a fuel issue to me. If this was my old 928, I'd suspect that it was losing fuel pressure while cold somewhere, could be a problem with the check valve or a leaky injector. Maybe a really dirty fuel filter.

Anytime I've had a vacuum leak big enough to cause a stall, there are always other symptoms like shaking as if its misfiring.
Mike
Strange things I have in head, that will to hand.
2000 Ocean Blue 996 Carrera 2
1982 928 (sold)

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5chn3ll
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Re: Weird-ass Jeep Cherokee stall issue - resolved

Post by 5chn3ll » Tue Jan 29, 2019 12:53 pm

I was half-convinced it was a loose or dying fuel pump - that was consistent with at least some posts I'd seen on the Grand Cherokee forums - but there was nothing at all with the 2017 Cherokee and fuel delivery problems. When the fuel pump died on my Corvette, the symptoms were nearly identical. However, the starter on the Cherokee was also not QUITE sounding healthy, enough that I tested the battery as step #1 to ensure I wasn't dealing with a battery chemistry issue.

If it crops up again, I'll figure out how to log fuel pressure.

Understeer: You will hit the wall with the front end.
Oversteer: You will hit the wall with the rear end.
Horsepower: How hard you will hit the wall.
Torque: How far you will move the wall.

"If, after coming to an abrupt stop, your wife is not whimpering in terror, you're no longer a Porsche driver."
-Mrs. 5chn3ll


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32wildbilly
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Re: Weird-ass Jeep Cherokee stall issue - resolved

Post by 32wildbilly » Tue Jan 29, 2019 1:54 pm

5chn3ll wrote:
Tue Jan 29, 2019 9:08 am
Mrs. Schnell's '17 Cherokee started exhibiting an unpleasant issue about a week ago. The car would start cold and run normally, but was consistently stalling when pulling away from the first full stop. It was reluctant to restart - the starter would crank MOSTLY normally, although it sounded a little strained. Eventually - after a LONG crank cycle (it's push start, so you don't get any input over how long it cranks) it would catch, fire up, and be fine for the rest of the day.

On Saturday morning, it got much worse - driving it for my morning caffeine infusion, it stalled at EVERY stop sign, and it damn near didn't restart. I was REALLY grateful this happened to me and not the wife.

The Google, of course, provided loads of useless and terrifying information about serial crank position sensor failure, incurable vacuum leaks, etc., so I tossed the iPad aside and started poking at shit.

There were no codes at all, nothing pending, no emissions issues, nothing suggesting any problems with fuel, air, or spark. Dammit.

I don't have a handy way to check fuel rail pressure, and thus no way to test a hunch that it might be a fuel pump issue (either a bad pump or debris from recent off-roading and/or bad gas in Big Bear), so I figure the easiest place to start is tracing grounds to the ECU, engine, and transmission. Since I'm lazy, I start with the battery end of the harness. The 10mm nut on the negative terminal is as tight as it will get...yet when I pushed back from the engine bay in disgust, the negative clamp rotated about an eighth of a turn around the battery post.

This is so damn typical of "modern" vehicle design. The "old style" battery terminal clamp - a semicircle of soft metal (probably lead) with a bolt through it - could just have been tightened. Jeep switched to using a cam-lock terminal clamp that uses a tapered bolt and a nylock nut - but there is no slop in the Jeep clamp; it' can't just be tightened MORE. This wouldn't have been a problem with an old-style clamp, but the new cam-lock style clamp CANNOT just be snugged up. It's ridiculous:

Image
(Random Google image, not my car)

In the photo above, the nut closest to the battery post is what locks the clamp to the post. As you can see, there is no way to adjust the clamping force. What a stupid "upgrade."

For the time being, half of an Autozone terminal post shim is working perfectly to make the post thick enough for the clamp to work effectively. Next weekend, I'll remove that abomination entirely and replace it with a $5 lead clamp that will never fail.

