Carpokes Porsche Forum

All -

Can someone with a little more tuning experience than me help me with a couple of issues related to my latest tuning log?
About 300 seconds into this, the car stalls while driving. I am not sure what is causing it as things just kind of go crazy at that point in the log.

The car has been doing this lately about once or twice per drive. As I am coasting, I can restart the car and everything will be fine.

Im not sure if it is a voltage issue causing MS to crash, a fuel problem, or what. This entire drive was on the way home from work today and there is another stall a bit later with the same result. So just trying to piece this story together.

Thanks for looking and sharing the expertise!
This is my 1986 951 unmodified exceot for the MS setup and required sensors, etc for that.

-scott

I'll add this to the newsletter to see if we can coax some MS users out of the woodworks....

I think I have MS on my work laptop from a past customer project, I will look tomorrow if I have time.

Ping me here if you have not heard from me after a couple of days.

Hey Tom and Chris thank you!

A response to my cross post indicates a reset each time of the MS controller. I’ve got to do some investigating. I have lately been messing with accel enrichment, but have always seen a voltage drop at the gauge when the cooling fans kick on. I don’t think the reset is happening at the exact same time but will have to check that.

I’ll keep this thread updated but please let me know if you see anything odd. I don’t think I have AE set quite right yet so would appreciate any comments there as well.

Thanks again!

Seems to be quite a few occassions where you MAP sensor is dropping to basically 0 which is dropping you off the bottom of your fuel map. (Injectors duty cycle goes through the floor). No way that should be happening at 2000rpm. Why is your Idle valve cycling like that?

Hi Scott --

Amateur outsider non-944 owner here.

On similar installations, your symptom has shown up when the controller ground is to chassis rather than the block. Ideally, that single clean shiny ground point on the engine would include the controller itself, injectors (via that controller ground), and any engine-mounted sensors that don't ground directly to the block.

You can inspect the battery ground to chassis and the engine to chassis cable, but my advice after some extended testing and troubleshooting that is to just replace them with new.

Consider adding dedicated feeders for the cooling fan(s) directly from battery + to the relay and controllers. This will let the battery 'soak up' the starting current inrush without having to wait for the alternator to react with more current


I endlessly preach about maintaining excellent primary cable and ground connections at the battery. Clean, shiny, new looking. There's an Annual Electrical Maintenance protocol that includes inspection and cleaning of all ground points in the car, the primary cables and connections, and an easy test of battery condition via a specific-gravity test of the the electrolyte fluid (read: acid). A little cheap parts-store floating-balls tester is perfect for this.

Hope this offers you some help!


Bob

While Bobs advise is good there is nothing in that log that indicates a voltage issuse at the ECU, voltage sags a little but not below 12V, the only time it really dips is when you restart the car. This is a red herring and I wouldnt spend time on it, the continous log shows no evidence of the ECU reseting or restarting.

I would follow up on the MAP sensor failing, the lowest it should read is 10-20kpa on overrun unless your tuning Elons Tesla he launched into space and you have multiple occurances of it being close to 0. Does your MS have an internal MAP sensor or are you using a MS3 with an external sensor?

Can you see if you can add the 5V output voltage to the log at all? (I cant remember if you can do this on MS)

Also it is required by pretty much every aftermarket ECU manufacturer to run sensor grounds to the sensor ground pin on the ECU not directly to the block, like Bob says the main ECU ground should be to the block.

In my experience, with a bad ground or voltage issue there are a LOT of nasty spikes and dips in the voltage (sometimes other) signals and a lot of resets. I only see 2 real resets in yours. Both after the engine shut off.
The MAP signal definitely looks interesting. 32% TPS and 1.6kpa is suspect just before the first reset. Then right back to 98.3 after the engine shuts off, as it should ... interesting. Could be a problem with the MAP signal causing the resets.
Injector PW also looks very low and AFR high. Not quite into over run fuel cut though. I assume that was downhill just before shutting off? What's your req fuel setting at?
I'll keep poking around to see if anything jumps out.

Edit: The more I look at this, the more I suspect the MAP has a problem. What is the manual mark 002 @658 seconds? Did you notice something there and intentially mark it?
Like said, your idle valve is kicking open when it shouldn't be too. Probably just a setting though.
Around 206s, 269s, 492s +/- 10s your TPS is near zero, but the RPM is oscillating. That shows in a few other spots too. Any insight as to what was happening there? All looks like times when the idle valve is open. I'd turn off closed loop idle for now.
Care to share the tune/project folder?

P.S. Getting the AE correct is very difficult. We'll call that a later problem

Bergerac wrote: Mon Jul 27, 2026 5:51 am Seems to be quite a few occassions where you MAP sensor is dropping to basically 0 which is dropping you off the bottom of your fuel map. (Injectors duty cycle goes through the floor). No way that should be happening at 2000rpm. Why is your Idle valve cycling like that?

Screenshot 2026-07-27 204441.png

@Bergerac might have the winner winner chicken dinner. There is no reason the ICV should be active during this portion of the drive.

Check your TPS physical adjustment and settings, and try to do a re-learn of the TPS via the MS software---ensure that 0 and 100% throttle are what they should be key on-engine off.

EDIT: as he says, check the basics as in sensor and power grounds all being properly located and secured.

Thanks all as always for the responses! I have found another issue I have been working through as well that may have an effect....

My knock trigger has been activated on engine start for some time now and staying high, meaning I have been continuously losing 10 degrees of timing from the settings every time I started the car. I think I have that fixed now, and everything runs much better, cooler, more stable AFR readings, etc. Not sure if that will directly address the stalling. Here are some follow-up answers....

1. All electrics are new and were installed with the engine rebuild. All cables are new (ground, power, etc.). Fan controller and fans wired direct to battery via a fuse and enable relay.

2. I am open to all suggestions re: ICV. I had some struggles getting that to work (via Tom's added circuit for 3 wire ICVs), and it is working but I may need to adjust something in settings to keep it from spiking. I will post a screenshot of what I have now for settings when I get home.

3. All of the señsor and power grounds should be good. The MAP sensor is new and is plumbed into the port on the manifold that fed the original Bosch Knock computer. It is a 3Bar GM sensor. But - I am intrigued with the thoughts on the incorrect readings, although I think this may be most related to the timing issue with the knock sensor, as I was running consistent 8-10 vacuum. Now that it is fixed I am back to 20 with a warm engine and the correct knock settings.

Here's the current tune - thanks all!