Initial Fuel System Priming

[Rob_the_Sparky 9]

No more than an irritation really but my 309 has never primed the fuel system so it takes quite a few rotations of the engine to start. Would be nice to figure out what sends the signal to prime the system when the ignition is first turned on so it starts normally.

 

If I had to guess I'd say the tachy relay but can anyone confirm this?

 

Cheers

 

Rob

[pacey205 2]

Im pretty sure it would be the tachy relay as well.

 

Michael

[pug_ham 242 3 Cars]

I also think its the tachymetric relay but my old 309 was just the same as yours & never primed but it used to start pretty quick, second or third crank woulkd see it fire.

 

I was looking at the Haynes diagram yesterday & I think it gets a quick kick closin the relay from wire #32 & then once its running the feed from either the coil or rev counter takes over to hold it closed which would explain why it stops the fuel pump running when the engine is stalled.

 

Graham.

[jonah 1]

Mine doesn't prime either, never has, but starts in an instant from cold no matter how long it's been standing.

 

The tachy relay closes whenever the key is turned to start, so the fuel system will pressurise very quickly (unless it's been drained), by the time the starter motor gets up to speed. After the key is released the relay stays on as long as there are still pulses from the coil LT (igntion amp). So as you say Graham if the engine stops for any reason the fuel pump will cut out too.

 

I keep hearing that some Pugs do prime the fuel system when the ignition is switched on, so this is either caused by a different tachy relay, or possibly a different ignition amp. If some brand of ignition amp produces a pulse through the coil when first switched on (quite possible given how it works), then the tachy relay may be detecting this and switching the pump on for a moment.

 

IMO the fact that your car doesn't prime when you turn the ignition on doesn't indicate a fault.

[Rob_the_Sparky 9]

All the 205s I've got certainly prime the system when the ignition is turned on and fire almost immediately. My 309 doesn't prime and it takes a few seconds before it catches, 4 or 5 rotations at a guess.

 

The ignition amp idea may be correct though as I think the 309 stopped priming when I had to replace it...

 

Rob

[Beastie 1]

I thought that I understood this once but now I'm less sure My CTI hardly ever primes but it always starts first compression hot or cold. I had assumed that this was because it is used regularly and the system mostly has full pressure. However when I replaced the fuel hoses last month it didn't prime when I switched the ignition back on and it still started instantly. The "already up to pressure" theory is often suggested and I believed it but probably wrongly so: The fuel pump runs continously when the engine is running and excess pressure is relieved back to the tank. There is no pressure valve or regulator on the car which could switch the pump on and off according to priming pressure so I guess that it could just be a timed prime triggered by the tachymetric relay.

[jonah 1]

Rob, if it takes 4 or 5 revs before it catches then the lack of priming definitely isn't the problem. As soon as the pump starts the system will pressurise almost instantly, no matter whether the pressure had held up since the engine last ran. Compare the fuel system to a brake system... both are sealed systems that are always full of (incompressible) fluid - when you jab the brake pedal, how much fluid volume are you shifting and long does it take for the calipers to react?? Same with the fuel system, it takes next to no volume of fluid movement to build up pressure (provided you haven't let air into the system since the car was last run), and the pressure will travel from the pump to the fuel rail at basically the speed of sound!

 

Worth checking though that the tachy relay really is switching on as soon as you turn the key to start. If it's only working on the coil signal then the pump and ECU won't get any power until after the engine has started cranking over, and this would cause some delay in starting. Easy to test - just pull the spade terminal wire off the back of the starter and turn the key the start - you should here the pump come on.

[futura 3]

A friend of mine has a strange fuelling issue:

He gets fuel and pressure to the injectors' rail, but nothing comes out the injectors when he turns the ignition on.

Pump is definitely running and when he turns the ignition off he gets a small drop through the injectors.

 

He's just fitted the engine so it could be anything really.

Any thoughts?