740 in tank pump

[davidw6092]

Does the in tank pump only run when the main pump is running?I thought it would come on with ignition,rather than when starter is triggered.Got poor cold starting,sounds like its not getting enough fuel pressure until pumps have run for a while.Quick edit,B230 f engine.

[Clan]

Originally Posted by davidw6092

Does the in tank pump only run when the main pump is running?I thought it would come on with ignition,rather than when starter is triggered.Got poor cold starting,sounds like its not getting enough fuel pressure until pumps have run for a while.Quick edit,B230 f engine.

yes both pumps run together ...

[Laird Scooby]

There are a lot of other things that cause poor cold starting, plug gaps (0.65 - 0.7mm), loose, missing or perished vacuum pipes, blocked breathers and umpteen other things including rotor arm, dizzy cap and HT leads.

Both pumps are meant to run for about a second to prime the system when the ignition is first switched on then run continuously while being cranked and for 1-2 seconds after the key is released from the start position.
Try switching the ignition on and waiting a second before turning to the start position, you should hear the pumps run briefly. If not flick the key into the start position so the start just begins to turn the engine and release, you should again hear the pumps run.

Now try starting it.

[davidw6092]

Pumps will only run when starter is engaged,they don't run initially when ignition first turned on.I have replaced the pump relay and its still the same.I think further investigation required.Thanks for the replies.

[Laird Scooby]

If you've had the pumps running, does the engine then start easier?

[davidw6092]

Yes.after pumps have run for a bit it starts well.This is what is leading me to believe that the pumps are not priming on initial switch on of key.(working on modern audis,they actually prime the in tank pump when you open the door!)

[Laird Scooby]

Originally Posted by davidw6092

working on modern audis,they actually prime the in tank pump when you open the door!

Hmmm, now that's an idea! I have some timer relays somewhere that would do the job. I had thought of a flick switch mounted somewhere to do a "manual prime".

The problem sounds like it's one of three things - first, ignition switch, second the ECU (if yours has one) and third, the fuel pump relay in need of resoldering.
Adding a manual prime or alternatively an auto-prime triggered by the door being opened (or unlocked) would be a work-around though.

Or you could do like i do and just flick the starter to prime the pump then start it normally.
It could also be the fuel pressure regulator valve is getting weak and allowing leak-down while not running, similar for the main pressure pump.