E38 996 Swap, revs slow to come down when clutching in

[Amille28]

Car is an 01 Porsche 996 with 08 5.3 E38 ECM.

I installed a speed sensor/VSS on the axle from Dakota Digital.
As I'm driving when I clutch in RPMs either stay stuck or very very slowly come down.
Also when driving at around 25-30mph revs get stuck and feels like cruise control is on (its not).
When I brake the RPMs are slow to come down until I'm almost stopped then the car jumps to idle RPM.

When I disable the MAF and go into VE, this does not happen and RPMs drop right off when clutch is pushed in like a normal stick should drive.
I'm not sure what settings I need to change.

Any help greatly appreciated.

Porsche LS tune vette 24MAR24 maf turn on.hpt

[LS ROB]

Car is an 01 Porsche 996 with 08 5.3 E38 ECM.

I installed a speed sensor/VSS on the axle from Dakota Digital.
As I'm driving when I clutch in RPMs either stay stuck or very very slowly come down.
Also when driving at around 25-30mph revs get stuck and feels like cruise control is on (its not).
When I brake the RPMs are slow to come down until I'm almost stopped then the car jumps to idle RPM.

When I disable the MAF and go into VE, this does not happen and RPMs drop right off when clutch is pushed in like a normal stick should drive.
I'm not sure what settings I need to change.

Any help greatly appreciated.

Porsche LS tune vette 24MAR24 maf turn on.hpt

More than likely this can be solved with your Base Running Airflow table. Do you have a log? Just lower the number @ whatever gear & rpm it happens. Are you running a 5.3 or something different? Need the details of the build/swap.

[Amille28]

Rob, Thanks for the reply.

Apologies, not sure which table is the base running airflow table. Is it under Idle -> Airflow?

I am running a Gen IV 5.3 mated to the Porsche M96 manual transaxle. The Porsche used the wheel speed sensors for speedo and vss functions.. so when you do the swap there is no VSS output on the trans.

Dakota digital makes this (https://www.ebay.com/itm/193874535635) which is just a magnetic pickup with some magnets that mount on the axle. As the axle rotates it creates a VSS pulse.
I was told this VSS signal is needed for the ECM to do its thing.

I'd like to get a clean coastdown of RPM when pressing in the clutch. It seems like the VSS signal is acting like an automatic trans car and is slow to RPM down when the clutch gets pressed.

[LS ROB]

Yes, that's correct. Idle->Airflow->Base Running Airflow also known as (BRAF) You can try this as a test to see if this helps. Took out about 20%
Porsche LS tune vette 24MAR24 maf turn on.-braf exp.hpt

[Amille28]

I've never touched this table. I probably need to steal a table from someone's tune to get this better in line with what it needs to be.

[LS ROB]

just attached a file to my prior post. Give that a shot and see if it helps.

[Amille28]

Thinking about this for a minute. Since the M96 Porsche transmission is completely mechanical, my ECM has no way of knowing what gear the trans is in. I should somehow make all of these values the same for all the gears?

[blindsquirrel]

Or do this: https://www.hptuners.com/forum/showt...e-tuning-guide

[LS ROB]

Thinking about this for a minute. Since the M96 Porsche transmission is completely mechanical, my ECM has no way of knowing what gear the trans is in. I should somehow make all of these values the same for all the gears?

I do not know if that would be best practice but i do know that hanging idle/cruise control feel can be fixed by adjusting the BRAF.

[obthomas]

First, you did good to add a VSS to the car because it does not have one. Next you did good to use an E38 for the gen 4 corvette and hopefully the VIN is for a manual transmission.

If the E38 is not for a manual transmission it is expecting VSS information via CAN from the transmission controller. Of course that is not going to happen.

If the E38 is for a manual transmission You can then connect the VSS to pins 71 and 72 on the E38. If you did this then the tune will operate using VSS information when needed.

Good Luck

[obthomas]

Is the VSS you added working? Does speed register when you log the vehicle while it is moving? A closer look at your E38 speedo tab leads me to believe you need work. Probably need to set output to wheel speed instead of vehicle serial, and final drive ratio doesn't seem right. I think your engine running issues you started this thread with are because your VSS is not working. I could be wrong and just trying to help. Good Luck.

