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can someone pls explain how the f... VK 308 fuel system works?

ari666

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hi all

as the title says i am pulling my hair out trying to understand how the fuel pump operates in a VK. i bought a Vb-vk manual which covers the mechanical fuel system in depth, but only explains how to drop the tank and re-fit for electric.

my problem is that it appears my fuel bowl is running out of juice when i tramp it. it takes off like a rocket then stutters at which point i back off, then it runs normally again.

this thing has an electric fuel pump in it which dosnt get power when the car is in the 'on' position, but somehow gets power when it is running.

also, i always thought that electric fuel pumps need to have a fuel return hose fitted to avoid overfueling which is unnecessary on mechanical fuel pumps as the pressure is determined by the camshaft, but the carb doesnt have any means of returning it. is there some kind of pressure switch on the fuel line which switches the pump on and off?

there is a tiny metal pipe which runs back to the tank. i am assuming it is meant for fuel return, as my friends HQ 308 ute (mechanical pump) does not have it. would running a 't' piece near the carb in the fuel hose act as a return? or would it just pump half the petrol back to the tank and not to the carb?

it has also started to act up in the morning if it has been parked for 24 hours. i have to connect power to the fuel pump fuse till the carb bowl fills up, then it will start, otherwise it just cranks with no fire.

blah. cars bite.
 

Michelson4301

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The fuel pump should only operate once you crank it (ie give the tachometric relay a signal) or the motor is running, this is a safety feature because if you roll the car, crash, etc and the motor dies then so will the fuel pump. If the sytem is hard wired and in an accident a damaged fuel hose to allow the pump to suck the tank dry and paint the ground in flamable fuel.

Unsure of the return hose you talk of, it is possible as other vehicles of the same era have a return hose on electric f/p systems, although tee'ing this pipe into the supply hose will not be a good idea unless you trace this pipe back to point of origin. You believe your car is running out of fuel then bleeding off fuel via this "T" idea will only cause you more problems.

If your car is running out of fuel in the bowl overnight then that would be the first problem to rectify. Fuel does evaporate but shouldn't clear the fuel bowl overnight!.


A point to note is all fuel pumps electric and mechanical have a regulator system/circuit to keep a certain pressure. These systems can fail. If these regulating systems were not in place then an electric fuel pump would burn itself out if back pressure was too significant and a mechanical pump would either fold the pump arm or grind the lobe off the cam in seconds.
 

ari666

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Unsure of the return hose you talk of, it is possible as other vehicles of the same era have a return hose on electric f/p systems, although tee'ing this pipe into the supply hose will not be a good idea unless you trace this pipe back to point of origin. You believe your car is running out of fuel then bleeding off fuel via this "T" idea will only cause you more problems.
the small pipe goes back to the tank from the engine bay and from all the pics i have googled appears to be connected just above the fuel supply line to a round cylinder thing i assume is a filter of some kind in front of the carb. i dont have one of those cylinder things on my carb, so was wondering if a t-piece would suffice.

the reason i thought it might help was if the pump was shutting off due to pressure, then when i accelerate it would have to play catch-up. and being a pretty flimsy pump pressure-wise maybe that is why it is stuttering. the return hose may help by keeping the pump active.

i havent got enough cash atm to fit a better fuel pump, so i am gonna hafta think of a fix for this one in the meantime.

i have a mechanical pump here. would it help to fit that? lol. what else could cause the car to stutter? a better description would be;

full throttle runs great revvs freely up to 6500rpm, then just before the end of 2nd, roughly 4000rpm car just dies, no engine power at all, if you continue to hold full throttle it accelerates for a split second, then dies again, till you take your foot off the pedal, then returns back to normal.

i know internet diagnosis is hard. thanks.
 

Michelson4301

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your small metal hose you speak of is for the charcoal canister. The purpose of the charcoal canister is to store the fuel vapours from the fuel tank and then feed them into the engine on the next startup. I would not push fuel back down that line because if your tank can't breathe than you'll get vapour lock. Basically the fuel expanding and shrinking with heat also as you drive the deminishing fuel level causes an imbalance in the tank pressure (tank should be almost zero pressure and/or vacuum to say 0.5psi +/-) so this metal line is a balance line for the tank. if you Tee up to it you will create more problems for yourself.

