A4 PD BEW fuel cooling

DanG144

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Aug 2, 2007
Location
Chapin, South Carolina, USA
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2005 A4 Jetta 5spd
A friend's car is running up to 90 C or hotter fuel temperature. The fuel system is intended to maintain 70 degrees C.

The ambient temperature comparison test passed. All three sensors read the same. (fuel, IAT, coolant)

Replacing the fuel thermo Tee did not make any changes.

There is some indication in the Bentley (generic presentation of a PD overview,page 2-8) of a fuel cooling relay (J445) and fuel cooling pump V166. However I cannot find these components, either in the car or in the detailed section of the manual.
Searching the Bentley CD did not bring up any hits, either.

I have come to believe the components are not actually present on a 2004 PD BEW or a 2005 PD BEW, that perhaps they are on other models. UPDATE: I have confirmed they are not on our cars.

The thermo tee is supposed to send all fuel back to the tank once fuel temperature reaches 70 C and it looks as if it all goes to the fuel cooler under the passenger side of the car (without further manipulation by the phantasmagorical J445 or V166).

Can anyone shed any light on this benighted TDI brother?

DanG
 
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DanG144

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Chapin, South Carolina, USA
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2005 A4 Jetta 5spd
I will take a log on my car for comparison - perhaps this is normal summertime operation?
I have only logged fuel temperature on my car once, and it was not summer weather.
DanG
 

pruzink

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Sep 25, 2004
Location
Granbury, Texas
TDI
GLS, 2004, silver
This is a copy & paste of the 67 page PDF file on this engine "Fuel Cooling System
The high pressure generated by the pump/
injectors heats up the unused fuel so much
that it must be cooled before it gets back to
the fuel tank.
A fuel cooler is located on the fuel filter.
It cools the returning fuel and thus prevents
excessively hot fuel from entering the fuel
tank and possibly damaging the Sender for
Fuel Gauge G.
Fuel Cooling Circuit
The heated fuel returning from the pump/
injectors flows through the fuel cooler and
its heat transfers to the coolant in the fuel
cooling circuit that also flows through the
fuel cooler.
The auxiliary water cooler reduces the
temperature of the coolant in the fuel
cooling circuit by dissipating the heat in
Pump for Fuel Cooler V166 is an electric
recirculation pump. It circulates the coolant
in the fuel cooling circuit through the
auxiliary water cooler and the fuel cooler. It
is switched on by the Diesel Direct Fuel
Injection Engine Control Module J248 via
the Relay for Pump, Fuel Cooling J445 at
a fuel temperature of 158°F (70°C).
The fuel cooling circuit is largely separate
from the engine cooling circuit. This is
necessary because the temperature of the
coolant in the engine cooling circuit is too
high to cool down the fuel when the engine
is at operating temperature.
The fuel cooling circuit is connected to the
engine cooling circuit near the expansion
tank. This enables replenishment of the
coolant for fuel cooling at the coolant
expansion tank. It also allows
compensation for changes in volume due
to temperature fluctuation."
Just out of curiosity, how full was the tank? When I have done the diesel purge procedure (I do that every year when I replace my fuel filter) I was very surprised at how fast and how hot the fuel getting returned from the injectors gets. I would think that a tank near empty would reduce the cooling of the oil mixing in the tank quite a bit. I'm not really sure that 194 degrees F (90C) is too much higher (30 degF) than where it starts sending cooling water to the cooler.
 

DanG144

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Chapin, South Carolina, USA
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2005 A4 Jetta 5spd
Pruzink,
Thanks for the reply and reference. I have a copy of this also, and have read it.

Have you found any of these components on the car? It appears my car only has an atmospheric fuel cooling heat exchanger. I have not located any of these other components. And the Bentley has no mention of J445 or V166, no schematics for power, nothing.

Mine looks much closer to the fuel circuit on page 8, with a fuel to air heat exchanger, than the description on page 14 or page 49 of the PDF you reference.

I would be ecstatic to have a truly authoritative source for how this works on the 2004/2005 BEW PDs. I do intend to figure this out.

