guys iv got 0 pressure in coolant tank new thermostat new water pump and replaced all the seals in the Vally pretty stumped no leaks to be seen anywhere also changed cap to known good one and recently had new tank also my coolant fans never seem to kick in please help

 

btw its 2007 3.0tdi engine code bug

 

When you say zero pressure in reservoir tank, under what circumstance are you referring to specifically? Have you pressure tested the reservoir to verify if it holds pressure or not? I'm just not understanding your point-of-view or concern about the pressure inside the cooling system based on lack of details. Is it losing coolant?

There is zero pressure present in the cooling system on a cold engine, and as the engine warms up/coolant heats up, the sealed cooling system builds up pressure internally. The reservoir cap has a calibrated pressure release valve built in to allow any system overpressure to escape. The whole point of a pressurized/sealed cooling system is that it prevents the coolant from boiling when the engine is really hot or under adverse operating conditions, and also makes engine management more predictable, etc.

The cooling fans never coming on is definitely a problem though. If you turn on the A/C and crank it down low, both fans should run even when the motor/coolant is cold, because they pull air past the A/C heat exchanger in front of the radiator, etc. I'd do quick verify with the A/C on test, and if those fans don't run immediately, then you need to scan the car for fault codes using VAGCOM type scan tool. If you don't see any fault codes, then next you'd need to view live data on coolant temp sensors and try to test run the cooling fans from the scan tool functions, which requires a decent scan tool with diagnostics functions included...not just a code reader.

Cooling fans and/or their variable resistor or controllers can die, but normally you don't lose both of the radiator fans at once, so just keep that in mind. They are thermostatically (feedback loop) controlled based on the coolant temp sensor at front of engine bay on the radiator outflow hose. You'll just need to check a few things to figure out what's gone wrong there, and don't forget about rodent damage to wiring as a possibility too. That creeps up a lot on these older Q7s when you've got odd symptoms.

 

So when the engine is hot I can open the cap no hissing at all I don’t have access to pressure tester unfortunately but I guess I should buy one I have brilliant coolant curculation in car gets warm and stays warm I have a autel scanner I have a code For coolant temp sensor high no open circuit just high witch is the same sensor you was talking about in your reply no codes there for the fans witch worries me because in live data says that there both at 10.3% at operating temp

 

o I managed to do a active test for the fans they come on but only for one test so I’m not sure if that’s a issue but with the test iv seen them spin up if I come out of active test and back in select fan and test it will spin up again but just leaves my lack of any coolant pressure and my coolant sensor 2 high circuit

 

On positive side, you were able to test run both cooling fans and have visually observed them in action or felt airflow from each fan as it ran...that's what I'm understanding at least.

IMHO, the temp sensor at base of radiator that's looped into the fan feedback popping fault code is a big clue waving at you. Either the sensor(s) itself is bad ($8/each part when I ordered mine), which you can easily verify via scan tool live data monitoring, Engine module, live data, ... (check both coolant temp sensors, BTW). The second coolant temp sensor is less easy to access at back of engine same side, but lower down and on a coolant pipe running horizontally across back of engine. That sensor is the one that sends to the ECU, which in turn, ECU uses to feed signal to the temp gauge inside the car.

Verify those coolant temp sensors are both reading correctly when cold & hot. Those are both common failure points as the car ages.

Assuming your 'known good' reservoir cap is actually good (a $7 part when I ordered/replaced mine recently)...old pressure caps have old internal valves and old, compressed/hardened sealing O-ring on them, so the cap and/or O-ring on it would be my suspect as to lack of pressure building in reservoir, given all the much more expensive parts you've replaced already, to include the tank. Occam's Razor Theorem...the answer lies in what remains.

If not losing coolant, there's really no need to pressure test the cooling circuit, but just FYI, many auto parts stores will have tools on hand available to loan as customer support and incentive to shop there. Any tool I don't have for a DIY job I will normally find is available as loaner from my local parts store. Granted you are in the UK, so your mileage may vary in that regard, but never hurts to ask. Parts stores here have big window placards saying they loan tools out and big laminated sheets at the counter listing all the tools available to loan out.

Hopefully your problem is as simple as old, bad temp sensors and an old reservoir cap.

Be sure to let us know & good luck!

 

So I got a new sensor and code has cleared, do you know the operation temps for the cooling fans as maybe being in the uk it’s not hot enough for them too kick in, ima bite the bullet and put new cap on for the sake of what we’re trying to figure out what’s going on, did notice a slight coolant smell in the car on my first start up this morning so this has puzzled Me so slightly is it possible to loose coolant pressure though the egr cooler I have it blanked off at exhaust manifold and intake manifold

 

Coolant smell.... maybe just check other known areas to verify all is good, such as the engine valley, the front of the engine around the water pump, at the thermostat housing, etc.

 

LOL, the plot thickens! So you blanked off the EGR inlet/outlets...did you get a tune to code out the actual EGR functionality? What did you do about the vent hose from the oil separator?

Regardless on that, yeah the EGR Cooler can crack internally and consume coolant, which now has nowhere to go except back into the coolant system or find a way out. My prior question was focused on the moving armature on coolant shut-off valve for EGR is an external source of coolant leaking on the early TDI engine design, and that definitely will drip down into the engine valley, but it's normally a pretty slow, steady leak once it starts.

Still on about the cooling fans are we?
In their 'normal' operation you won't even hear/notice them inside the cabin over the TDI engine noise with the windows closed/radio on, but you can easily detect them upon exiting the car and there will be obvious airflow movement passing through the grill, which you can feel. If they weren't working, then your car would be overheating in any number of driving scenarios, regardless of ambient temperature being cold or hot, which does not appear to be an issue at all based on your statements. Do be mindful that getting a strong 'whiff' of coolant smell via the air vents inside car regulary can also relate to a failed heater core, but again, that would result in a very strong odor of coolant and it would be a repetitive thing if the heater core is leaking.

On mine, I would sometimes catch a little whiff from coolant sitting on top of the engine valley, through the air vents, but it was random.

 

So yeah it’s tuned, I have no idea what oil separator Pipe you talking About buddy I guess I need to get over the fans I guess your right and they probably do work when there ment to. Sunday ima strip the intake and few other bits and delve into the valley but as I said I did do all the seals so maybe somethings a miss
il keep you posted