Hey guys,

Quick question...Simply due to forgetting a couple things in my house, I had to turn my ignition on and off (just the electrical not the engine) a couple of times before starting my car today (cold start)...all within 2 mins or so. When I went to start the car (1990 962 C2) she fired up like a beast on the first crank. She usually cranks 2-3 times and then fires up...

Do our systems prime with fuel before starting? Did I unintentionally load her up with fuel by turning the ignition on and then off a couple times pre starting? Im curious, because if thats the case, it may be a good idea to give her a 10 seconds to fully prime up before starting for a quicker better start?

Cheers!

Jack

 

The fuel pump does run anytime the ignition is "on". Waiting a couple of seconds before cranking should get the pressure up to full, so "priming" some minutes in advance of starting should make no difference over waiting just a few seconds before cranking.

 

I don't know if it's relevant, but I have a C4 and I make a habit of waiting anything up to a minute for the hydraulic pump to get up to pressure before attempting to start the engine - and I have NEVER had my car start on anything but the first crank.

I would agree that the fuel pump must build up pressure although it's unlikely to take much longer than a few seconds.

Regards

Dave

 

I agree with dave, just a few seconds to let the pump prime is a good habit. An old BMW mechanic I used told me way back ( way back) that letting the fuel pump run a few seconds before turning over the engine will prolong the life of the pump and keep the fuel injectors cleaner. Don't know if any of this is true but I never have had to replace a fuel pump since he replaced one for me.

 

I do the same just for the hydraulic system... it is like part of the ritual...

...and if I am in such a hurry that I can't wait for a minute before it is all ready then I need to either take another car or re-visit life's priorities... It is like getting an double expresso to go... man! some things are meant to be enjoyed without any rush...

 

I have a C4 and I start the car after a few seconds. I'd rather have energy from the engine build up the pressure in the brake hydraulic pump than the battery. You never know by the time the pressure is up you may not have enough juice left to crank it up

 

That is why my wife asks me if I have a hybrid car... I plug it in everytime it is in the garage...

 

I think the fuel pump only runs when the engine is turning over at >~30 rpm. Anybody have Adrian's book handy?

 

Yep, as far as I know the fuel pump only starts to run once the engine is cranking. Before I started using Motec I could never hear the system 'prime' with only the electrical on. Now with Motec, I can actually hear it prime for a second or so before I crank. Its a long time since my car was standard but I am pretty sure the pump doesn't run on standard car pre-cranking...

 

I think I've read this >30rpm rule before, but I can't find it now. Checked Adrian, Shop Manuals and two Bosch books.

Interestingly, I recently R&R'd my injectors and on the first start thought to turn the key to on and hold it for a short while to get the fuel pressure back up to spec thinking that the pump would run. Instead it took a couple of start attempts to get it to kick over. Makes sense in light of the >30rpm rule, but I can't say for sure.

 

I don't understand why you would need to prime the system unless there's a leak in it.

 

I found the reference in Adrian's book - page 126: "Fuel pump earth (ground) is switched on by the DME control unit whenever the flywheel speed exceeds 30rpm".

 

Page 126 explains it. It doesn't mention the 30 rpm bit - just "runs while cranking" but it's assumed that cranking will be >30rpm since the DME will not fire the ignition until that speed is reached anyway.

Regards

Dave

 

My copy of Streather has the line the ormy quoted . . .though I didn't see it first pass through that page earlier this morning!

 

I recall running the fuel flow test without needing to crank the engine. Perhaps it times out, but the fuel pump definately runs when the key switch is first turned to "on".