DanG
 

whitedog

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Jul 12, 2004
Location
Bend, Oregon
TDI
2004 Jetta that I fill by myself
Dan, this stuff is for industrial engines, I believe. Since a stationary engine doesn't have the passing airflow, they need to use a different cooling solution.

Of course this is all just guesses, but none of that auxillary cooling is found on our cars. Just check that the cooler under the car isn't plugged with debris, blocking airflow.
 

pruzink

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Sep 25, 2004
Location
Granbury, Texas
TDI
GLS, 2004, silver
Sorry Dan. You are correct, I just looked at the fuel cooler on my car & there isn't any supply of engine coolant going to it or from it so it is strictly an air cooled fuel heat exchanger. I do know that the temperature of the fuel returned to the tank on the PD engines does run quite hot, but I have never paid any attention to just how hot. I bought one of those Kerma widgets (http://www.kermatdi.com/servlet/-strse-51/KERMA-widget/Detail) because I felt that the amount of time that the 3 way thermostatic tee valve would be in the recirc through the filter position and not just back to the tank would be so small that it wasn't worth the risk of it not working correctly (plus the widget is made out of aluminum and I have read a fair amount of posts about the plastic Tvalves cracking). So far, I have not gotten around to installing the widget. Did you verify that the hoses to the fuel filter are all hooked up correctly with the T-valve going the right way. It is possible that 90C (194F) is normal; I just don't have any data to support that.
 

Zero10

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Jan 16, 2004
Location
Calgary, AB
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05 Golf TDI PD, Tiptronic
If it is above 25-30*C outside and the fuel tank on my golf is empty sometimes I'll see fuel temps in the high 80's to low 90's, but if there is fuel in the tank (over 1/4 tank) or the temp is below 25*C it doesn't normally top 82-85*C on my golf. All of these measurements are around town.

I only measured it once on the highway and it was ~-20*C that day. Fuel temp was 55*C.

You are correct, only fuel cooler is the air-fuel heat exchanger under the passenger-side of the car. It doesn't do a whole lot from what I can tell, maybe on the highway it matters.
 

DanG144

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Location
Chapin, South Carolina, USA
TDI
2005 A4 Jetta 5spd
A graph of fuel temperatures on two different BEWs, a 2004 Jetta and a 2005 Jetta.
[/IMG]

The ten degree C difference is enough for Jim's engine to cut back timing enough that he noticed the performance change, seat of the pants.

And Jim's car has had fuel temps up to 93 degrees C, and that was on short trips. We are not sure how high the fuel temperatures would get on an interstate trip.

Seemed like something worth checking into.

We have already replaced the fuel thermo Tee, and verified that at ambient temperatures all three sensors on his car read the same thing.

Refer to page 8 of http://pics.tdiclub.com/data/517/114..._PumpeDuse.pdf
to see the components mentioned below. Take this drawing with a grain of salt; it oversimplifies some components, leaves some out, and may show some that do not exist. But it gives us something to start checking from.



This was sent to the owner of the other car:
The thermo tee is intended to move hot fuel to the tank through the
cooler when it sees 70 degrees. Before that there is only fuel
recirculating via the tandem pump to the filter and back to the engine. After 70 degrees you have a constant turnover to and from the tank. The fuel is supposed to move fast enough that it does not reach 90 degrees before leaving the head passage. Mine does this.

There are several of possibilities:
1) There is a restriction in the line back to the tank, so the thermo
Tee, even when open, moves very little flow.
2) The fuel pump assembly in your tank may not be dumping the hot fuel
as it should. I have not pulled a PD lift pump, yet, but the assembly
probably has some of the same features as the ALH fuel pickup. UPDATE: I have pulled a PD lift pump. Please see post #14 below for the link to how it works.The return fuel is dumped into a canister in which the pump sits, but it should be flushed out by the eductor pump flow. (Much erroneous conjecture deleted.) (This seems the most likely problem as of 9/22/08)
3) PD pump flow may be minimum, not supplying excess to the tandem pump. (disproved, by good tandem pump flow.)
4) Tandem pump flow may be minimum, not reaching the overpressure
condition required to cause excess flow to be sent to the thermo tee.
Clogged strainer is possible, as well as bad pressure control device,
allowing pump discharge to short cycle to the suction. (disproved, 1.3 liters per minute to the thermo tee from the engine.)
5) The pressure control device, that maintains backpressure on the fuel
passage in the head, may not be working properly. (disproved, 1.3 liters per minute)
6) The temperature sensor may not be accurate at higher temperatures,
even though it is right on at ambient temps. (not likely, in my
opinion.)
7) The new thermo tee was failed in the same manner as the last, again,
not likely.

