R33 GTS25 NA - Stalling when vehicle is at operating temp

[DRoc81]

Hey all,

The skyline I have is stalling when the engine is at normal operating temp.  When starting it while it is cold, it'll run just fine.  I have replaced the battery (bad cell), I have replaced the fuel pump, fuel filter and clean the MAF sensor.  Any other suggestions?

[TurboTapin]

Do the following first and get back to us.

- Clean and verify your IACV

- Verify for vacuum leaks/post MAF leaks. (Pressure test is the best and cheapest way)

- Do you have a BOV with stock ecu/MAF? If so, remove it. 

 

 

[Dose Pipe Sutututu]

Unplug your stock O2, does the problem go away?

[MBS206]

Also check the factory 2 wire coolant sensor is functioning properly and plugged in. Ignore the dash, the dash uses a different temp sensor.

[DRoc81]

22 hours ago, TurboTapin said:

Do the following first and get back to us.

- Clean and verify your IACV

- Verify for vacuum leaks/post MAF leaks. (Pressure test is the best and cheapest way)

- Do you have a BOV with stock ecu/MAF? If so, remove it. 

 

 

IACV, (Intake Air Control Valve)??

Sorry, what is a BOV?  and I believe it is a stock ECU/MAF.  So, by removing it, just unplug it or take it out completely?

[DRoc81]

19 hours ago, Dose Pipe Sutututu said:

Unplug your stock O2, does the problem go away?

I will try this and let you know.

[DRoc81]

18 hours ago, MBS206 said:

Also check the factory 2 wire coolant sensor is functioning properly and plugged in. Ignore the dash, the dash uses a different temp sensor.

I will check this as well, and get back to you.

[GTSBoy]

BOV = Blow Off Valve.

Better referred to as compressor bypass valve on stock RBs.

[MBS206]

5 hours ago, DRoc81 said:

IACV, (Intake Air Control Valve)??

Sorry, what is a BOV?  and I believe it is a stock ECU/MAF.  So, by removing it, just unplug it or take it out completely?

Ignore the BOV / recirc valve question.

You have an NA and shouldn't have one.

If there magically is one though... It should be removed, though it shouldn't really effect it in the way one will for a turbo motor with it venting to atmosphere while running an afm.

[TurboTapin]

2 hours ago, MBS206 said:

Ignore the BOV / recirc valve question.

You have an NA and shouldn't have one.

If there magically is one though... It should be removed, though it shouldn't really effect it in the way one will for a turbo motor with it venting to atmosphere while running an afm.

Touché. I had not noticed it was an NA. 

[DRoc81]

with the car running (runs fine when temp is cold), I unplugged the MAF and the car stalled.  I have already cleaned it a week ago.  I unplugged the O2 sensor from the left side of the vehicle, and the car was still running.  I let it idle for about 5-7 minutes and shut it off, to see if it would restart, and it did not.  It cranks but doesn't start after engine temp gets up to normal range.  

I still need to look at the coolant temp sensor, is this one at the top or bottom of the radiator? Or is it one of the sensor threaded into the T-stat housing?

[GTSBoy]

41 minutes ago, DRoc81 said:

Or is it one of the sensor threaded into the T-stat housing?

Yes.

[DRoc81]

On 4/8/2025 at 12:09 PM, DRoc81 said:

I will try this and let you know.

I unplugged the O2 sensor, the problem did not resolve itself.

[DRoc81]

I removed the IACV, cleaned it and reinstalled.  I found a video on youtube that helped with the whole process, the guy then said the idle needed to be set, so the process was to get the vehicle up to operating temperature, unplug the TPS, set the idle screw in the IACV so the RPMs were around 600, then plug the TPS back in.  After I reinstalled the IACV, the car started (because, it was a cold start), once it warmed up, the car died, I adjusted the idle screw to see if that helped anything, it did not, I had to wait about 3-5 minutes, then the car would start back up. Only to shortly cut out, and only crank.  I loosened the TPS to see if adjusting that would do anything, and when I would rotate the sensor clockwise, there was a humming noise, but it would go away when I got the sensor in the horizontal position.  It only made the hum noise with the key on.  What do you guys think?

[GTSBoy]

It's (the TPS) buzzing? Or something else is buzzing in response?

Remove the IACV from the plenum. Block the hole into the plenum with your hand. Get someone to start the car. Let some air is. Do the revs flare up? This is checking that the path into the plenum is clean.

After that, is the air hose leading to the IACV fuly clear?

After that, either you didn't clean it thoroughly enough to allow the valve to move, or it is broken.

 

[dbm7]

It's actually one of the worst bits of Nissan nomenclature (also compounded by wiring diagrams when the TCU is incorporated in ECU, or, ECU has a passthru to a standalone TCU).... the gripe ~ they call it the TPS, but with an A/T it's actually a combined unit ...TPS (throttle position switch) + TPS (throttle position sensor)....

