MP4.2+6.0

Just to confirm from your experience, the mechanical/bimetalic component of the thermostat can fail separate from the electrical? And can/does fail in open mode and with no codes?

Great practical technique/disassembly info in your follow up BTW. Have you gotten to the bottom of your recent issue w/ running temp and the fuel trim overlap?

volvopentaman

There's no bi-metal component, just a normal wax-thermostat (with a 4mm hole in the middle for the electricak heating probe).

If the heating probe fails, it will throw a fault code (P1292, MAP controlled coolant thermostat, open circuit). I've never seen this code, apart from disconnecting the thermostat probe connector under the rear of the manifold. The connector CAN be connected and disconnected with just removing the plastic cover.

I haven't still got to the bottom of the issue. The car is consuming normal amounts of fuel, I'm getting normal mileage if i check the trip-meter, but not quite as good mileage as with the 2006 W12.

The fuel trim is where it should be, a few percents on minus at the moment, and idle fuel trim has set to near zero too. Just suspect there was some kind of a glitch.

The only "problem" is that the engine temp is just at around 90ēc while driving, and i get it up to 104-109ēc By disconnecting the thermostat heating probe.

So something is making the ECM (J623) to open the thermostat. The engine heats up at exactly normal speed compared to the 2006 W12 (8,5km to 83ēc = 90ēc on the gauge)

I would want to know, what makes the ECM regulate the temperature that low, because as said, the engine temp is regulated to correspond with outside temperature and engine load. So the ECM has to have an internal fault in the transistor regulating the thermostat heating probe, OR then it reads either outside temp or load wrong. The ECM receives the load and outside temp fairly right, so ruled down to a faulty transistor in the ECM. Tough, i doubt I will replace an otherwise working ECM because of that small fault, especially when I'm not 100% sure about it.

MP4.2+6.0

On yours, yes what you say pointing toward ECM makes sense. They are twinned as I'm sure you know, and ECM 1 (the inner one toward the HVAC filter) owns the MAP calculation for the thermostat per the W12 SSP. I see a fair number of W12 ECM's floating around German eBay depending on the motor code you have if you ever want to go there.

Have you also looked at ALL the sensors that influence the load and temp calculation and kick the thermostat to 100%? You looked at water temp several times--sensor against IR actual readings--but what about other sensors? I think you looked into knock sensors too? What about the oil temp sensor? That goes into the thermostat opening related calcs. per the SSP. Thus read it in VCDS against an IR reading at oil sensor body or filter housing? Also maybe the ambient temp. coming off the MAF's shown in VCDS, even if the fuel calc's seem correct per our prior post exchanges? Anyway, consider checking things that may not be recognized as faulty per VCDS for something electronic but are just sending the ECM erroneous data enough to affect the thermostat load calc.

volvopentaman

Shall check the oil sensor, and hope the controller has same measuring value block locations as S8 V10 BSM engine.

I cannot find a MVB chart for the W12 engines anywhere, all other engines (V6, V8 gas and diesel and V10) found here:
Audi Portal: ECU Diagnostic . Audi A8 4E (2002 -) . Engine Control Unit J623

Measuring Value Blocks 130-136 are for the MAP cooling system in the V10 engine.

MVB 134 is for temperatures, 1. OilTemp, 2. AmbientTemp, 3.IntakeAirTemp, 4.CoolantTemp

BTW. MVB 132, there is a "hot countries setting coding", this could be my problem, have to check it, as i have no extra radiator in this, as i had in the prior W12.

Had problems interpreting the cylinder-specific knock sensors (integrated in the coils), when comparing to the V10 engine already. Most MVB:s are the same, but some are way off.

carl viking

Thank you very much for excellent info! If I can avoid removing the fuel rail that would be a good thing.

volvopentaman

Checked the temperatures today, everything normal.

Channel 134 displays oil, ambient, engine and intake temperatures.

Replaced the tranny thermostat with a new one, no luck, temp going down again on the highway.

I think i found the reason my car runs just 90ēc, measuring value block channel 132 shows various data, engine fans on or off in binary format. I had a binary 00001101, this means :
No fault in system
Thermostat control not active (=the temperature control probe in the thermostat)
Fan actuator active
Temperature deviation (= the engine set temp is something else than it should be)
Fan speed 2 active
Fan speed 1 active
Vacant
Hot countries function encoded

And the last one is a ONE, this means my car is coded for "hot countries". I suppose this is why it runs 90ēc when the temp control probe is connected, and not 105ēc as it should.

I found another channel 130 (i think) with at least engine temp, expected engine temp (=90ēc), engine load and a "vacant". I think this tells me enough, that the "fault" is the hot country coding.

Now I'd just need to find a "map" for measuring value blocks for the W12 engine, and where to code off the "hot countries", or better, which channel to adapt in the ECU.

carl viking

Vovlopentaman, maybe this is useful to you?

I also do not see temps higher than about 90 when driving (in motion), while it reaches 102 or so standing still (in the winter at least).



Looked at my 132 block, and I think I'm NOT coded as hot climate car and I see the same temps you do?

volvopentaman

After blocking the ATF thermostat with two O-rings the temp was roughly around 90ēc on the highway and city.

After replacing the ATF cooler thermostat core (not the plastic casing itself, just the contents) the temp sailed on the highway again between 75-80ēc. This was because i had missed to put the thermosat core in place, so it was still leaking.

Today i took a fast trip, 100-150km/h for about an hour at the motorway, and i just thought about connecting my handheld universal OBD reader. The engine temp was at 107ēc when the speed was at 150km/h constant (had been for 15min). Took her down to 120, 100, 80, 50, idle, and still 106-108ēc.

Haven't done anything else than rectified the thermostat core in the housing. Circulation pump and main thermostat probe are connected as far as i remember (at least the circulation pump, and i have a memory of connecting the probe too, at least it showed no OBD faultcode on the universal reader, but I shall check that tomorrow).

I noticed considerably lower fuel consumption, like 5-10% on the highway, even if i had higher speed, and wasn't driving too economically (can you with a W12 )

Anyway, the idle mixture is again messed up, so next problem

dvs_dave

So in short, the problem was initially caused by a failed ATF cooler thermostat? And the problem remained as you had incorrectly installed the replacement part, but now after a revisit, it's back to normal?

carl viking

Interesting - you say my data log in my post above. That would lead me to believe my ATF thermostat is likely broken as well? I never saw temp above 90ish (degree C) while driving. The temp gauge is of course smack in the middle...