Another Starting Issue

[PeteRR]

I wonder if anyone can shed some light on my starting prob please?.

The issue is, when starting the engine from anything but hot, it will crank over readily but struggle to run if/when it does catch (If it does catch it will stumble at minimal revs for a couple of seconds before it cuts out).

A second turn of the key and it will fire right up immediately every time, and runs/drives as good as gold on warm up.

I've done various searches, and it sounds to me like the temp sensor for the ECU at the back of the block was maybe at fault. But I checked it's resistance and it's within spec.
No fault codes on my hand-held reader either. Connections to MAF, TPS etc are all good.

I'm led to believe that it has a fairly recent Bosch 044 fuel pump and filter, so discounted those for now....

Could it be something like a sticky fuel pump relay (would have thought that that would also affect general running though)?

Does it sound like a classic symptom of some sensor failure or other issue, any other ideas chaps?

[twoqu]

I wonder if its fuel related. If the pressure is not being held after shut down and bleeds off perhaps the pump struggles initially to refill the system before the car stalls.
There is a residual pressure valve usually incorporated at the pump. Unless of course it has not been refitted during the pump change. A fuel pressure check would tell you.
Or if you actually have an RR then you should be able to take a look at the pump and see whether it has the valve fitted.
Just a theory!

[PeteRR]

Thanks mate. Yeah, it certainly seems to be fuel related to me too. And your description of a valve does feel like the kind of thing that is happening...
Though it fired straight up on leaving work tonight.

This is for an ABY by the way, I have had two RR quattro's in my time (hence the name!) and it certainly was easier to inspect the pump on those!

[UrS4boy]

Definitely sounds like the fuel is draining back to the tank, not staying in the fuel rail, pressurized, after shut off. The longer the shut-off period, the more that drains, and therefore, the longer it takes to refill to the point there is the proper pressure in the fuel rail.

Suspects: Fuel Pressure Regulator = http://forums.quattroworld.com/s4s6/msgs/21108.phtml

Fuel pump: your 044 might not have been fitted with a proper external check valve OR the external check valve has failed.

[PeteRR]

Great info as ever, thanks Dave! That certainly sounds like one to check. My Mityvac may come in handy here methinks.

[twoqu]

Just wondering if, after the car has sat, you carefully remove the cover from the fuel pump relay and close the contacts for a few seconds, that should prime the fuel system. If the car then starts and doesen't misbehave that would suggest its the fuel system.
Although a fuel pressure check would prove it.

[twoqu]

The holding pressure should be:
3.5 bar cold
3.8 bar hot
Thats after 10 mins.
A more hazardous test would be to leave the car and then crack a fuel union to see whether there is at least some pressure in the system. Leaky injectors could also allow the pressure to drop.

[PeteRR]

Nice one, I shall look into that too
Hopefully it's not the injectors, they were replaced with RS2 ones fairly recently by a decent specialist (the engine is now basically RS2 spec.).

[twoqu]

If the pressure regualtor has been changed for the correct RS2 one it effects the pressure.
3.3 bar cold
3.6 bar hot.
These are minimums after 10 minutes as were the previous set which are based on the ABY 4 bar fpr.

[PeteRR]

I don't think it was changed, but I'll check it to see.
I wonder if it ideally needs an RS2 one now?
Thanks for the info

[kulster]

I would also check the fuel lines under the car are not leaking fuel maybe

[PeteRR]

I would also check the fuel lines under the car are not leaking fuel maybe

They were apparently replaced a couple of years ago, but good point and worth checking out too