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Troubleshooting SPI TU3M 1.4

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Hey all,


Since three years I have a 205 Roland Garros with TU3m SPI. In the past year the engine began pulsing when idling. Sometimes the amplitude gets so high that the engine stalls. The problem seems to get worse under large electric load, e.g. all the lights and window heating on. So far, I have changed the sparkplugs, air filter, idle motor, coolant temp sensor, lambda. What is left is TPS (which is integral part of the throttle body), air temperature sensor and injector itself.


To understand better what is happening, I've 'hacked' in the ECU by reading all analogue data with arduino (see the data in the image), however I cannot really make sense out of it. So you can see this oscillatory motion of rpms (blue line). When it reaches peak, the ECU reduces the injection time, when rpms start dropping, it increases the time again. Basically the two are perfectly in the opposite phases.


The puzzling part for me is that TPS voltage is constant, which means a constant air intake, which also means that the ECU tries to control RPMs only with more or less fuel. The idle motor, as mentioned before, was replaced, and it is working.


Furthermore, lambda values are also of a question. I understand, that it is usually the case that it is oscillating back and forth, because it is a narrow band sensor here, but I would expect maybe a smaller amplitude? Cause here it is doing the full swing. Also, when lambda is at peak (fuel rich), the ECU increases even more the injection time and then lambda drops, which with a constant throttle angle, should give an opposite effect. 

Is my understanding correct? Also, can anyone point me in the right direction what to check more? And why/how the electric load affects the engine work? Any insights/tips would be appreciated :)





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Max Nilsson

Hey Bronius! Are you still there? I just got myself a 205 1.4 (Tu3m). And I need some tips. Let me know if you see this and we'll take it from there. Thanks! /Max


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