i have just received my new complete ST170 inlet manifold in the post and there is an odd thing on it that i cant figure out what its for.....

The ST170 manifold is split in two parts - the top section which people use for throttle bodies, and the bottom section which contains the variable runners...

on the bottom section ( the variable runner section ) just under the throttle body appears to be a fuel return line.

This is odd, cause the fuel rails on these manifolds dont have a retuern line - the fuel rail pressure is maintained by the pump.

the top and bottom sections of the inlet manifold are joined by the runners, nothing else, so what is this fuel return all about???????

in this reference photo, you can see there is n orange rubber bung over the fuel inlet/outlet just under the throttle body ( has a white cap on the throttle body )...

mine, has a fuel line actually on it ( red ) so it seems to be some sort of fuel return, but from where???????



st170_1

anyone?

Any chance of a better picture of the area in question - have enlarged the pic and its gone all pixelly!

I know that the Duratec V6 has two different designs of fuel rail (returnless and erm ... returned ) and maybe your engine is similar. Personally I would prefer the return version as it is easier to monitor fuel pressure

no, i think the ST170 only had one type which was return less.

plus, the fuel rail only feeds the injectors, nothing else, so im not sure how fuel would actually come out of the fuel return line, there is no connections from the second part of the throttle body to the upper section...

its weird.... its like there is a fuel return line for no reason...

Eh? So the feed goes to the rail (and therefore the injectors) so where is the return fitted?

I might be stating the obvious here but a return system would have a fuel pressure regulator in the return line on its way back to the fuel tank.

thats my point!

thee should be no return from the rail, because its controlled by the pump...

yet there is a fuel return, no where near the fuel rail, and not connected to the fuel rail - but its there!

Maybe Ford reused an existing manifold from another car, or were putting in some insurance in case the funky fuel pressure regulating pump didn't pan out in the real world.

well, i had a poke about last night with the manifold....

i wanted to see if it would fit in my chassis, but alas, its too wide, and too long..

anyway, i took the throttle body off ( which was not bolted on, but screwed on in to the plastic! ) and felt down inside, and basically, the fuel return line is fitted on what looks like just a take off from the manifold????? perhaps the fuel pump in the tank uses the return fuel line to know how much pressure is in the manifold????

who knows...

at the end of the day, im not going to use the main part of the manifold anyway...

Are you sure it's something to do with the fuel lines? If the pipe leads into the plenum/throttle body it's far more likely to be a pipe for e.g. brake servo or MAP sensor, or maybe even the the fuel pressure regulator.

Crankcase breather?
Connection to the carbon canister?

My bike carbs have a 'de-icing' connection that linked into the bike's coolant circuit - hot water went in, did a quick 'there-and-back' around a very short loop, then back out into the circuit.

Would a bike throttle body have a similar system?

my guess is with mikerj, it is a vacume take off from the plennum