1995 Q45t Intermittent Stall / RPM Plunge when Braking to a Stop (Live Consult Data)

[WhiteCursedQ45t]

Hey everyone,

It's my first time posting in NICOclub, so I apologize in advance if I do something wrong or missed a rule!

I'm looking for some insight from the VH45DE / G50 experts on an intermittent stalling and severe RPM dip issue when coming to a stop on my 1995 Q45t. I have an ECUTalk setup and have been tracking live data, but I'm currently lost and need help diagnosing the root cause.

Regarding the symptoms, when I have the car adjusted to a factory 650 RPM hot idle, coming to a normal stop causes the engine to violently plunge to ~525 RPM, and sometimes it goes below 300 RPM, causing the car to stall out completely. To try and prevent the drop, I tried opening the IACV mechanical screw manually to force a 900–950 RPM idle; with this adjustment, the RPM still dips violently when braking, but it only drop tk around 550–600 RPM, which allows the engine to survive the stop, though after a restart. I also tried performing a 100% emergency panic stop, and the car actually does not stall; the RPM drops to 475 RPM and then immediately recovers.

What has already been checked and ruled out:
• TPS Calibration: Dialed in. Closed Throttle Switch = 1 (ON) at idle, and resting voltage is rock solid at 0.46V–0.48V.
• IACV/AAC Valve: Completely removed and cleaned.
• Vacuum Leaks / Brake Booster: Tested via the official Factory Service Manual (br.pdf, page BR-11 / page 733) on-vehicle service procedures. The system passed both the "Operating Check" and "Airtight Check" perfectly, completely ruling out an internal booster leak or vacuum diaphragm failure.
• Error Codes: Cleared a temporary Camshaft Position Sensor history code (4 starts counter), and it has not returned. No active codes present.

Live Data Observations via ECUTalk:
I caught a data log of the car driving and dipping down to a brutal 362 RPM without completely dying yet. The data shows the AAC % opened to 56%. However, when the car does successfully stall, the timeline looks like this: the AAC stays stuck at its 10% floor while the engine drops from 687 RPM down to 625 RPM. The exact millisecond the engine touches 600 RPM, the AAC finally peaks to 18%, but the RPM goes down to 212 RPM (at 35% AAC) until it eventually dies.

The part that confuses me the most is that why does the RPM drops so low, enough for the car tk stall right before a stop.

Based on these live data parameters, what could be causing this bottleneck and forcing the engine down like this? What components should I look at next to properly diagnose this? Has anyone else had this happen before, and if so, what fixed it?

Thanks in advance for any guidance, really appreciate the help!

[VStar650CL]

Have you cleaned the Throttle Body itself? Something has to be "sticking", and my first reaction to problems like that is to look for problems in the TB. If cleaning it out fixes anything, replace the PCV Valve because that's probably where the carbon came from. You can't necessarily tell much from that livestream because the ECM may be reacting to something else, so you may be looking at a symptom and not a cause.

[WhiteCursedQ45t]

Have you cleaned the Throttle Body itself? Something has to be "sticking", and my first reaction to problems like that is to look for problems in the TB. If cleaning it out fixes anything, replace the PCV Valve because that's probably where the carbon came from. You can't necessarily tell much from that livestream because the ECM may be reacting to something else, so you may be looking at a symptom and not a cause.

I did cleaned the throttle body the same time I cleaned IACV a while back when I replaced the knock sensors , so it's completely clear of carbon, though I did use red gasket maker to seal it back up. I also noticed the coolant lines to the TB are cut off and bypassed , but our weather here stays pretty mild between 15°C and 20°C anyway. I haven't touched or inspected the PCV valves yet, so I'll pull those next to check them.