Peugeot Diesel Fuel Tank Pressure Build Up

I can't look but I guess the fuel filler cap includes an air vent.

Any fuel tank must be vented to allow air in as fuel is consumed, and to allow air out as the fuel expands with heat. In a diesel engine the system pumps very much more fuel than is burnt, in part to lubricate and cool the injector system, and this excess fuel is returned to the tank. So the air pressure in a diesel tank can increase as well as the tank warms. There are other possibilities for an increase in pressure in the tank such as that minute amounts of air are being sucked into the suction line between the tank and pump, and being returned to the tank.

However, any such changes in pressure (+ or -) should be accommodated by the tank vent. I suspect that this tank vent is in the filler cap. If the engine runs fine without the filler gap gasket in place then that suggests that tank venting is the issue, and that the tank vents through the filler line and the tank cap.

I would try cleaning the filler cap, or try a new cap.

v8oholic wrote:

kevjan wrote:Hi, every time I fill up with diesel i get a woosh of air going into the tank it has happened ever since i had the van thought this was normal for it to build up pressure. Kevin

If that's all we're talking about, "they all do that sir". The van does it, cars have done it, my current petrol Lexus does exactly the same. If there are no other symptoms or problems, then it seems normal to me with modern fuel systems.

I'm not sure I would rest easy on ignoring the venting issue if that is what you are inferring.

For example, I was out in my little outboard motor sports boat the other day and I forgot to open the air vent screw on the plastic fuel tank's cap (portable tank, so you close the vent screw when taking the can to fill it up). Having run a tankful through (11 litres) with a bit of energetic wakeboarding (not me : I was steering the boat!) the tank had imploded. Modern high pressure fuel injected engines have quite powerful pumps. How strong are Peugeot tanks?

In the case of the OP there may still be a separate issue somewhere which caused his injector problems, but the fact that the glove retained pressure when the engine was stopped suggests that his tank vent is in the filler cap, and if the tank is either building up pressure or vacuuming in normal driving (for whatever other reason) it suggests that the vent is not working.

However, the OP has said : The same garage fixed diesel leaking from unused fuel return pipework and also from one of the high pressure injectors (perhaps caused by the pressure build up) so no leaks.

It is conceivable that if the fuel pump was pushing returned fuel against a build up of pressure in a sealed, unvented tank then that could account for the leak in the excess fuel return line and perhaps the issue with the injector.

v8oholic wrote:There is no vent in the filler cap. The fuel cap will not vent to the outside, by design. It's been that way for decades now. Some vehicles even throw a check engine light if the cap isn't fitted tightly enough to seal, as they expect a certain amount of pressure in the tank, or they assume there's a leak. There will be a breather elsewhere to allow air in, and there might be a vent somewhere. But it's not in the cap.

I am happy to stand corrected. Partially!

Just got home and checked my van. The filler cap has an air admittance valve built in, which will allow air in to compensate for fuel used. It is one-way, so the cap will not allow excess pressure out. That is one theory busted thank you! I will let somebody else try and guess why the OP has positive pressure in the tank, unless indeed a degree of pressure in the tank is as you say a design requirement. I have to go and wash my mouth out, literally!

In boats we have to have vents that work both ways. Free air flow. That is a regulatory requirement. A diesel tank under pressure is considered be a fire risk if diesel vapours are pressurised since they could become atomised upon release of that pressure, e.g. when releasing the filler.

Yes, I know what causes an implosion!

But EVAP is a completely new one on me as a mere marine engineer! Another complex system to go wrong! Thankfully not applicable to boats yet!

Here is something to read for others who may not have heard of this:

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