5 replies to this topic

[NLinPEN]

Since a while now am I trying to figure out why the engine stalls when I engage either forward or reverse gear. I am running out of ideas and am hoping for some help here.

The car is a Japanese SPi model, with automatic gearbox and A/C. I have it since about two years. The troubles started to develop a few months ago and gradually became worse. What happens is that I can start the car in neutral and it will idle fine. However, when I then put the gear lever in either forward (any gear) or reverse the engine stalls. I mentioned that it became worse gradually: initially it only stalled when selecting reverse gear on a cold engine. Forward gear was no problem, and with a warm engine there were no problems at all. Nowadays it stalls on a cold engine in both forward and reverse gear. Meaning that I can't leave the house anymore ...

 

What steps have I taken to analyse the situation?

(*) I have ensured that I have the correct oil for this engine/gearbox/clutch. As this is a wet clutch it requires a mineral oil, 20W-40, complying with the JASO MA2 specification.

(*) I measured the oil pressure in neutral, forward gear and reverse gear. The oil pressure is sufficiently high. I was filming when I measured this. The slow motion made it clear to me that the engine stalls once the gearbox engages. Thus after the clutch&gearbox have selected the appropriate gear wheels. This means that the engine stalls when a load is placed on the crankshaft.

(*) This made me focus on the engine ECU (MEMS). The SPi does not have a separate idle control valve (only the MPi has that). The ECU is supposed to keep the engine running during such a change in load by adjusting the throttle. But it is apparently not able to do so.

(*) I connected an adapter to the diagnostics port and analysed the signals with the memsfcr online software tool.

(*) All engine parameters seem to be fine: the intake air pressure (MAP, i.e. the vacuum) is at a good level, the Lambda voltage is nicely oscillating and there is a closed loop, the idle rpm is stable at 850 when warm, and the ignition advance timing is fine. All is in the green, and nothing weird is being observed.

(*) The battery voltage is fine, the coolant system works nicely (thermostat and fan switch work), and the ambient temperature is about 30 C.

(*) When I push the throttle the car revs up. This is also visible in the so-called IAC Position (steps) parameter. This is the parameter that indicates how the ECU is adjusting the throttle. So in neutral the ECU is able to control the engine properly.
(*) When I change from neutral to any gear (forward or reverse) I see strong swings on the IAC Position (steps). It fluctuates strongly reaching (almost) zero and 100 and fluctuates two or three times to these extremes before the engine stalls. This happens very quickly: well within one second.

 

So it appears to me that the ECU does try to correct for the load being placed on the engine but is unable to act sufficiently to keep it running.

Having done all this, and having arrived at this point, what else is there to investigate? Or otherwise to do to make this again a running and driving car?

[viz139]

The idle is controlled by a stepper motor on the throttle . More info here

https://www.theminif...otor-operation/

[NLinPEN]

The idle is controlled by a stepper motor on the throttle . More info here

https://www.theminif...otor-operation/

Thanks, yes I am aware of that. It appears that the ECU thinks that everything is fine. Maybe my next step should be to remove the stepper motor and inspect it according to the message thread you link to.

[bpirie1000]

Start with simple things..

Check the engine earth strap. Sounds like you have an electrical gremlin.

The earth pointon the left inner wing near the servo may be a good shout. Or the main earth strap usually found under the engine mount. ( or try adding an extra one, or jump lead to the body and block)

Would also look at the main relay.

https://www.theminif...unusual-pinout/

These get any form of water/damp in them electrical wizardry appears and causes many issues..
(Only because it sounds like when movement starts the components get shaken about)

[FlyingScot]

 

The idle is controlled by a stepper motor on the throttle . More info here

https://www.theminif...otor-operation/

Thanks, yes I am aware of that. It appears that the ECU thinks that everything is fine. Maybe my next step should be to remove the stepper motor and inspect it according to the message thread you link to.

 

Can you see the stepper linkages moving ? - appreciate you need to get someone else to operate the controls whilst you look under the bonnet with airbox

removed.

 

 

FS

[NLinPEN]

In case anyone stumbles upon this thread in the future: I was finally able to get the car running.
I wanted to remove the stepper motor to inspect its solder joints. However, to get access to one of the torx screws that hold it in place you also have to remove the upper portion of the throttle body: the part that holds the fuel injector and pressure regulator. This part needs to be removed carefully as there is a gasket underneath which is prone to break. Luckily it didn't get damaged in my case.
Measuring the resistance of each of the four stepper motor windings show that they are all identical, at 15 Ohms. This gave me hope that the motor itself isn't broken. The solder joints didn't look bad or cracked, nevertheless I re-soldered them all. Also the connector did not look bad but I sprayed some contact spray on it for good measure. With the top of the throttle body separated from the bottom I could clearly see the butterfly valve. There was a bit of dirt, but not impressive. I cleaned that as well.
After assembling everything it all worked. Now I can change the gear lever between Park, Reverse, Neutral, and Drive, without the car stalling. Neither at cold start, nor with a hot engine. So I think that either the electrical connections of the stepper motor, or the slightly dirty butterfly valve were the reason for stalling.
 
A very helpful video was this one below. It shows the assembly process and is therefore helpful in disassembling these parts.
https://youtu.be/jGN...8GXXsGIkMktBrMA

Edited by NLinPEN, 02 October 2025 - 06:28 AM.