Cylinder head/block temp sensor

[Bilkob]

My GTI Turbo passed it's MOT today-(hooray!) and the drive there and back was fairly congested and lumpy and slow. Some may be familiar with my car's Hot idling/cutting out issues, I solved this on this trip by just turning it off if the traffic looked really bad!
Anyhoo, after a lengthy discussion with my ace auto electrician we're now suspecting that it may the fuel is being cut, shut down ,by the ECU due to a temp signal from the sender....
Make any sense???
And if so, where EXACTLY is this sensor located?

[marc61]

I don’t know the answer, but I’ve always thought it was ignition related rather than fuelling. I get a stumbling idle when it’s hot, but one thing that definitely helped recently was cleaning up the male HT connections on the coils and cleaning the flywheel sensor. It’s less variable now but the hot idle is still a long way from rock steady.

[Bilkob]

I unplugged the sensor and it ran like utter sh@#e!!!! It's running new Ford crank sensors, and replacement coils and good leads and plugs......
I WILL get there. I think next thing is to fit fuel pressure guage

[marc61]

Dont know if you’ve looked at this already but I was wondering if it might be the auxiliary air valve?

When the engine is cold it lets a bit more air in and the car idles smoothly at about 1200rpm, no problem. But once it warms up the valve closes and then the engine usually doesn’t idle well - maybe it’s related to the AAV?

To get over that I turned the idling screw to raise the revs a bit, but now when it’s hot I either get an unsteady idle at 900-1000rpm or a steady idle at about 1200rpm (which is way too high). All I want is a steady idle at 800-900rpm but it seems like that’s asking too much!

[Bilkob]

You've got an SM??!!!!!!!!

[Bilkob]

I've had the AAV off the car, checked it's all working ok, and even spaced it off the block a tad with nylon spacers
Still the same

[wurlycorner]

What sender is the auto electrician suggesting is giving a bad signal?

Has he looked at the wiring diagram for the car or is he just guessing, going off experience in more modern cars?

The control of the fuel pump on a CX isn't like a 'modern' car - there isn't a sender in the fuel system that tells the ecu's what the fuel pressure is (in fact the ecu has no idea whether there is fuel pressure or not).

The fuel pressure is mechanically regulated by the regulator on the fuel rail.

The pump runs dependant entirely on the two relays behind the drivers side headlight ('big blue' and the other one) which either get a feed from the 'start circuit' or the injection ecu.

When ignition is first switched on, the pump runs briefly (IIRC - tbh I can't remember off the top of my head if this step ocurrs or not - however the following steps are definitely correct). This would be driven entirely by the timer circuit in 'big blue' first getting ignition live.

Then when the ignition switch is turned to start, that provides a feed into 'big blue' (pin 1/terminal 50 on the relay) to put the fuel pump on. Once the key is turned back to ignition, this feed is lost (albeit the in built delay timer on 'big blue' holds the pump running for a second or so after the starter signal is removed from the key).

When the engine fires and runs (over 250 rpm) a feed from the aei ecu into the injection ecu tells the injection ecu that the engine is running and the injection ecu then directly holds the fuel pump running by a separate feed (by-passing 'big-blue')

When engine speed drops below 250 rpm, the fuel pump is cut because the aei drops it's feed to the injection ecu, which then removes it's feed to the fuel pump.

You can hear whether the fuel pump is running or not, so it should be fairly obvious if that is cutting out first or not.

[marc61]

You've got an SM??!!!!!!!!

Yep and it never ceases to amaze me how a distributor and triple carb setup idles nice and steady hot or cold. Meanwhile the CX’s electronic fuel injection and ignition can’t be relied on to do the same!