[Alvin]

More than likely this can be solved with your Base Running Airflow table. Do you have a log? Just lower the number @ whatever gear & rpm it happens. Are you running a 5.3 or something different? Need the details of the build/swap.

This appears to be the problem to me also.

This looks like 6.2L base running airflow. A 5.3 would take a good bit less airflow. Usually 6-8 g/sec full warmed up if it is stock.

A log would be helpful. Maybe I missed it.

[TheMechanic]

Try dropping [ECM] 2214 to 1,100 along with LS ROB's BRAF

[Amille28]

Thanks for all the replies. I did want to say that initially (one drive to the grocery store), the BRAF edits seemed to fix the problem. I also just copied the values in Neutral, to all the other gears and Reverse, as the E38 has no input telling it what gear this is in. This just helps me down the road as I tune the table to make sure I hit all the blocks in the table the same.

blindsquirrel - I may try this when I have more time the concept looks promising. I've battled idle issues the whole time.

ob thomas - the E38 I have is for manual transmission and hooked into the correct wires. I got it all from PSI. The VSS is working, but I haven't touched the speedo stuff because the Porsche speedo runs from the wheel speed sensors and doesn't even touch the GM ECM. If this output matters I'll work on it for the tune if required. Drive ratio is correct for M96 transmission 3.44.

Alvin - I didn't post a log sorry, but this setting adjustment seems to work.

TheMechanic - I'll give it a shot, thanks.

[obthomas]

Amille
The VSS you added has nothing to do with the Porsche and everything to do with the E38. You have not provided a log so I cannot confirm it is working. Even if it is working it has to be wrong because you are using an axle and apparently two magnets to tell the E38 your speed. Your new VSS does not provide speed information going through the M96 so why would you skew the speed information with an irrelevant final drive ratio. Being a suspicious person I question if your new VSS is providing the right information to the E38 and a wrong VSS will cause most of the problems you started this thread with.

[Amille28]

Thanks for your response although I think part of the problem is my lack of understanding in some ways of how the E38 operates. Some of the Porsche swap related forums state that the fidelity of VSS isn't super important just "the car is moving" and "the car is stopped." The transmission has no way of telling what gear its in to the E38 other than the reverse. Someone else did the same setup to get the VSS signal. Attached is a pic of a G96 which is like the M96. It mates up to the LS engine with an adapter plate and has the gear and final drive all in one. The axle where the pickup is attached on the side of the transmission/axle output. I did a small run to the grocery store max speed 30 mph and you can see the speedo comes alive but doesn't have good fidelity because of only 4 pickups. On the other tune I posted I had this set to 2 to see if it would double the speed and never remembered to change it back to 4. There are physically 4 small magnets on the axle which is after the final drive. In all honesty I will say adjusting the BRAF table helped tremendously.

Porsche LS tune vette 24MAR24 maf turn on max idle VSS 4.hpt31MAR24.hplg9600.png

[LS ROB]

If you want, you can try this to see if it feels any better. Although it was a short log, I tweaked the MAF and VE so that should have you heading into the right direction now.

Porsche LS tune vette 24MAR24 maf turn on max idle VSS 4.-maf ve tweak.hpt

[TheMechanic]

Use this for your VSS. Works like a charm and specific for that trans.
https://www.deloreanmidwest.com/prod...ed-sensor-kit/

[obthomas]

Use this for your VSS. Works like a charm and specific for that trans.
https://www.deloreanmidwest.com/prod...ed-sensor-kit/

That is the one I have on my Porsche 986/LS3. An accurate VSS fixed all my problems. Many of the E38 tables have speed as an axis. There probably are some tables that use VSS that you cannot see with HPT

[Amille28]

TheMechanic /obthomas- Yes! thats exactly what I need. one of the 944 forums used to have these but they stopped carrying them. This is way better than what I currently have.

LS ROB Thanks I will try out this tune and let you know!