Thread: How warm is too warm. How cool is too cool?

Originally Posted by BradC

The fuel cooler cools the fuel from the return line (which is essentially where it is hottest but still at a manageable pressure).

Agreed, but still not sure why the fuel is getting that hot. If the engine starts to derate at 85deg fuel temp and I'm seeing 110-110 under moderate towing conditions then that waves a red flag to me. At 85-90km/hr level road towing in cool ambient temps (ie <20deg) I am seeing stabilised ~90deg fuel temp. That doesn't compute at all as the airflow should be huge and the fuel cooler should be handling this with ease and the engine really isn't working that hard. The moment any load goes on the engine the fuel temp spikes up, and then drops back just as quickly when the load comes off. This you'd expect, but it's the magnitude of the temperature values that concerns me. Even without towing, and driving in cool temps (~15deg) I see more than the magic 85deg just going up a reasonable grade. I saw 90 deg fuel temp yesterday going up Pretty Sally out of Wallan before it dropped back to 50 deg as I coasted down the other side. Apart from under load the fuel temp seems to track along fairly consistently around 20deg under coolant temp, which seems about right given what others have seen. Fuel, coolant and engine oil temps all match at start-up after resting for a while, so it seems the sensors are good.

Looks like the TDV8 uses a standalone cooler radiator, as you said.
Could it be it's blocked with muck?
Give it check and/or hose it out to be sure?

So with this fuel cooler radiator setup, does this mean that the fuel itself is coursing through the system, and not a coolant transfer system... like the TD5(and my dads Peugeot).
Strange setup, having fuel running at high pressures so perilously close to the most common impact point of a car.

Originally Posted by DiscoClax

Agreed, but still not sure why the fuel is getting that hot. If the engine starts to derate at 85deg fuel temp and I'm seeing 110-110 under moderate towing conditions then that waves a red flag to me. At 85-90km/hr level road towing in cool ambient temps (ie <20deg) I am seeing stabilised ~90deg fuel temp. That doesn't compute at all as the airflow should be huge and the fuel cooler should be handling this with ease and the engine really isn't working that hard.

The fuel temperature is measured at the spill-return lines coming from the injectors. The heat is imparted by the HPFP compressing the fuel and any heat soak from the hardware and injectors. So the higher the fuel pressure, the hotter the fuel is going to be, and to a degree it accumulates with the thermal inertia of the whole system.

Under any form of (more than short term point and squirt) load, I often see temps > 85C. I don't see how you can compress fuel from essentially atmospheric pressure to > 500 bar and not get it fairly hot.

The fuel cooler is actually mostly irrelevant to the temps you are seeing (unless it's not working at all in which case the fuel in the tank is going to eventually get quite warm) as the temp is measured *pre* cooler(s). There is no measurement of the fuel being returned to the tank and therefore no way to gauge the effectiveness of the fuel coolers.

Originally Posted by AK83

Looks like the TDV8 uses a standalone cooler radiator, as you said.
So with this fuel cooler radiator setup, does this mean that the fuel itself is coursing through the system, and not a coolant transfer system... like the TD5(and my dads Peugeot).

If you look at the coolant diagrams the fuel cooler aux radiator takes "subcooled" coolant from the bottom of the radiator and further cools it before pushing it into the actual fuel cooler (a plate heat exchanger). The fuel never goes near the front of the car.

Some TDV6 have this extra cooler arrangement also, although I don't believe it was specced for the Australian market.

Looking at the diagrams, the 2007 TDV8 uses the fuel filter assembly with the thermostat in it to circulate warmed fuel in the filter to prevent waxing/gelling. The 2008-> seems to use the same filter assembly as the EU4 TDV6, so the return would go straight to the tank.

The 07/08 crossover year makes it a bit ambiguous to look up in Microcat.

Thanks Brad. That makes a lot of sense. Of course the operating injection pressures for modern diesels are bonkers. Didn't even think of that, I guess I'm still stuck in a MPFI petrol mindset... I can only assume that the ECU calculates a value of likely fuel temp based on the measured outlet temp and engine operating conditions and then decides to derate or not.
Apart from a few bugs the cooler core is clear. And the temp drops back quickly off-load indicating that the bulk fuel temp is OK.
So I'll stop obsessing about fuel temperatures and concentrate on the real issue. Coolant temps... Car has to go back to the Indy soon for a touch up (the new ACE pipes are weeping very slightly at the valve block so they want to have another look and sort it. They'll plug the bypass at the same time (leaving a 1/8" bleeder hole) and we'll see how that goes.

Could the underbody stuff be causing the heat problem...

Originally Posted by Pedro_The_Swift

Could the underbody stuff be causing the heat problem...

I'm not sure what you are referring to? Tyres? Bash plates?

The protection plates,, could they be sealing the engine bay more than originally designed..