Rpm drop when stopping.

[Doubeleive]

@NickTransmissions any idea's? could it be something in the transmission, tc lockup, etc
this should be the 4l60

 

[Dustin Jackson]

6l80e?

 

[NickTransmissions]

@NickTransmissions any idea's? could it be something in the transmission, tc lockup, etc
this should be the 4l60

Could possibly be TCC dragging a bit but he mentioned having a Dorman throttle body installed and ordering an OEM one..I'd await to see the results of the swap there as I bet that fixes the problem..Dorman makes garbage for the most part...

 

[Doubeleive]

Could possibly be TCC dragging a bit but he mentioned having a Dorman throttle body installed and ordering an OEM one..I'd await to see the results of the swap there as I bet that fixes the problem..Dorman makes garbage for the most part...

he also stated he tried his friends TB and same results, also had the Idle bumped up and same result.
would the tuner be able to TC status on a drive test?

 

[mattbta]

What are your volts doing when the RPM drops?

 

[NickTransmissions]

he also stated he tried his friends TB and same results, also had the Idle bumped up and same result.
would the tuner be able to TC status on a drive test?

Thanks for the additional info - I didn't see those comments, just looked at the previous page.

Yes, you can monitor TC and all transmission control parameters via HP Tuners to see what's going on at any given point in time and tell if the TC is being commanded on when it shouldn't and vise versa. That said, TCC dragging usually doesn't last long before it completely fails and you stall out when coming to a stop (assuming it's stock on/applied) or the TCC lock up solenoid has malfunctioned...Additionally, you usually store P1870 or similar DTCs if this is happening for more than two consecutive drive cycles with no code resets.

Ok, bumped the idle from the factory 550 in gear, and 500 in park or nuetral up 50rpms on both. So rpms are still dropping down to 350 when braking, and now its starting to do it even at low speeds, but i think its the temperatures causing it. When it was cooler out it only did it on medium to hard braking. It did make it a little peppier off the line, so will keep the new settings, or do you guys recommend setting it to 650, which seems alot.

Have you put a vacuum gauge on the engine at idle? Looks like you replaced and/or checked quite a few things (just read your first post)...

Also, do you have a 6L80 (six speed)? Those converters are much more failure prone than the TMBX converters that went with the 4L60Es.

 

[Hodgeee]

Ok, datalogged the driving. Engine load at idle in gear and braking is 23% to 25%, regardless of braking force, which is normal for these tahoes. This means nothing with the tranny, and no extra drag from pullies, even AC on and off was only maybe 2% more drag.

My tuner only allows 50rpm increase increments. But it showed base idle values at 600rpm idle in park, and 500rpm idle in drive. So i bumped it to 600 and returned the park idle to 600.

Now braking it only drops to 450rpm, and 400 when hard braking. This is much more acceptable and didnt feel like it was shaking that much or surging when it dipped. Plus off the line in summer in Vegas makes it feel more like a Power Wheels, than a truck. It takes off nicely now if i need it to. Im gonna just give up at this point. It drives much nicer, i can live with it.

If there is any change with the new TB, ill post, but not expecting anything.

Thanks for helping all, it did give me routes to take that escapped me.

 

[Hodgeee]

Oh, by the way no voltage drops or spikes.

 

[NickTransmissions]

Ok, datalogged the driving. Engine load at idle in gear and braking is 23% to 25%, regardless of braking force, which is normal for these tahoes. This means nothing with the tranny, and no extra drag from pullies, even AC on and off was only maybe 2% more drag.

My tuner only allows 50rpm increase increments. But it showed base idle values at 600rpm idle in park, and 500rpm idle in drive. So i bumped it to 600 and returned the park idle to 600.

Now braking it only drops to 450rpm, and 400 when hard braking. This is much more acceptable and didnt feel like it was shaking that much or surging when it dipped. Plus off the line in summer in Vegas makes it feel more like a Power Wheels, than a truck. It takes off nicely now if i need it to. Im gonna just give up at this point. It drives much nicer, i can live with it.

If there is any change with the new TB, ill post, but not expecting anything.

Thanks for helping all, it did give me routes to take that escapped me.

