S4 starting operation: That "delay"...
11-13-2013 | 09:58 PM
#16
Under the Lift
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Joined: Mar 2002
Posts: 18,647
Likes: 49
From: Buckeye, AZ
Reading through all the 928 literature and Probst book on Bosch fuel injection about the LH, there is no mention of any delay. Simply that the pump is energized by the ignition switch in the START position and the pump will run for a few seconds, stopping if it does not get a signal from the CPS that the engine is turning above a certain RPM (indicating it is running).
The check valve does not retain fuel pressure forever. The test spec is 0.5 bar loss in 30 minutes. Most I have tested barely make that spec if even that. So, it usually takes some time (a few seconds) to build up fuel pressure.
Most of the time my car starts after a few cranks. Occasionally it has surprised me and started instantaneously.
The check valve does not retain fuel pressure forever. The test spec is 0.5 bar loss in 30 minutes. Most I have tested barely make that spec if even that. So, it usually takes some time (a few seconds) to build up fuel pressure.
Most of the time my car starts after a few cranks. Occasionally it has surprised me and started instantaneously.
11-13-2013 | 10:27 PM
#17
Drifting
Joined: Jun 2006
Posts: 2,707
Likes: 73
From: Indianapolis Area
On my 1990 S4 the fuel pump runs for a fraction of a second when i turn the ignition on. Then the pump does not run again until the engine is cranking. I suspect this initial fraction of a second of pump running just with ignition in the on position is enough to pressurize the system so that it 'catches' as soon as the starter turns the crank.
Starts instantly after a month or more of non use.
Maybe differ ent fuel pump relay electrics in the relatively later cars.
Much conjecture on my part.
Dave
Starts instantly after a month or more of non use.
Maybe differ ent fuel pump relay electrics in the relatively later cars.
Much conjecture on my part.
Dave
Maybe that's not proper operation, but that is what mine has been doing for years... prior to an post fuel pump replacement. Also, mine almost always fires on the first revolution and certainly by the second when situation normal.
11-14-2013 | 03:26 AM
#21
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Joined: Oct 2001
Posts: 20,506
Likes: 549
From: Bend, Oregon
11-14-2013 | 04:41 AM
#22
Pro
Joined: Feb 2006
Posts: 693
Likes: 12
From: Australia
I don't really understand how ignition timing can affect cranking speed before the engine starts. I can understand how it affects how quickly an engine starts, but not the 'non-stRting' cranking speed. I would have thought valve timIng would affect cranking speed, with it's impact on 'effective' compression ratio. But ignition timing? I'm no expert, so would welcome an explanation
Cheers
Cheers
11-14-2013 | 01:45 PM
#25
Rennlist Member
Joined: Feb 2002
Posts: 7,055
Likes: 38
From: Cambridge England
I have seen perhaps two LH2.3 ECU that "flash" the fuel pump briefly on igntion switch on, before the key is turned further to crank the engine.
I think it is more "normal" for a car that has stood long enough for the residual fuel pressure to decay
that the engine will need to turn over a 2-3 revolutions in order to build fuel pressure and the engine to fire.
Maybe the "quick starters" have not been left that long and then is some raw fuel in a couple of cylinders which starts the engine quickly ? I know that happens with my car which has a couple of leaky injectors.
I think it is more "normal" for a car that has stood long enough for the residual fuel pressure to decay
that the engine will need to turn over a 2-3 revolutions in order to build fuel pressure and the engine to fire.
Maybe the "quick starters" have not been left that long and then is some raw fuel in a couple of cylinders which starts the engine quickly ? I know that happens with my car which has a couple of leaky injectors.