Corolla EGR System

Just trying to understand how the egr vacuum switching valve works. 4afe engine, 92 Corolla.

It says the ECM opens the vacuum passage of the vacuum switching valve and vacuum is not transmitted to the EGR valve, but vented to the atmosphere, causing the EGR valve to stay closed when coolant temp is low or at idle position or engine is at full throttle.

Would you say the VSV is shown in the closed or open state? Is the VSV solenoid powered or not in that state? Why does the egr valve have a small opening right where the VSV tube comes off from it? Why would the computer completely close the egr valve at full throttle?

The VSV is shown in the open state. Not powered at open. The small air passage on the EGR valve seems like a small controlled bleed hole as a pressure balance. Seems like the VSV is additional vent hole area so the EGR closes.

Looks like EGR pressure closes the Modulator so the engine vacuum can open the EGR valve if the VSV is commanded closed.

To me it looks like the vsv plunger is blocking off the path to the atmosphere (which I presume to be the square box below ), so I’d say it is closed, not open. I see no path to outside air as it is drawn. It makes sense what you say about a bleed hole at the EGR valve, so the EGR valve can’t get stuck in the open state. It’s just shown out of proportion to its actual size I guess.

As far as the modulator goes, I think you are right. It appears the modulator allows atmosphere pressure along the white path to the egr when the exhaust pressure is low, which keeps the egr closed. When the exhaust pressure is high, the modulator plunger pops up and closes off that path, allowing a vacuum source to the egr valve, along the black path, which opens the egr valve. I still don’t understand why the computer would close the EGR valve at full throttle though. Is it thinking at wide open throttle you are trying to pass a truck and there’s oncoming traffic so you need all the power possible?

No, I think the plunger is raised to block the outlet port on the right that is open to atmosphere. Follow the pipe from the left into the chamber with the plunger. That chamber is open to atmosphere through the pipe to the right not from the box below. If the plunger is raised, it blocks off the pipe to the left. The box below is the solenoid coil, the plunger shaft looks like it passes out the bottom to a weight that keeps the valve normally open.

And yes, I think you are right in that the EGR is blocked at WOT so it doesn’t compromise power.

I don’t see any pipe to the right of the VSV. There’s a black line on the right going to the ECM, but that’s an electrical wire I think. That’s why I thought the solenoid coil is where that black line meets up w/the VSV, and below that is a filter to the atmosphere. The path above the plunger could go to the atmosphere, if that’s what you mean by the right side port, but that pipe is blocked off by that black thing. That black thing really exists b/c I’ve seen it on the Corolla’s VSV, it is a black plastic cap blocking off that port. Presumably that same VSV is used in other applications and that port wasn’t needed for this one.

The ability to prevent EGR during WOT makes sense, now that you’ve explained it.

On top of the valve, going to what looks like a black block-off. As you say that is actually there. Is it a seal for the line or a vent? You wouldn’t want vacuum sucking crud from the engine through the valve itself so I figured that as a vent and not a sealed cap. But the bleed hole int he EGR would seem to present a similar problem so I’m stumped!

You’re not the only one stumped. The diagrams aren’t drawn very well, that’s the problem I think. For example, take a look at the EGR modulator. To test it, you plug off the two ports on the right (called p & r), and blow in the port on the left (q, which goes to the egr valve). Air is supposed to come out the air vent. Air vent? There is no air vent shown on the EGR modulator diagram!

The other test for the modulator, again you plug off p & r and blow in q, but this time with a hand-held vacuum pump attached to port s (the one out the bottom going to the exhaust stream). The test is, when you blow in q, blocking off p & r, you aren’t supposed to be able to draw a vacuum on s. What? I don’t see why blowing in q would have any effect on whether you could draw a vacuum on s. s looks like a totally sealed area in the diagram, so you should always be able to draw a vacuum on s.

One has to wonder why they even bothered to draw the diagram. Or if they asked anybody to review it to see if it makes any sense.

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