TBI Regulator bypass?

[EagleMark]

Looking inside the injector pod lid that holds the injectors in as well as the top half of the fuel pressure regulator there is a brass jet cast into the fuel delivery line. Looks like it's there in case the regulator get's stuck shut so fuel can still get to injectors.

Is that the real purpose? In testing some huge springs that will not open the regulator I know fuel bypasses, but what's it really for?

[Roadknee]

I'm not familiar with the fuel flow path through the TBI, so can't comment with any certainty. Could it be an air bleed to remove trapped air from the system?

[Fast355]

I plugged mine with JB weld when running a TPI pump. Would hold fuel pressure after shutdown that way.

[brian617]

Have a TBI off older Cadillac 4.9L that does not have the bleed in it. I thought it was odd as all the others I have do.

[EagleMark]

I'm not familiar with the fuel flow path through the TBI, so can't comment with any certainty. Could it be an air bleed to remove trapped air from the system?

I don't think so? It's got no where to bleed to, it's just a small brass (?) jet cast into the fuel passage. On picture posted, the fuel flow comes in the top hole, the hole under it is a bolt hole. The it travels over the brass jet to big hole in center of regulator, then to injectors.

As Fast has said he JB Welded it shut with no ill effects, just held pressure when pump was off. And Brain found one without...

The only thing it looks to do is let fuel in if regulator was stuck shut. Like if a fuel pump was weak it would still run... although probably not well it would get enough fuel to idle and drive slow. Like a fuel system LHM?

[pmkls1]

You've got the flow path backwards. The fuel has already reached the injectors and is on it's way out to the return line at that point. I do not know the purpose of the little orifice, but if the regulator were to be held closed then fuel would flow through the orifice into the return passage. It could be an air bleed as suggested.

[pmkls1]

Here is your pic with notes illustrating what I mean. It's a little sloppy, but I think you can get the idea.


tbi-cover.jpg

[one92rs]

wow I to believed it to flow the other way around till I went out and blew through the extra top piece. since I have an extra think I will block it off and throw that on with a new spring. heck why not.

[Byron454]

ya like has been said before, the orifice lets the fuel pressure bleed off of the line/injector side back to the return and that lets the pressure drop to 0 with pump not running. the reasoning behind it is up for question? given the caddy type that doesn't use the bleed orifice, probably a carry over from when GM used the return type system on there mech fuel pumps with carbs or possibly worried about vapor lock with lower TBI 9-13 psi.

[pmkls1]

The vapor lock theory sounds like another very likely reason for the little orifice there. I hadn't thought of that, but it makes more sense than an air bleed. I never did understand why the TBI systems bled off pressure so fast after the ignition was turned off, but I never noticed that little orifice before.