Didnt realize you were in Vegas - so am I....PM me if you need any help and we can take a look at it together at some point

 

[Hodgeee]

May consider especially if you have a full active scanning pad. My handheld tuner can datalog and then i look at it in Data Viewer. But i cant look at the pedal control values, or mid TB values properly, only going over driven data, same for MAFS.

 

[NickTransmissions]

I have a TECH2 and Snap On Verus so between them we should be able to gather some useful information...let me know..

 

[Hodgeee]

Holy poopies, im in the stone age compared to you. Ill send you a message.

 

[Hodgeee]

Interestingly, i have a friend mechanic that works at the local Chevy dealer. Turns out that he is pretty good friends of the main mechanic that does all the Police Tahoes under contract. He took the truck first thing this morning, went over everything he could related to the braking issue, testing all sensors and main electric components. He found nothing wrong, BUT, he found that the fuel pressure would drop at the last few moments to 43psi from the steady 48psi, when braking. He said 43 to 48 is right in the range of where Flex Tahoes are supposed to be, but said there is no reason the fuel should drop. After stopping, the fuel pressure would immediately go back up to the 48psi. BG1988 stated that it was the DFCO, the mechanic stated the same, and stated it may need to be reprogrammed, but unsure, he believed that only the Hybrids had it. He asked if anyone I knew used HP Tuners and could turn it off, or reprogram it. I dont know anyone who does, but the local shops told me $580 to program it. He then also told me that in nuetral and manual gear selection it is deactivated but only on hybrids again. He said if it is on mine, there are no specific alternate settings, just turns on at about 1200 to1600 rpms, and turns off the same. This would explain why mine does it in nuetral or manual gear select. He stated on 2013 and up models, they added Hill assist braking, that auto down shifts your tranny so you dont go over the listed speed of the cruise control.

I don't suppose anyone in Vegas in the Forums has HP Tune?

 

[Dustin Jackson]

Interesting

 

[Hodgeee]

Just wondering if anyone else has vapor lock issues, not the topic of this post. We had a break from the normal 105 degrees today, it was rainy and overcast and was only 78 degrees outside this monring. While running errands, no idle issues, and the rpm drop at braking, seemed noticable less. Still there, but not the vehicle shuddering at every stop like usual.

 

[Dustin Jackson]

@Hodgeee Back when I had Cherokee XJ's vapor lock was a common problem with the 4.0 straight 6 motors, amazing things by the way. I have never heard of vapor lock being a problem with these motors and I've been a member of several Chevy forums for the last 4 years and haven't heard a peep about vapor lock until you mentioned it. I am open minded and haven't jumped to discount your concern but this is my experience of the subject.

 

[Dustin Jackson]

To add to the vapor lock subject, It was a problem when turning off a hot motor and attempting to restart the motor again after the gasoline had a change to "cook" above the hot motor. People would have difficulty starting their vehicles after shutting them off when it was really hot outside.

 

[Foggy]

The DFCO settings aren't doing it... The DFCO kicks in rarely and usually only above
about 1600 rpm. I actually set my DFCO to be MORE reactive to try and help out
my fuel mileage when i cammed it and got rid of the DOD crap

 

[Hodgeee]

It aint bad, once again, the weather was cool today, only 90 out, and the problem was much less noticeable. Plus raising the idle rpm to 600 made it drive better, and less affected by the rpm drop. Im kind of accepting that its normal maybe. Just waiting on the new AcDelco TB. mine is Dorman, and the one i tested out was a Dorman also.

My chevy mechanic friend told me that the reset procedure doesnt work on non Gm throttle bodies. He said when he tried the reset on the throttle body, the scan tool stated incompatability error. He is wondering if the TB is off a little and not allowing the computer to adjust it properly. I forgot to add that in my previous post. Ive also read the aftermarket TB dont work the same as oem on other forums, and Youtube.

 

[swathdiver]

He stated on 2013 and up models, they added Hill assist braking, that auto down shifts your tranny so you dont go over the listed speed of the cruise control.

GM calls it Cruise Grade Braking - Normal Mode. Older GMT900s have it in Tow/Haul Mode as well.

These trucks run at 43.5 psi with little fluctuation on a good pump.