Speedtoys

Well...I have.

No other cars I own wait a # of rev's before letting itself start.

If a fuel line breaks when it gets pressure, what did it safe me from whether it broke at key on, or after 3-4sec when it fires up?

Just asking..

jpitman2

Unless I have misread things, on an S4 the pump relay is a std one (53), and the pump relay is triggered by the injection ECU sensing air moving through the AFM. If air stops moving, the pump stops. Not sure if there is a prime run at switch on, suspect not.
With the earlier cars using the tach signal to the relay, the 2 second run at switch on should prime the fuel circuit for starting, its not trying to delay the start. The cut off when the engine stops turning is to prevent the pump potentially spewing fuel all over the engine bay after an accident - isnt that safer?
jp 83 Euro S AT 56k

Speedtoys

Oh...after a crash, indeed...But the pre-start prime..the later models doesnt have this?

Alan

I really don't think this is the way it works... I think you always get the priming pulse solely as a turn on pulse triggered after the 15 circuit becomes active. I have tested one of these relays on the bench a few years ago. I actually then built a version for my GTS (from scratch) that does priming (pinouts required are different) - again a timed pulse based off 15 becoming active. I designed my own circuit for this - pretty easy. Had to add a new ignition connection to the relay socket too.

Anyway changing the delay is only possible if you are willing to perform surgery on the fuel pump relay innards...

Alan

Alan

The late models don't have it - which is why I built one - I think its actually a quite good idea.

On these Pre-S4 models the fuel pumps turn on almost immediately after you start to crank so there really isn't much delay anyway. Without priming they start as fast as an S4+, with priming its faster...

Alan

Speedtoys

STRIKEMASTER

The pump relay is a 'latched' relay, It needs an input from the tach ( my K-Jet) or the F.I computer (L-Jet) to activate the electro-magnetic 'latch' and keep the relay closed. No 'latch' signal and the relay opens after about 2 seconds. That way, if you crash, the pump shuts off, instead of spraying fuel at 75 psi into the engine compartment.( Ka-BOOM!!) You just have to say " The Shark Driver's Prayer": Key on-"Some sharks do 160'-key-off---Key on- "some do 159" -key off--- key on-"If I can make mine run at all"- key off---key on-"I think I'm doing fine"- Key off--- then it will start. Always works with my '83 K-Jet.. LOL

dr bob

I've disected a few CIS FP relays over the years, and found that the initial-priming was indeed a hit-or-miss proposition within the relay itself. In specific installations, there may be something in the tach pulse generation that causes it to regularly start out pulled low, subsequently causing the relay to close until it times out, but there was nothing in the circuitry in the relay itself that provided that function.

My last personal CIS car (analog KE-jet in a 900 Turbo, not a 928) ultimately had a 556 dual timer driving the relay, with half dedicated to initial pressurization and the other half performing the safety shutoff function. With that, the engine would start with just a bump of the starter motor. Were I using the 928 for DD duty with lots of engine starts, I'd probably be looking to add that feature to the existing relay circuitry.

Wisconsin Joe

Well, you could always rig up some sort of switch to prime it, couldn't you?

A push button would be simplest. Just run power through it to the pump. Hop in, push the button for 3 or 4 seconds, let the button up and hit the key.

If you wanted to get really fancy, you could rig up a timed relay attached to something. The Cayenne (at least the early ones) primes the pump when the driver's door is opened.

Again, a little ingenuity, a bit of effort, and you could rig a timed relay to the interior light circuit to prime the car when the door is opened.

Personally, I'd think it's more effort than it's worth. My 85 Euro (LH) fires up quick enough. Maybe 4 or 5 seconds of cranking if it's been sitting. A lot quicker (like the first full revolution) when it's only been shut down for a few minutes. But even the "long" crank is not that much.

jpitman2

With my K-jet car, after standing for more than a few days, it might take 5-8 secs of cranking to fire, no matter the temperature. This doesnt bother me at all. Starts are instantaneous if at all warm.

jp 83 Euro S AT 56k

Ed Petry

Hey everyone,
Can not really tell you what caused the V-8 to start but it did and is getting better. But, this shows me the torque tube and tranny are in need of attention.

Question...should the front of the tranny have a complete bearing around the input shaft? If yes...Why? lol

Thank you all.

Ed