1.6 GTi engine cuts out when ECU coolant temp sensor plugged in

[Archie's mid life crisis 0]

Hi, I'm hoping some of the knowledgeable folks on this forum can help diagnosing a curious issue I am having with a 205 1.6 GTi that I have been rebuilding for the last few years. The car is completely standard. The engine has been completely rebuilt and starts and runs great ONLY when the ECU coolant temp sensor is unplugged. What appears to be happening is the injectors stop firing when the sensor is plugged in. I have checked the injector wiring with a test light and get a good strong pulse with the ECU temp sensor unplugged; with the sensor wiring connected there is a very weak glow from the test light. I have replace the ECU temp sensor with a new Bosch one but with no improvement. Could this be caused by a short circuit in the loom somewhere that gets grounded when the wires are connected to the sensor?

Thanks for any help or advice in advance.

[Tom Fenton 1,525]

Are you plugging the correct plug to the temp sensor? There is another one in the same area for the SAD (idle valve). I would bell out wiring back to the ECU from what you think is the coolant temp plug and check which pins it is going to.

[Archie's mid life crisis 0]

Hi Tom,

I did double check that I hadn't got the SAD and ECU temp connectors mixed up. The coolant temp wire is connected to the correct pin out on the ECU, so I have discounted that. With all connectors on the injectors, mad, throttle switch, sad and ECU temp disconnected I am getting continuity between the supply to the in injectors and the return wire from the ECU temp sensor to the ECU plug, so it looks like I have a bit of damaged wiring somewhere.

[Archie's mid life crisis 0]

Finally got to the bottom of this. I checked the loom over and couldn't find any faults. I had a spare fuel pump so I thought I would give it a go. When I pulled the old one out I noticed that the rubber connector from the pump outlet to the plastic casing was distorted, the hose clamp only acting on a small portion of the rubber, and was half off at one side of the outlet. I dismantled the pump and managed to seat the connector properly and put the clamp back on. Car started up first few turns. I originally bought the car partially dismantled so had never heard it running. Strange thing is that it ran without ECU temp and SAD sensors connected. Perhaps the ECU increases the injector rate to compensate in this scenario and this was just enough to make it run when the fuel pump was putting out low pressure due to the bad connector pipe. 

[Tom Fenton 1,525]

They do run very rich with a bad coolant temp sensor so your theory probably does fit.

[welshpug 1,622]

S.A.D is purely that, just a slow acting supplementary air valve that gets a switched 12v, there is no sensor.