Try the mechanical pump, remember though that if the electric pump is "sticking" or shutting off all together then it will become the restriction under full power. Have you checked your fuel filter ? remove it from the car and using your mouth back flush onto grass(this is great for the enviroment:D) if fuel comes out looking like dirt instead of having a yellow or purple tinge then replace filter.

What carb do you have bolted onto this motor ?
 

ari666

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its a quaddy.

it has a Z200 fuel filter which i use for fuel injected skylines, 180sx, silvia etc so they are free :) its brand new. well, 1000 kms or so now.

the charcoal cannister is connected as normal to the tank and the purge, the additional line i am refering to has no involvement with the canister and it is half the width of the fuel delivery and runs next to it to the tank.

p.s. good point about adding the mech pump... :bang:
 

Not_An_Abba_Fan

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All electric fuel pumps should work in tandem with a fuel regulator. This sends fuel back to the tank when there is too much pressure for the injectors/carby to use. The small round cylinder thing you see in the pictures is probably the regulator, does it have a vacuum line going from it as well?

I would say your problem could be a knackered fuel pump. It will pump enough fuel with moderate driving but runs out of puff at more spirited driving. Either that or your fuel bowl level is too low.

We have a VC with a 308 and injected heads, it runs a VN electric fuel pump. It died a while ago with similar symptoms as you. It would idle and take off slowly, but when you pushed it, it would die. It was making noise too, so if your pump is noisy, it could be that.
 

Smitty

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hi all

as the title says i am pulling my hair out trying to understand how the fuel pump operates in a VK. i bought a Vb-vk manual which covers the mechanical fuel system in depth, but only explains how to drop the tank and re-fit for electric.
late VKs had an electric pump added to overcome a hot fuel handling issue with police and HDT vehicles. the main mech pump does all the work
but the add-on pump overcomes fuel vapour issues

my problem is that it appears my fuel bowl is running out of juice when i tramp it. it takes off like a rocket then stutters at which point i back off, then it runs normally again.
sounds like your carb is not set correctly or the main fuel pump is struggling.
Remember ...VHs and VKs (and VLs) use a higher pressure mech fuel pump
(with VKs and VLs getting a helper leccy job)

this thing has an electric fuel pump in it which dosnt get power when the car is in the 'on' position, but somehow gets power when it is running.
yep...correct and as Holden designed it

also, i always thought that electric fuel pumps need to have a fuel return hose fitted to avoid overfueling which is unnecessary on mechanical fuel pumps as the pressure is determined by the camshaft, but the carb doesnt have any means of returning it. is there some kind of pressure switch on the fuel line which switches the pump on and off?
return?...only needed with high pressure pumps without regulators.
The VK pump is low-pressure (max 7psi)

there is a tiny metal pipe which runs back to the tank. i am assuming it is meant for fuel return, as my friends HQ 308 ute (mechanical pump) does not have it. would running a 't' piece near the carb in the fuel hose act as a return? or would it just pump half the petrol back to the tank and not to the carb?
the small ID pipe is the fuel/vapour return to the tank.
The VK Quaddie has an in-line dashpot with a small outlet for separating
vapour. A thin rubber hose goes from that to this metal line.
The genuine Improved Perfformance Engine Manual shows all this

it has also started to act up in the morning if it has been parked for 24 hours. i have to connect power to the fuel pump fuse till the carb bowl fills up, then it will start, otherwise it just cranks with no fire.
sounds very much like the fuel level in the float bowl is TOO low
and also like your mech pump is on its way out.

hth
 

ari666

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late VKs had an electric pump added to overcome a hot fuel handling issue with police and HDT vehicles. the main mech pump does all the work
but the add-on pump overcomes fuel vapour issues

sounds very much like your mech pump is on its way out.

hth
umm, it doesnt have a mechanical pump!?! it never has had one since i bought it...

i have a spare one here off a vb-or-c, should i fit that and see how it goes?
 

Pub247

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get a mechanical one aswell we had so many troubles getting a vh to run just on an electric pump. We ended up ditching it and putting the mechnical back on
 

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umm, it doesnt have a mechanical pump!?! it never has had one since i bought it...

i have a spare one here off a vb-or-c, should i fit that and see how it goes?
fit it (makes sure it works)
I think you will be surprised how well it goes when the carb has fuel
(assuming the carb is in good nick)
 
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