Suddenly won't start after recent tandem pump change

PlasticMac said:
It becomes less dense as it warms up. For the same energy a larger volume is required for warm fuel as for cold.
Not much though, diesel coefficient of thermal expansion, typically around 0.0008 per degree C.
You get more energy for your money if the fuel you buy is cool, as it's sold by volume.
Avoid above ground storage in summer, assuming we get one ...
Most storage is below ground, so not a problem.
I think mpg will be marginally better in winter, in summer, you'd need a gnat's more right foot for the same power.
Mac.
Click to expand...
While I agree with your physics, I think there are other factors that make a much bigger difference to power and economy. For example, I think the improved spray pattern with hotter less viscous fuel provides a significantly greater difference to performance than fuel density.
Also I have noticed significantly poorer fuel economy in winter than in summer. I think this is due to the higher injection quantities required in a colder engine coupled with the longer warm up times due to cooler ambient temperatures and increased use of cabin heating (ie engine cooling). Thus these effect significantly outweigh the benefit of increased density of the cooler fuel.
 
Every day is a school day eh, I've certainly learnt plenty today. I had no idea there was so much going on in these modernish diesel fuel systems..
 
I suspect - not found any technical proof - that the return diesel is connected to the filter housing but may not fully be entering the housing. Then when the thermal bypass on the filter housing opens all the return fuel passes through the fuel cooler on the way back to the tank.
 
The fuel goes around a loop thus: Filter, Tandem Pump, Injector rail, Fuel Temp Sensor.
When fuel is used by the injectors the filter is topped up from the tank and the engine runs just fine.
Problem is that the fuel in the loop ends up getting too hot, so the thermal bypass valve opens @85F to return fuel to the tank via the fuel cooler under floor.
This basically maintains an optimal fuel temp and prevent waxing in the filter when fuel is cold.
 
I had the same issue after tandem pump change, towed the car with a friend about 15 meters & started fine, air gets in the system & you need a really good battery or tow it,
 
Ami said:
I had the same issue after tandem pump change, towed the car with a friend about 15 meters & started fine, air gets in the system & you need a really good battery or tow it,
Click to expand...
The issue here is that air is getting into the system somewhere.
Its always a pig of a job to start the car after a filter change if the fuel lines aren't clamped or a pump change.
 
audifan said:
The fuel goes around a loop thus: Filter, Tandem Pump, Injector rail, Fuel Temp Sensor.
When fuel is used by the injectors the filter is topped up from the tank and the engine runs just fine.
Problem is that the fuel in the loop ends up getting too hot, so the thermal bypass valve opens @85F to return fuel to the tank via the fuel cooler under floor.
This basically maintains an optimal fuel temp and prevent waxing in the filter when fuel is cold.
Click to expand...
20240402_223908.jpg

Top detective work
@audifan.
:) 👍
FUELCANOSTER.JPG

Workshop Manual Pg.31
Audi A2 2001 ➤
Fuel supply system, diesel engines
Engine ID AMF ANY ATL BHC
 
Last edited by a moderator:
DaveySprocket said:
All seems a bit strange to me, but what do I know.. :rolleyes: Surely if the return fuel is going into the filter, then how would it get down to the cooler? Maybe it only gets diverted to the cooler after a certain temp is reached.. You can just about see the return connection area of the filter housing in this picture. There's something metallic in there.View attachment 121698
Click to expand...
I missed this post, that metallic thing may well be the key to understanding this. Is it a bimetallic strip? If you blow a hairdryer on it does it bend diverting flow?

Also the tandem pump filters, as best it can, air from the fuel supply before the fuel reaches the injectors. I thought this was good as I'm fairly certain it dumps the air in the return fuel which I thought went back to tank. It appears warm return fuel mixes with cold fuel in the filter housing until fuel from the tank reaches a certain temperature. So get a small amount of air in the fuel filter before starting and it is going to go round and round for some time causing starting problems. Every day is a learning day, thoughts?
 
Last edited:
The filter connections are

Top Tandem feed
Middle Cooler feed
Right Tank feed
Bottom Tandem return

All of the ports n the filter housing have an arrow on them showing direction. The thermal valve can only be operated by a bi-metallic strip as there are no electrical connections to operate it.
 
We have some knowledgeable contributors to this thread; I have some question please:

Air gets into the filter housing and it collects at the top. it is going to get sucked into the tandem pump when the engine starts. The tandem pump, I think, attempts to filter it out but can't. The cause of non starting is air in the injectors?

Air returns to the filter housing and because it is a cold start mixes with the any fresh fuel from the tank. There is not much fresh fuel from the tank because the engine has not started. Around the air goes again but this time it is more finely distributed with the fuel tandem pump can filter it out and the engine starts. Correct?

The air is still in there doing the rounds until the fuel in the filter housing reaches temperature and a thermostatic valve opens to allow fuel to return to tank via the cooler. The air can now escape. My question here is why does warm fuel not take the suction route back to the tandem pump, particularly if the tank is full? Is it just the pressure drop across the filter combined with the take off point giving sufficient head for the return to overcome pressure from the tank?

If you do a filter change and don't clamp the pipes correctly, starting is very difficult. How does the air get out? I'm going to take a further look at the tandem pump. Perhaps it dumps air to the vac side and into the crankcase? Has anybody had two clear plastic pipes fitted, big bubbles in, no bubbles or micro bubbles out? Does all of the air have to be injected into the engine?

Thank you.
 
DaveySprocket said:
I don't think this is correct @rotifer, to me the top outlet of the filter housing is the feed to the tandem pump? The lowest connection is the return from the tandem pump no? 🤔
Click to expand...
Hello,

Well spotted @DaveySprocket I was too tired to cross check my work.

Please use the Audi A2 dedicated workshop manual data for fuel related repairs.
Data obtained from 7ZAP should be cross check as @DaveySprocket did. Use 7ZAP with caution as its not always 100%.
Edit: human error or the errors I introduce will be most common.
:)👍
 
Last edited by a moderator:
As I have just blown into the tandem return port on my housing air is coming out the cooler input port. So from that I conclude yes fuel always returns to the cooler and tank but guess as the fuel heats up more or all directly returned to the tank.
 
I've just blown air through a tandem pump casting. The air draw off capillary feeds into the return fuel from the injectors. The only possible ways out I can think of for air are:
Back to the tank, see @audifan post above.
Into the cylinders via the injectors.
 
audifan said:
As I have just blown into the tandem return port on my housing air is coming out the cooler input port. So from that I conclude yes fuel always returns to the cooler and tank but guess as the fuel heats up more or all directly returned to the tank.
Click to expand...
Hi @audifan ,

Was the test done using the new filter housing shown in your photographs?
From your results to this test if the air can return to the tank as the bimetallic valve is not closed at room temperature.
I wonder what the bimetallic valve does if you put the filter housing in the freezer?

Or as @Little Dog said earlier on, if the metal is heated up does to open the return fuel path to the cooler?
 
You must log in or register to reply here.
Back
Top