I did 800 miles at a steady 75-80 mph on Saturday and my trip says 22.3mpg......

But I was running on LPG so it's all pretty much irrelevant.

I would have gone about it a different way. Retain your existing main PCB and satellite board with the message centre on it but swap the actual gauges with their faces. That way, any differences between sensor voltages, will still drive the gauges to the same place. GEMS uses two temperature sensors, one for the ECU and one for the gauge but Thor has a single one containing two separate senders so it is quite possible the temperature/resistance outputs are different.

If it was a fuel pressure problem it would affect all cylinders equally so that would seem to rule that out. I'm assuming there is only one MAF sensor on your engine and not one per bank, so again that rules that out. That leaves the lambda sensors as there will be one, or possibly two, per bank. Can your scanner do live data? In which case log, if it can, or watch the outputs from the lambda sensors. They should flip flop between 0 and 1V about once a second. If either hangs at one end or the other, that is likely the problem. In saying that, the sensor gives 0V when lean and 1V when rich, so with a dead sensor giving a permanent 0V, the ECU would interpret that as lean so richen rather than lean off the mixture on that bank.

Does the diesel have a crank position sensor? On a V8 when they start to fail they do it when hot but will start working again once left to cool down. If it does and it is accessible, when it stops, spray it with cold water to cool it down. If it restarts immediately then that is likely the problem.

Is it the hose or the clip? I've found that the constant tension spring clips are fine if they are fitted exactly perpendicular to the stub. If they are on at a slight angle they will leak. Easily done considering what a pain it is to get to them.

I normally use the Diagnostic function and move them from 0 to 100%. You can hear them operating and see what the feedback pot is reporting. It will often go to 105% (which is a good trick if you can do it) but you'll be able to see if you are getting full travel and the correct feedback.

I remember somebody saying on rr.net that one from a Saab of some description fitted and worked. No idea on the part number or how well it worked though.

Pipes not pushed fully home on the valve block?

But if you did read from the beginning, or even from halfway through, you'd see that if he plugs the SAI pipe with his finger it runs normally...... Definitely a problem with the SAI system rather than anything else.

Do you have any diagnostics James? As you've found, the temperature sensor on the Thor is two sensors in one body (rather than 2 separate sensors on the GEMS). One feeds the dashboard temperature gauge while the other feeds the ECM. If you connect OBD diagnostics that can display live data and look at what engine temperature is shown. If it is obviously wrong, that is the problem.

Unless your lower grade fuel is really poor, a P38 doesn't need top grade stuff. Recommended fuel is 95 RON which equates to 90 in the US as a different method of grading is used (AKI or (R+M)/2).

As it appears to be an SAI problem, there is limited knowledge here to help as only NAS cars had it fitted. Reading about it in RAVE, it appears that it should only operate on a cold start and not a hot start so it could be caused by a temperature sensor problem. I understand on later NAS cars, there is an additional one in the radiator, presumably earlier ones just used the engine sensor.

RAVE says:

The engine control module (ECM) checks the engine coolant temperature when the engine is started, and if it is below 55°C (131°F), the SAI pump is started. Secondary air injection will remain operational for a period controlled by the ECM and is dependent on the starting temperature of the engine. This varies from approximately 95 seconds for a start temperature of 8°C (46°F) to 30 seconds for a start temperature of 55°C (131°F). The SAI pump operation can be cut short due to excessive engine speed or load.

As it's operating a solenoid, I would suspect at a pretty low frequency or it wouldn't get chance to operate before it released. Then again, I would expect the SAI to operate for a set time rather than being pulsed.

I've got a spare UK Thor ECU..... You only need the Nanocom to copy and paste the initialisation code from ECU to BeCM.

@LPGC, no it isn't a PWM signal, it is a permanent live directly from the fuse (via a splice) with the ground,either permanent or pulsed supplied from the ECU. So provided he is testing the voltage with respect to ground and not across the pins, it should show full battery voltage.

If the SAI isn't needed to pass emissions in your particular US State, then it can be removed. All you need to find is some blanking plugs to blank the holes in the cylinder heads. Once they are blanked the rest can be taken out and removed. Not sure if that will throw a fault code or not though. If you disconnect it and check for any SAI related codes you may have to put a resistor in place of the solenoid to fool the ECU. The purge valve is a separate system and even on a UK car if that is disconnected you will get a code thrown.

Ground is supplied by the engine ECU. They both share the same supply from fuse 26 which also feeds the MAF, ECU box fan, O2 sensors and cam position sensor on a Brown/Pink wire. See pages A1, page 4 for a description of how it works and A1 pages 8 and 15 in the ETM for the relevant bits.

Dodgy connection somewhere.....

Can't you just plug the pipe and leave it at that? The rest of the world survives quite well without any of the SAI stuff.

Doesn't does it. I tried to edit your post so the picture appears but don't seem able to do it, so there's a click-able link now instead so others can see it. Moisture gets under the insulation and rots the copper so you end up with a high resistance or no connection at all.

Mine would do that a couple of years ago. I found that if I switched off and restarted, it was fine. One day I had the Nanocom plugged in and checked for any stored codes and found one for the EVAP solenoid. Had a look at it and noticed one of the wires had corroded and pulled out of the plug. Found a suitable plug, soldered it to what was left of the wires into the old one and it hasn't done it since.

From what I've found online, it could be down to a dead SAI pump but I don't really know enough about it as it is an NAS only feature.

Misfires will be as a result of running rough, so the only one that points to the actual fault will be the SAI code. It decodes to P0413 OBD-II Trouble Code: Secondary Air Injection System Switching Valve A Circuit Open so that is where you need to be looking.