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Discussion Starter #1
Hey guys, was exploring my CATA the other day, and found the dinky little fuel cooler located by the fuel filter.

Would it be efficient to add in a front mounted cooler? I have one off a mk5 Jetta that mounts under the car. Could be easily mounted behind the grill or somewhere on the radiator support. (Haven't looked into location yet).

That little cooler can't do much, although it is a water cooled unit. But I see that being a heater at some point.

Just thought about getting rid of it and going with an air only cooler.
 

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Is it actually a cooler? Or is it a heater? The latter makes more sense as the fuel gets cooled when it goes back to tank, but needs to be heated in winter for more efficient combustion.
 

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Discussion Starter #4
But I guess that's what I'm here to figure out. I've been reading into other Tdi models. Cooler fuel is more dense, and gives better performance. As the fuel heats up the ECU pulls timing back and performance. But it would seem better economy would be achieved with a hotter fuel. In the end, water meth lowers cylinder temps, and more performance as well as other good properties of having meth. You'd think cooler fuel would have a similar but not as noticeable effect.
 

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Some the info on the engine: http://vwts.ru/engine/cata/pps_941803_3l_tdi_cata_w_clean_diesel_system_eng.pdf

See page 20. It draws from the expansion tank. It is the fuel return line (from the injectors) that is run through an exchanger, so it does indeed appear to be a cooler and not a heater.

That same document makes only one reference to fuel heating, and it's how the fuel metering valve somehow makes it unnecessary. Pretty vague there.

What are you looking to gain by cutting it out? Rather, I should ask, how does it benefit the Jetta? Are you trying to reduce coolant line paths?
 

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The excess fuel the engine does not use is under high pressure and gets hot so it is returned to the fuel tank via the cooler.

Elsewhere on here, when discussing expensive HPFP failures, it seems that the cooled fuel probably isn't cooled enough by that cooler and relies on further cooling when it is returned to the cooler contents of the fuel tank.

Therefore the recommendation on here is that you do not run the fuel tank under 1/4 full on Tdis to assist in the cooling of the excess fuel and to avoid possible HPFP failure which can cost up to $16,000 to fix.
 

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Discussion Starter #7
So it would sound like a cooler between the filter and pump would be useful if nothing else to keep the pump safe?

But cooling the fuel after the fuel temp sensor could pose mapping issues as the fuel would be colder than the ECU thinks
 

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The excess fuel the engine does not use is under high pressure and gets hot so it is returned to the fuel tank via the cooler.

Elsewhere on here, when discussing expensive HPFP failures, it seems that the cooled fuel probably isn't cooled enough by that cooler and relies on further cooling when it is returned to the cooler contents of the fuel tank.

Therefore the recommendation on here is that you do not run the fuel tank under 1/4 full on Tdis to assist in the cooling of the excess fuel and to avoid possible HPFP failure which can cost up to $16,000 to fix.
I find it hard to believe the return fuel could be that hot after even a modest cooler and the big reduction in pressure when returned to the tank. I would expect all of the temp added by pressurizing the fuel should be lost at the point of depressurization.

Has anyone taken any type of measurements or have a reference to return fuel temperature being high when it re-enters the tank? Perhaps there are other issues with low fuel levels that could damage the pump such as air ingestion.
 

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If this is like the V10, that cooler gets its own coolant loop (on the V10 it's shared with the alternator). I'd say there's no reason to fiddle with it.

The cooler is on the return side of the fuel system. The high pressure system does make the fuel get quite hot. On the 4 cylinder PD TDIs the fuel rail is a passage inside the cylinder head which gets quite hot!

It would be easy enough to see what temp the fuel going back to the tank is... just strap a temp sensor to the return line at the fuel pickup module.

-J
 

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If this is like the V10, that cooler gets its own coolant loop (on the V10 it's shared with the alternator). I'd say there's no reason to fiddle with it.

The cooler is on the return side of the fuel system. The high pressure system does make the fuel get quite hot. On the 4 cylinder PD TDIs the fuel rail is a passage inside the cylinder head which gets quite hot!

