91-92 Vacuum logic diagram? (1 Viewer)

[mct75]

Does anyone have a vacuum logic diagram from a 3FE FJ80? I have been searching for one that shows vacuum lines from a component and connection standpoint instead of a spacial "how it looks in the engine bay" layout.

This is the best spacial diagram I can find, but it's not easy for me to actually confirm what connects where. Like, how in the world is anyone supposed to trace all this out?

For example, here's a proper vacuum logic diagram for my 22R. You can actually see what connects where, instead of what it looks like and where it is. Does a diagram like this exist for the 3FE?

 

[retrofive]

Per chance is it on a sticker under the hood?

 

[retrofive]

Just looked at mine.....

 

[mct75]

Thanks for the pic. Here's mine! While a little more helpful, I really need to "connect the dots" as to what connects to what. I suspect that most of my vacuum lines are no longer needed as I'm removing the emissions equipment that the PO smashed with a hammer to disable.

 

[retrofive]

@jonheld is the 3fe guru. Perhaps he has something on his website or will chime in.

JH Home

 

[mct75]

I'm suspecting that the BVSV on my thermostat housing just controls the EGR (Until engine is up to temp, disable EGR).

If that's the case, I can remove it without issue. But it'd be really nice to know for sure. Also, I'd like to remove all 3 VSVs on my DS fender, but I'm not positive on the function of the one for fuel pressure. I read that it exposes the FPR to atmosphere in order to prevent fuel pressure from dropping during cranking, and is then completely inoperative while the engine is running. Still, I'd rather know for sure and do all this the educated way.

 

[jonheld]

I'm suspecting that the BVSV on my thermostat housing just controls the EGR (Until engine is up to temp, disable EGR).

The BVSV runs the VSV for venting the charcoal canister fumes back into the intake.
The VSV for the EGR is run by the ECU.

 

[mct75]

Awesome, now I know!

 

[mct75]

The BVSV runs the VSV for venting the charcoal canister fumes back into the intake.
The VSV for the EGR is run by the ECU.

It's hard to tell, but based on this chart it looks like the BVSV controls both EGR and Evap. This would make sense as both systems are only to be active when the engine is warm. Is it possible the diagram I'm using is for a different model?

 

[jonheld]

You are using the correct diagram. The EGR valve is controlled from the EGR VSV that is triggered by the ECU. The primary function of the BVSV is to open the vacuum path to vent the charcoal canister vapor back into the intake. There has to be vacuum present at the EGR port for this to work.

 

[LS1FJ40]

@jonheld How the hell do you know all of this stuff? Seriously. I've always wanted to ask you.

 

[mct75]

You are using the correct diagram. The EGR valve is controlled from the EGR VSV that is triggered by the ECU. The primary function of the BVSV is to open the vacuum path to vent the charcoal canister vapor back into the intake. There has to be vacuum present at the EGR port for this to work.

Very cool.

So, the BVSV is connected to the EGR, but it does not control the EGR. Instead, the EGR port controls the evap system. Evap is only active when the throttle is open enough for EGR to be engaged. This makes sense as you probably do not want evap discharging when there's so little airflow in the engine.

If I follow, anyone who removes their EGR by capping the lines at the throttle body also disables their evap system. But, if you cap the hardlines at the intake runners evap will not be disabled.

Interesting too that the evap canister discharges past the throttle body as this is unmetered air. Maybe the fuel vapors are close enough to 12:1?

 

[jonheld]

@jonheld How the hell do you know all of this stuff? Seriously. I've always wanted to ask you.

If you sound like you know what you're talking about, people believe you...

 

[LS1FJ40]

If you sound like you know what you're talking about, people believe you...

Yeah, but you can only fake until you make it for so long...

 

[jonheld]

Very cool.

So, the BVSV is connected to the EGR, but it does not control the EGR. Instead, the EGR port controls the evap system. Evap is only active when the throttle is open enough for EGR to be engaged. This makes sense as you probably do not want evap discharging when there's so little airflow in the engine.

If I follow, anyone who removes their EGR by capping the lines at the throttle body also disables their evap system. But, if you cap the hardlines at the intake runners evap will not be disabled.

Interesting too that the evap canister discharges past the throttle body as this is unmetered air. Maybe the fuel vapors are close enough to 12:1?

The purge port is 3mm with flow restrictions, so not much unmetered air is getting in.

 

[mct75]

@jonheld, one last question. Can you verify what the fuel pressure VSV does? I have heard that it does nothing more than increase fuel pressure during cranking. If that's the case, I'd like to remove it, and also remove just about every vacuum line that is on the truck. If I could get the vacuum hardlines off I would be ecstatic.

 

[jonheld]

@jonheld, one last question. Can you verify what the fuel pressure VSV does? I have heard that it does nothing more than increase fuel pressure during cranking. If that's the case, I'd like to remove it, and also remove just about every vacuum line that is on the truck. If I could get the vacuum hardlines off I would be ecstatic.

Personally, I would not screw with fuel delivery, but it's your truck.

The FSM does not appear to give details on the FPR VSV and why or when it operates, but I didn't spend any serious time looking. The FPR does require a vacuum line to operate. Fuel rail over pressure will kill your fuel pump and cause very hard starts.
If you're going forward with this, then you need to monitor fuel rail pressure and make sure you stay within spec.

 

[mct75]

Thanks, I too am concerned about tampering with the fuel system hence me asking. I will definitely keep the vacuum reference for the FPR.

My assumption is that if the VSV only serves to increase fuel pressure during cranking, any issues with hard starting could be resolved by opening the throttle slightly during cranking. I am again guessing that there's a chance of manifold vacuum pumping down during cranking, which causes the FPR to lower pressure, leaning out what needs to be a richer mixture. Again this is all conjecture based on what I've read.

If the hardlines didn't burrow into the manifold, it would be much easier to trace. I think based on what I can see is that the VSV only selects between manifold vacuum and atmosphere, choosing to reference the FPR against one or the other.

 

[LS1FJ40]

I will throw is some anecdotal experience. I experience hard cranking on COLD START with my 92 3FE. I don't recall it being so hard to start when COLD prior to deleting the EGR, VSV, air pump, etc.

If I give t throttle when starting it starts right up. I just ordered a new Cold start injector as I have some suspicions about it. I pulled it and cleaned it but it looked "rough". I will report back after replacing it.

It starts just fine when warm so I'm not certain that it's related. Maybe @jonheld can help clarify this.

It could be purely coincidental.

 

[jonheld]

An over pressurized fuel rail will cause the check valve inside the fuel pump to stick closed. This is commonly associated with a failed FPR.