It would seem the best thing to do would be to measure flow from the
fuel return line to the thermo tee, on your car and mine, if this flow
is comparable, it rules out failures 3, 4, and 5, leaving failures 1 and
2 as the most likely.

We could rule out 1 with a pressure gauge in the return line downstream
of the thermo tee.
We could rule out 2 by checking fuel supply temperatures from the lift
pump.
We need to do a visual inspection of your fuel cooler - make sure it is
there, under the passenger seat- clearly visible from the underside of
the car. Also look for debris damage closing off the line or cooler,
causing a choke point. It would just take one rock. ALH's did not have a
cooler, my 2005 has one. (visual showed all as it should be.)
DanG
 
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DanG144

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Location
Chapin, South Carolina, USA
TDI
2005 A4 Jetta 5spd
BEW return to tank fuel flow rate

We measured the flow rate for the return to the tank/filter line on two BEWs today.

1.3 liters/min or 0.34 gpm at idle.

They were identical, so return flow rate is not the difference in the two cars.
Remember that one has fuel temperatures of up to 93 degrees, the other works at 70 C consistently.

We swapped fuel temperature sensors in the two cars, and there was no change - the sensors did not change the fact that Jim's car runs high fuel temperatures.

So we are still working on a solving the issue.

DanG
 

Ski in NC

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Wilmington, NC USA
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2001 Jetta ALH 5sp stock
No pd expert here, but am around alot of engines with unit injectors. If the fuel has to run in passages in the head before reaching the injectors, the fuel will gather heat from the cylinder head. So if one engines head is at a higher temp, the fuel will get hotter too. Might check that.

I wonder why ecu bothers to measure fuel temp if the fuel gets heated by the head. Can't imagine it would make much difference what temp the fuel is in the tank. What matters is the fuel temp in the pumping element, and thus its density. If it is near head temp in any case, why monitor. Where is sensor picking up this fuel temp reading, discharge from head?

-Eric
 

DanG144

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Chapin, South Carolina, USA
TDI
2005 A4 Jetta 5spd
Yes, the fuel temperature is measured at the exit point from the head.

We verified that both engines are running 186-190 degrees coolant temperature. We verified all 3 temperature sensors read ambient on both cars after parking overnight - so the sensors seem good.

DanG
 

aNUT

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'01 TT (ALH-ish), B7 Audi gasser, '05 Golf
Have you tired by-passing the thermostatic 'T' on the fuel filter return line?

90C is too high, and will result in reduced efficiency as the ECU retards timing to limit NOx on hot fuel. 50-60C fuel is normal on a VE car, maybe a bit higher on a PD b/c the fuel temp sensor is after the lines go through the head.

If the T was stuck open, hot fuel would not go through the fuel cooler under the car and back to the tank. Hot fuel would re-enter the tandem pump, head, and injectors as it is dumped into the filter.

I think that by-passing the T would give useful information for diagnosing this problem. To do so, remove the fuel lines from the T, and use a barbed hose connector to join them. Loop both ends of the T with about 6" of 7mm ID fuel line.

The under-car fuel cooler isn't occluded with mud or other debris is it?
 

DanG144

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Location
Chapin, South Carolina, USA
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2005 A4 Jetta 5spd
Lift pump operation.

aNUT said:
Have you tired by-passing the thermostatic 'T' on the fuel filter return line?

90C is too high, and will result in reduced efficiency as the ECU retards timing to limit NOx on hot fuel. 50-60C fuel is normal on a VE car, maybe a bit higher on a PD b/c the fuel temp sensor is after the lines go through the head.

If the T was stuck open, hot fuel would not go through the fuel cooler under the car and back to the tank. Hot fuel would re-enter the tandem pump, head, and injectors as it is dumped into the filter.

