5.3 LM7 Dying @ Idle During Braking

ckruzer

ckruzer

Infidel
Joined
Jul 2, 2015
Location
asheville nc
Depends on how hard I am braking. If I shift to neutral or leave in a higher gear and stop the engine dies.

It drops below the typical 600ish idle and too low than bam dead.

Any ideas. Kind of scratching my head on this one.

It's very easy to duplicate. Literally unless I down shift perfectly and keep the engine rpms up while slowing down it will die. Sometimes when I finally slow down enough to shift to first that final stopping power mid shift is enough for it to die.

Crazy stuff. It has not always done this. Just started random and not after any mods or changes.

Halp please

:sad:
 
Sorry. Running manual. Yes to VSS. Pulled it and it has some flakes on it, and the tip had something on it besides the ATF. Wiped clean now.

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Mac5005 said:
Idle air control valve dirty/clogged/slow?
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Haven't checked. I remember when I was checking sensors before first start there was slot of oil in the TB area and the TPS had oil in it when I pulled it. Thoughts?

@XJsavage and have looked at this VSS and hope that the cleaning did the trick. I'll know more tomorrow.
 
If it was the vss, your speedometer would have read wrong as well.. check the iacv as said.. scan and see if any codes present. When you say running manual do you mean manual trans or manual valve body? Also if the latter, is the t/c lockup still under PCM control?
 
a_kelley said:
If it was the vss, your speedometer would have read wrong as well.. check the iacv as said.. scan and see if any codes present. When you say running manual do you mean manual trans or manual valve body? Also if the latter, is the t/c lockup still under PCM control?
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Manual trans right now. No speedo atm either. I just use my phone when driving through hot spots. But I'm a fair judge on speed based on rpm and gear.
 
Reading online. Apparently depending on loop status determines if vss or iac is controlling rpm. And loop status is determined via vss.

So hopefully this was just a dirty vss issue. We will know more tomorrow. Apparently stalling while braking is a common issue exp by those who run non electronic trans but also try to skip vss
 
Id clean the throttle body and iac with carb cleaner and an old toothbrush. Ive not seen this stalling, but I have seen them cause the idle to be slow enough that the computer will disable the a/c. Every summer we'll get customers at the shop complaining the ac isnt working, get in and its idling around 5 to 600. Clean the tb and its good to go.

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Tim C said:
Id clean the throttle body and iac with carb cleaner and an old toothbrush. Ive not seen this stalling, but I have seen them cause the idle to be slow enough that the computer will disable the a/c. Every summer we'll get customers at the shop complaining the ac isnt working, get in and its idling around 5 to 600. Clean the tb and its good to go.

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Click to expand...

That's what mine is idling around. What should it be at?
 
ckruzer said:
That's what mine is idling around. What should it be at?
Click to expand...
Im taking about the factory dash tach which may not be completely reliable, but most ive seen idle closer to 700 in park. A manual should be similar.

Another thing to consider if this was a swap for a wheeling rig or whatever, is that you may need someone with a tuner to adjust the throttle follwer settings in the tune. I know when I tuned a tpi motor for my dads truck and the tbi on my crawler it would stall occasionally when coming to a stop until I played with the throttle follwer settings, theyre different between auto and manual transmission. The throttle follwer basically opens the iac by % in scale with the throttle open %, its like a stall saver, and is why injected vehicles with a manal trans will hang at a higher rpm if you clutch in. For example my dads tpi 67 c10 will drop to and hang at 1500 rpm if you clutch in from a higher rpm, until the truck slows to 15 mph. Then the rpm will drop in proportion to the drop in vss, until it hits the desired idle speed.

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@Tim C ah gotcha. Thanks for the tips.

For the sake of diagnosis ill toss in the cleaned VSS today, take it for a spin and see. Then I will take gander at cleaning or replacing the IAC

Ive been meaning to, so today when i test drive ill spray some starting fluid around the intake to see if any leaks.
 
for what its worth, i had a similiar issue on our suburban. it was the mass air flow sensor.
 
Meh... warlocks are handy to have around sometimes. ;)
 
Its back. Not as bad. Basically as it was like when it first started.

Could this be just a really crappy vss that is too sensitive? Its a 90$ duralast sensor. (dual latch version)

ALT options are Airtex/Wells ($60) and Ultra-Power ($40). Both of which have probe tips which have a convex area for more probe protrusion...
 
Can't you run a standalone engine harness without a VSS?
 
Croatan_Kid said:
Can't you run a standalone engine harness without a VSS?
Click to expand...

Yes. I could remove vss if i wanted to, but the computer vss would also have to be deleted. but to my knowledge deleting this results in alot of issues, like my current example apparently.
 
What transfer case are you running and is the VSS in the transfer case?
 
It's either a VSS issue or maybe the tooth count is different from an NP261/NP263 to an NP231. I'm running an NP241 and apparently they're the same tooth count as an NP261.

Here's another though...are the wires from the harness to the VSS pigtail connector assembled correctly?
 
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