..by the looks of it (and considering car is A/T) you have this unit... https://www.amayama.com/en/part/nissan/2262002u11

The connector on the flying lead coming out of the unit, is the TPS (throttle position sensor) ...only the TCU reads this.

The connector on the unit body, is the TPS (throttle position switch) ...ECU reads this. It has 3 possible values -- throttle closed (idle control contact), open (both contacts open, ECU controls engine...'run' mode), and WOT (full throttle contact closed, ECU changes mapping).

When the throttle is closed (idle control contact), this activates what the patent describes as the 'anti stall system' ~ this has the ECU keep the engine at idling speed, regardless of additional load/variances (alternator load mostly, along with engine temp), and drives the IACV solenoid with PWM signal to adjust the idle air admittance to do this. This is actually a specific ECCS software mode, that only gets utilized when the idle control contact is closed.

When you rotate the TPS unit as shown, you're opening the idle control contact, which puts ECCS into 'run' mode (no idle control), which obviously is a non-sequitur without the engine started/running ; if the buzzing is coming from the IACV solenoid, then likely ECCS is freaking out, and trying to raise engine rpm 'any way it can'...so it's likely pulling the valve wide open....this is prolly what's going on there.

The signal from the connector on the flying lead coming out of the unit (for the TCU), should be around 0.4volts with the throttle closed (idle position) ~ although this does effect low throttle shift points if set wrong, the primary purpose here is to tell TCU engine is at idle (no throttle demand), and in response lower the A/T line pressure ... this is often described as how much 'creep' you get with shifter in D at idle. The way the TPS unit is setup (physically), ensures the idle control contact closes with a high margin on the TPSensor signal wire, so you can rotate the unit on the adjustment slots, to achieve 0.4v whilst knowing the idle control contact is definitely closed.

The IACV solenoid is powered by battery voltage via a fuse, and ground switched (PWM) by the ECU. When I check them, I typically remove the harness plug, feed the solenoid battery voltage and switch it to ground via a 5watt bulb test probe ; thing should click wide open, and idle rpm should increase...

...that said though, if it starts & idles with the TPS unit disconnected, and it still stalls when it gets up to operating temperature, it won't be the IACV because it's unused, which would infer something else is winking out...

 

[GTSBoy]

Nah, it's different across different engines and as the years went on.

R32 era RB20, and hence also RB26, the TPS SWITCH is the idle command. The variable resistor is only for the TCU, as you say.

On R33 era RB25 and onwards (but probably not RB26, as they still used the same basic ECU from the R32 era), the idle command is a voltage output of close to 0.45V from the variable resistor.

[DRoc81]

14 hours ago, GTSBoy said:

It's (the TPS) buzzing? Or something else is buzzing in response?

Remove the IACV from the plenum. Block the hole into the plenum with your hand. Get someone to start the car. Let some air is. Do the revs flare up? This is checking that the path into the plenum is clean.

After that, is the air hose leading to the IACV fuly clear?

After that, either you didn't clean it thoroughly enough to allow the valve to move, or it is broken.

 

It sounds like something else is buzzing in response to me moving the TPS.  I will do these checks when I can and get back to you.  Thanks for the info.

[GTSBoy]

Put a hand on an injector and see if that's what the buzzing is. You might be triggering the injectors by moving off idle command.

[dbm7]

23 hours ago, GTSBoy said:

Nah, it's different across different engines and as the years went on.

R32 era RB20, and hence also RB26, the TPS SWITCH is the idle command. The variable resistor is only for the TCU, as you say.

On R33 era RB25 and onwards (but probably not RB26, as they still used the same basic ECU from the R32 era), the idle command is a voltage output of close to 0.45V from the variable resistor.

For sure the later RB26 variants had a different setup (6 cam voltage sender?), but the RB25DE/DET share the same ECU pinout/PCB footprint, and at the ECU plug it ends up being 'throttle sensor in' & 'throttle sensor out' (to A/T TCU) ...you stick a multimeter on these pins and you'll find them directly connected (0 ohms) ; I had an pair of Nissan ECUs on the bench long ago, and I noticed this throttle sensor in/out link, actually linked to an op-amp ...which was unpopulated on the NA board, but present on the T board and I went 'ah-ha!' in some moment of clarity...

...it wouldn't surprise me if they changed strategy here though (in the software and actual monitoring of this signal), because they did with other stuff (A/T signals).. and 0.45v as a trigger point makes sense, as that's what the external TCU units expect at idle ; kudos, thanks for that insight...I suppose it boils down to whether or not Consult can display real time data of that signal's voltage...

...in any event, it presents the same target...ie; rotate TPS unit to achieve 0.45v to suit both ECU&TCU, and where do you goto from there? Just disconnect TPSwitch connector and/or that plus TPSensor connector....or do you go for the throat and disconnect IACV solenoid and see if it still stalls? (probably throws a recoverable fault code and goes for default idle strategy?)...