It would be easy enough to see what temp the fuel going back to the tank is... just strap a temp sensor to the return line at the fuel pickup module.

-J
Or you could monitor it with a VAG-COM. On my old dodge diesel I could the fuel temp using readings from the ECU. I am sure you could do the same with any TDI engine.
 

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The fuel temp is read before the cooler.
 

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This cooler functions to protect the plastic parts of the fuel tank, etc. The fuel DOES get pretty hot when it is compressed to 20-30 thousand psi before being injected. The fuel NOT injected is returned to the tank. If this hot fuel [line] is attached to or near plastic bits it will soften them significantly. Thus it needs to be cooled before it leaves the engine. After it reaches the tank it is cooled by dilution with the rest of the fuel.
 

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This cooler functions to protect the plastic parts of the fuel tank, etc. The fuel DOES get pretty hot when it is compressed to 20-30 thousand psi before being injected. The fuel NOT injected is returned to the tank. If this hot fuel [line] is attached to or near plastic bits it will soften them significantly. Thus it needs to be cooled before it leaves the engine. After it reaches the tank it is cooled by dilution with the rest of the fuel.
Is the fuel tank pressurized? Basic thermodynamics says that if the fuel or any fluid is compressed it will get hot, if it is cooled and then returned to low pressure, it should be cooler than before pressurization. This is the same principle that AC and refrigeration use.
 

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The fuel tank is not pressurized to any important extent. The thermodynamics you mention is gas thermodynamics - the fuel is, of course, liquid. It takes a while for the heat to dissipate meanwhile the fuel is moving to the tankand that is long enough for any plastic parts nearby to get pretty hot.
 

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It has been a few years since my thermo classes, but I am fairly certain that liquid fuel will follow the same adiobatic heating and cooling as a gas, just with different constants.

I would think that if the fuel needs cooling before returning to the tank it must be getting heated from something other than just the pressurization.
 

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Now I am even more suspicious of the hot fuel in tank theory.

According to this doc, pages 28 and 29 of the PDF (labeled 24 and 25) the return fuel goes back to the fuel filter and not the tank. Only the pump overflow returns to the tank. If this is accurate, the level of fuel in the tank would not impact the temperature of the fuel going to the HP pump, and the temp of the return fuel would not impact the temperature of the fuel in the tank. The return fuel temp would impact the fuel filter temp and the temp of the fuel feeding the HP pump and also the temp of the fuel in the rail.

http://www.natef.org/NATEF/media/NATEFMedia/VW Files/3-0L-V6-TDI-Engine.pdf

It would seem to me that the heat is probably coming from the engine not the pressurization and the cooler is there to regulate the fuel temp hitting the HP pump and the injectors. Perhaps for fuel pump life, but also for optimal power and efficiency. The odd thing is I don't see any reference to a fuel cooler in the document. Hmmm.
 

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I did some more reading on this. It seems the above diagram leaves out a few pieces.

So, current understanding is that the fuel leaves the tank and goes to the filter, and then to the HPFP and on through the the common rail and the excess returns to the filter. The heat from the engine and engine bay will warm the fuel before it returns to the filter despite the temp lost due to the pressure drop.

The filter has a thermostat so when the fuel in the tank is too cool, the fresh fuel is mixed with the warm return fuel to warm up the feed to the HPFP. Excess warm fuel is sent from the filter to the fuel cooler and then back to the tank.

The purpose of the system is to keep the fuel temp in an optimal band for the engine somewhere around 45 c. The engine runs around 90+ c so without the cooler and heat loss in the lines and from the surface area of the fuel tank, that would be the max temp the fuel could get. In practice it would be much lower even on an empty tank, even more so with the fuel cooler, so I don't think the fuel in the tank is going to ever get hot enough to cause any damage to plastics or the HPFP.

I also don't think increasing the size or efficiency of the cooler will do anything for performance. If you cool the fuel more, the thermostat will just mix in more warm fuel at the filter to hit the target temp or the ECU will adjust the time of spray based on the denser fuel.
 

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The fuel filer has a thermostat in it which only recirculates fuel when it's cold out. When the fuel is warm it's sent right back to the tank.

-J
 
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