I think that by-passing the T would give useful information for diagnosing this problem. To do so, remove the fuel lines from the T, and use a barbed hose connector to join them. Loop both ends of the T with about 6" of 7mm ID fuel line.

The under-car fuel cooler isn't occluded with mud or other debris is it?
aNUT, the cooler is clean and clear. The thermo tee has been replaced with a new one - no change in fuel temperature.

The fuel filter is barely warm, maybe 100 degrees F, again indicating the thermo tee is working.

We were suspecting something was wrong inside the fuel tank, in the pump assembly. I think I found out what it was today. See this thread.
http://forums.tdiclub.com/showthread.php?p=2286266

I suspect that Jim's lift pump eductor jet is plugged, so that the warm fuel is just recycling.

DanG
 
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DanG144

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Chapin, South Carolina, USA
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2005 A4 Jetta 5spd
Fuel thermo tee failed

The fuel thermo tee removed from Jim's car (the one with the high fuel temperatures) was found to be failed such that it no longer allowed any fuel to go to the filter - all fuel was directed to the tank.

I have seen this type of failure quite often on coolant thermostats. When they get overheated the wax pellet container (or bimetallic element in some temperature control devices) distorts or ruptures, and they no longer work.

We will check out his new thermo tee to ensure that it is working when we check his lift pump assembly out.

DanG
 

weedeater

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Joined
Mar 17, 2001
Location
Reston, VA
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Jetta, 2001, Baltic Green
90 isn't that hot. But as you say, it will affect performance. Having not seen one, does the under-car fuel-cooler have it's own thermostat in it that bypasses the cooler if the fuel gets too cold? This also could be stuck.
 

DanG144

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Location
Chapin, South Carolina, USA
TDI
2005 A4 Jetta 5spd
Cleaned another assembly on 2004 BEW

Dirty fuel screen on the electric fuel pump. This car had been running a bit high on fuel temperature. We thought that perhaps there was a problem in the fuel pickup assembly, causing a reduction of flow.

[/IMG]

Note that this fuel screen is caved in right at the pump suction point, the 2005 done earlier did not display this caved in area. This car's fuel screen was fairly clogged with what appeared to be mainly biologics, though there was a few threads and some flat thin flakes of biologics, too. Again this was thought to be an indication it might have low flow. And in fact this pump showed lower flow than expected when we were pumping out the tank.
[/IMG]

This is the screen after cleaning.

[/IMG]

The screen cleaned up nicely. This seemed to lower the fuel temperatures on the affected car from 93 C down to about 78 degrees C when running at 80 mph.
 

DanG144

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Chapin, South Carolina, USA
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2005 A4 Jetta 5spd
Some new information on flow, pressure

MY lift pump flows .85 gallons per minute, or 3.2 liters per minute. This is flow to the fuel filter; the electric pump actually flows more, but some is used in the eductor.

This flow rate is enough to equal the cam driven pump's potential flow (which is 1.3 lpm per 903 rpm) at 2200 rpm, but not higher. Above 2200 rpm flow through the fuel rail will be essentially constant, as the positive displacement tandem pump will be limited in flow to that supplied - it will have voids inside the pump. (As the tandem pump draws a vacuum on the lift pump assembly's discharge it will rob a slight bit from the eductor, not enough to matter to the fuel rail, but enough to impact the eductor performance.) The fuel rail will be held at about 15 psi by the outlet pressure regulator - so your injectors will never see any voids.

This may help explain why fuel temperatures tend to run over 70 C at speeds of over 65 mph or so. Below 65 my car will hold 70 C unless the fuel tank is near empty.

The shut-off head of my lift pump was about 15 psi - using a 150 psi gauge, so the reading was not too accurate. It did solidly peg my 10 psi gauge.

Dan
 

pruzink

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Location
Granbury, Texas
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GLS, 2004, silver
Thanks for the information Dan; very usefull stuff to know when trying to diagnose fuel delivery problems on the PD engines.
 

mctdi

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Location
se WI
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2010 Jetta
DanG144 said:
The fuel thermo tee removed from Jim's car (the one with the high fuel temperatures) was found to be failed such that it no longer allowed any fuel to go to the filter - all fuel was directed to the tank.
I have seen this type of failure quite often on coolant thermostats. When they get overheated the wax pellet container (or bimetallic element in some temperature control devices) distorts or ruptures, and they no longer work.
DanG
DanG144

I agree with you on the coolant thermostats.