The stall problem appears to be resolved, which is nice.
My guess is the post clamp is cheaper for the manufacturer. I can't believe all the leads hooked to the battery post. If it was a pic of your car I would know why, but you claim it is not!

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5chn3ll
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Re: Weird-ass Jeep Cherokee stall issue - resolved

Post by 5chn3ll » Tue Jan 29, 2019 2:29 pm

The car was fine until we went wheeling last Sunday. That jarring plus the cold mornings was just enough to produce the intermittent issue; once the engine and battery got up to operating temps, thermal expansion kept the post-and-clamp stuck together and juicing things along until the next morning.

It all figgers.

Understeer: You will hit the wall with the front end.
Oversteer: You will hit the wall with the rear end.
Horsepower: How hard you will hit the wall.
Torque: How far you will move the wall.

"If, after coming to an abrupt stop, your wife is not whimpering in terror, you're no longer a Porsche driver."
-Mrs. 5chn3ll


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32wildbilly
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Re: Weird-ass Jeep Cherokee stall issue - resolved

Post by 32wildbilly » Tue Jan 29, 2019 4:04 pm

5chn3ll wrote:
Tue Jan 29, 2019 2:29 pm
The car was fine until we went wheeling last Sunday. That jarring plus the cold mornings was just enough to produce the intermittent issue; once the engine and battery got up to operating temps, thermal expansion kept the post-and-clamp stuck together and juicing things along until the next morning.

It all figgers.
"cold mornings..."? Come to NEB...!

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5chn3ll
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Re: Weird-ass Jeep Cherokee stall issue - resolved

Post by 5chn3ll » Wed Jan 30, 2019 8:12 am

Nope. I was born in PA; I remember just enough of Pittsburgh weather to know I don't have any interest in it.

Understeer: You will hit the wall with the front end.
Oversteer: You will hit the wall with the rear end.
Horsepower: How hard you will hit the wall.
Torque: How far you will move the wall.

"If, after coming to an abrupt stop, your wife is not whimpering in terror, you're no longer a Porsche driver."
-Mrs. 5chn3ll


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32wildbilly
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Re: Weird-ass Jeep Cherokee stall issue - resolved

Post by 32wildbilly » Wed Jan 30, 2019 9:28 am

5chn3ll wrote:
Wed Jan 30, 2019 8:12 am
Nope. I was born in PA; I remember just enough of Pittsburgh weather to know I don't have any interest in it.
Good choice!

Wait...I thought you were a ridge runner from East Texas??

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5chn3ll
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Re: Weird-ass Jeep Cherokee stall issue - resolved

Post by 5chn3ll » Wed Jan 30, 2019 11:19 am

My dad's family and my birth mother are both from Armpit, Texas, a one-intersection sh*thole (the nearest Wal-Mart is 20 minutes away).

Pop moved to PA to work and to bury himself in the mechanical engineering Ph.D. program at Penn State. Shortly thereafter, he ALSO buried himself in OPP that would eventually become his second wife, but that's a story for another day.

Understeer: You will hit the wall with the front end.
Oversteer: You will hit the wall with the rear end.
Horsepower: How hard you will hit the wall.
Torque: How far you will move the wall.

"If, after coming to an abrupt stop, your wife is not whimpering in terror, you're no longer a Porsche driver."
-Mrs. 5chn3ll


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sweet victory
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Re: Weird-ass Jeep Cherokee stall issue - resolved

Post by sweet victory » Tue Feb 05, 2019 11:03 am

Just an idear - you could've checked the fuel trims if you had a hunch about a fuel delivery issue. If the engine wasn't getting enough fuel, I'm sure the ecu would've shown an increase in fuel trims and potentially misfire codes.


Glad it was something that appears to be simple. Why do you think the engine would die only at stop signs and not while moving? Did you do a voltage drop test across the post and terminal clamp/other points?
2001 996 C2 Aero

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