Fuel system will not purge air until thermostatic tee opens

You may all know this but it had me sort of stumped for a bit. I had some air in my system and figured it'd eventually purge and as I watched the clear line to the IP it was like carbonated water for a good 20 minutes. I figured it was sucking air from somewhere... not the case, it simply cycles the air in the system until the thermostatic tee opens allowing the fuel (and air in the system) to return to the tank.

So lesson learned today. In the future, I'll find a way around this.

My advice is to seal all the connections around, to, and from the fuel filter. In my experience (since yours may vary), that is where those air leaks are. There's only, what, at least 5 connections? The connections into the filter from the tank, and out of the filter to the injection pump, as well as into and out of the tee, can be tightened down with small stainless worm-gear type clamps that you screw down tight. The tee itself into the fuel filter needs new o-rings, 2 of them, and might need some vaseline (or a similar generic petroleum jelly product) around it to help seal small leaks. If you do this and install the tee carefully with the proper new seals, then the only way for air to still be leaking in is if the tee is cracked and you need a new one.

There is another alternative, which is to install a CAT 1 micron fuel filter and dispense with the thermo-T entirely. You'd still clamp the in and out fuel lines, and then you'd be able to quit worrying about the sometimes problematic tee. Just a thought.

Cheers,

PH

I'm pleased you recognized the issue of re-circulating fuel (and any air) until the valve opens and the return to tank line is open. I've been aware of it for so long that I'd forgotten that not everyone understands the process.
I'd use a hair dryer or hot air gun to speed up the thermal valve opening process after replacing fuel filters on the non-lift pump equipped 1Z, AHU, ALH.

Great idea. I figured on temporarily bypassing the tee somehow via plumbing. Your way is way simpler and way better IMO.

You can use a fuel filter from a 1986 Jetta diesel. It will not have the thermo T. I'm pretty sure it's the same dimensions as the stock filter so it's drop in. Then just splice the return line back together with a simple barb fitting. Fast, cheap AND reliable!

suck from the tank and push through the filter with both fuel lines between the filter and pump pinched off and the outlet line from the filter disconnected... once clear bubble free fuel is coming out the filter it is completely bled ... i use a short piece of hose and a plastic bottle to catch the mess.... by pinching the lines FIRST you can avoid having to bleed the pump and injectors when swapping filters ... i use the same method when replacing PD filters and it saves a ton of cranking.... difference with the PD is i just cycle the key until clear fuel flows, reconnect the line, unpinch and start....virtually no air gets introduced into the pump, injectors, tandem pump, or head using this method