The fuel thermostat has a thermal bi-metal disk to restrict flow between the inlet and outlet & a check valve to let fuel flow from the inlet to filter.

I will try to take some pictures of the parts and post them here.
 

mctdi

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Location
se WI
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2010 Jetta
BEW Fuel Thermostat

Here are some pictures of a fuel thermostat:



The check valve spring can be seen in the center port.



In the center of the lower item, the yellow plastic part that goes into the fuel filter, the ball of the check valve can be seen. This is a view of inside the thermostat. The six blind holes do not go any where.
The upper item is the top of the tee, with the inlet on the left that feeds fuel to the main chamber. And the center porting flows to the right connection, the tee's outlet.
The upper right item is the thermal disk, a bi-metal disk. The side seen is the side that goes against center porting in the top of the tee. At room temperature the disk is cupped to the center porting in the top of the tee. This lets the fuel flow back to the tank. When the disk is cold, as when I put it in ice water the disk went cupped to the other side. This will then seal off the outlet to the tank, so the fuel pushes open the check valve and flows back into the fuel filter inlet side.
 

mctdi

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Location
se WI
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2010 Jetta
DanG144 said:
Thanks for the pictures and the explanation.

It is actually supposed to transfer at 70 C, instead of colder, isn't it?

Dan
Dan

I have seen 70'C post in other posts, too. I am not sure where that came from. But I can live with it.

I just know that this old unit, some 271,000 miles, will change from cupped one side at 65'F to the other side when put into icewater.:) Not sure either if the disk will change as temp. changes, or at some temp just snap over.:confused:
 

joshgb

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Cachagua, California
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04 Jetta wagon black
DanG144 said:
The shut-off head of my lift pump was about 15 psi - using a 150 psi gauge, so the reading was not too accurate. It did solidly peg my 10 psi gauge.
Dan

FWIW, my hard-wired pressure gauge reads about 12psi with a new cat filter measured just downstream of the filter and with only the lift pump activated. I find that it takes closer to 3000 rpms, once the fuel is warm, for the tandem pump to overcome the lift pump and produce zero or nearly zero pressure.

The colder (denser) the fuel, the quicker the pressure drop, and a fresh fill of particularly cold fuel can be enough to starve the engine just a bit if I stomp on the accelerator without letting the fuel circulate a bit first.
 

DanG144

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Chapin, South Carolina, USA
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2005 A4 Jetta 5spd
Excellent information. Thank you very much.

I suspected this would happen, perhaps as low as 2300 rpm, but about 3000 eh? Do you have a compound gauge, one that can read vacuum? I would like to know how much vacuum it was drawing at 4500 rpm.

Dan
 

joshgb

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04 Jetta wagon black
I don't have a vac. gauge on the line, but I can tell you anecdotally that once the fuel is warm the system will maintain 3/4 psi cruising at about 65mph on level ground. Off the top of my head I don't remember what that corresponds to in rpms. Fuel temp./density makes a very big difference in how fast the pressure drops, when it goes negative etc.

It seems to me also, btw, that under normal circumstances the PD fuel system should not get as hot as fast as a VE system. The flow path in the tandem pump is much less constricted and pressurized than a rotary pump (the tandem produces nowhere near the pressure of the rotary, obviously, and lets the unit injectors do the serious pushing.) Just conjecture on my part, but the reason I have stuck with the vac. tee and think the cooler is probably unnecessary, at least in the temperate climates where I live.

One other piece of food for thought. The press./vac you would see at high rpms would of course also depend on engine load. Driving along in cruise control, I can watch the pressure drop when heading up a hill or hitting a head wind, etc. Just a reflection of the injectors sucking more fuel out of the circulating system for the same rpm.

I have some work to do on my fuel system and if the opportunity presents I'll stick a vac gauge in the line and see what happens at 4500 and idle.
 
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weedeater

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Reston, VA
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Jetta, 2001, Baltic Green
the PD fuel runs through the head itself. And the injectors are at higher pressures. These two items will make the fuel hotter.
 
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