91 Jetta with AAZ - starts/runs cold but after warm - stalls/bubbles in fuel

[SR Heer]

With my 91 Jetta with true AAZ motor/pump/injectors it will start cold and after bit runs fine for a bit but then when it warms up a lot of air bubbles start to pass through the fuel line into and out of the pump - I was thinking of maybe a clogged fuel filter but now I am thinking that it may be the pump that when it warms it starts to loose its ability to draw fuel from the tank but why all of the air bubbles in both in and out of pump lines??? The pump itself is not leaking externally - I am thinking that after warm up something is expanding and loosing prime or suction strength - what could it be - it had been running fine!

[Doug]

The obvious is to first replace the fuel filter. Next bypass the water/sediment trap to the front of the fuel tank.

[Quantum TD]

Remember, you can suck air in due to a leaking return hose. I'd check all the fittings and replace any braided-return lines (thin or thick). I had this problem on my Rabbit truck. I finally replaced the hose from the top of the IP to the metal line back to the tank. No more air-bubbles.

[8v-of-fury]

If you can see air coming in and air going out its not the pump leaking. Even the smallest air leak will starve the pump of fuel, which will in turn stall the engine.

You need to check all the lines for cracks, check all connections, replace any pieces that are even close to questionable.

No air in the fuel line is acceptable.

[grimfrandango]

I myself just adopted a 91 Jetta turbo diesel.  It wouldnt start...hooked up battery to a running car so I could pump pedal & rotate engine constantly allowing the starter to cool & rest.  Finally fired up, but rough, with air bubbles visibly move through the feed & return lines for the injection pump...Runs...Dies. Over & Over.  Popped the trunk & guess what?  Fracking Moron that owned the car before me thought the fuel gauge sending unit it the trunk was a fuel pump & thought was NFG!  So he bypassed it in the trunk with an aftermarket fuel pump..IDIOT!

Diesels DO NOT have fuel pumps. They have injection pumps under the hood timed with the motor.  So I ripped out that external, aftermarket "fuel pump" 7 it started quicker & ran better only until air would find its way back in. 

I luv dubs, & over the years I heard legendary tales of a dub bible published by Bentley Publishers.  It knows all!

After many nights of decifering the great knowledge this book has to offer, me & my Dubber Bible figured the only way there would be air in the fuel system, was through cracked & deteriated fuel supply & return lines.

Diesel is VERY corrosive to any plastic or rubber.  Throw onto the mix A car that has been on the road for 20 plus yeard & you have very corroded connections & cracked fuel lines.  Plenty of tiny cracks add up to whole lot of air leaks.  Take into factor that diesel requires a very high pressure to ignite & you have a motor that sounds like you have your tool box in the spin cycle till she dies.

I went & bought 8 feet of 5/16 & 8 feet of 1/4 Diesel fuel line & 2 boxes of hose clamps.  I replaced every soft line from the tank to the filter, using double clamps where I could.

Sat in the driver seat...primed the glow plugs...turned the key...Brrrrrrrrbrrrrrrrrbrrrrrrrrrbrrrrrrrrrr...vvvvvvvvvv...brrrrrrrrrrrr

;p

& they lived happily ever after...

p.s. I just want to get the word out on older dubs.  The ground cables from the battery to the block are factory, braided cables which over time lose their potential to ground, especially if you strain your dub with new killer speakers.  I went through a battery, alternator, engine module(& BukaBucks) before I figured that the source of all these failures was a simple ground issue.  I bought a 5 dollar ground cable & she thanked me for it.

Peace Dubbers

Long Live Bubba Samson

[macka]

The obvious is to first replace the fuel filter. Next bypass the water/sediment trap to the front of the fuel tank.

   I had to do this because those seperators are notorious for leaking. I also replaced a crap load of lines all the way up to the pump.