What does VSV do that modulator doesn't?

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kerrye

kerrye

10 Year 15 Year
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Feb 13, 2010
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Been working on my EGR system recently and it's not clear to me why and when the VSV operates. I know it simply opens the vacuum system to atmosphere and prohibits the operation of the EGR. (currently I have a bb in the VSV line so it is permanently closed). Why does the VSV exist? The vacuum modulator already controls the vacuum to the EGR. Can anyone explain when and why the VSV has to override the modulator?
With my VSV currently closed, I can't detect any difference in the operation of the engine but I'm assuming there are times when the EGR is open when it wouldn't be if I had a functioning VSV.
 
VSV turns EGR on or off while modulator determines the degree to which EGR is opened.

The earliest EGR systems were bang/bang all open or all closed.

The combination of VSV and modulator is one implementation that allows finesse.
 
That does not seem correct to me on the basis of my vacuum readings. At idle, with my VSV closed, there is no vacuum going to the EGR. Vacuum increases as the engine increases in speed and goes back to zero at idle. That all occurs just with the modulator since my VSV is out of the circuit at the moment.
 
blkprj80 said:
VSV is to purge your charcoal tank when the engine is up to temp. I don't think it does anything with the EGR.
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HMMMMM? It definitely controls the EGR. When the VSV is open, the EGR cannot operate. I've confirmed this with vacuum readings. There may be a different VSV which controls the charcoal tank.
 
blkprj80 said:
VSV is to purge your charcoal tank when the engine is up to temp. I don't think it does anything with the EGR.
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That is a different VSV.

kerrye said:
That does not seem correct to me on the basis of my vacuum readings. At idle, with my VSV closed, there is no vacuum going to the EGR. Vacuum increases as the engine increases in speed and goes back to zero at idle.
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Actually, idle is max vacuum while increased RPM reduces vacuum.
 
Parsnip said:
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Actually, idle is max vacuum while increased RPM reduces vacuum.
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That is true for intake manifold vacuum but definitely not true as regards to the output of the modulator. I've monitored it with a vacuum gauge in the line. There is no vacuum at idle going to the EGR from the modulator.

That is one thing about the operation of the modulator that puzzles me. It's acting in reverse from the behavior of the intake manifold vacuum.
 
kerrye said:
That is true for intake manifold vacuum but definitely not true as regards to the output of the modulator. I've monitored it with a vacuum gauge in the line. There is no vacuum at idle going to the EGR from the modulator.

That is one thing about the operation of the modulator that puzzles me. It's acting in reverse from the behavior of the intake manifold vacuum.
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Just so. I wasn't clear. I was speaking only of the relationship between manifold pressure and the throttle plate. As the plate opens manifold pressure increases (vacuum decreases).
 
Parsnip said:
Just so. I wasn't clear. I was speaking only of the relationship between manifold pressure and the throttle plate. As the plate opens manifold pressure increases (vacuum decreases).
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Yes. So you need the modulator to modify that process otherwise the EGR would be full open at idle. What I don't understand is why you also need the VSV.
I'm assuming that since it's controlled electrically, there must be some situation in which you don't want the EGR to operate and the modulator is incapable of accommodating those circumstances. I just don't know what those circumstances are.
 
kerrye said:
Yes. So you need the modulator to modify that process otherwise the EGR would be full open at idle. What I don't understand is why you also need the VSV.
I'm assuming that since it's controlled electrically, there must be some situation in which you don't want the EGR to operate and the modulator is incapable of accommodating those circumstances. I just don't know what those circumstances are.
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As far as I know, the times the ECM wants ERG totally off are at idle and when the engine is cold. It would also make sense, at least as I understand it, to want EGR turned off when the throttle is near open as that would be a case where the driver is looking for max performance. But that last bit is only a guess.
 
Parsnip said:
As far as I know, the times the ECM wants ERG totally off are at idle and when the engine is cold. It would also make sense, at least as I understand it, to want EGR turned off when the throttle is near open as that would be a case where the driver is looking for max performance. But that last bit is only a guess.
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I agree but it's only when cold that the VSV seems necessary. There is no vacuum at idle coming from the modulator and while I haven't tested it with a vacuum gauge, since the intake manifold vacuum is almost non-existent with the throttle wide open, it seems like you wouldn't need a VSV to stop the EGR then either.

So, that leaves the need for the VSV strictly when cold. Why is it needed when cold?
 
kerrye said:
I agree but it's only when cold that the VSV seems necessary. There is no vacuum at idle coming from the modulator and while I haven't tested it with a vacuum gauge, since the intake manifold vacuum is almost non-existent with the throttle wide open, it seems like you wouldn't need a VSV to stop the EGR then either.

So, that leaves the need for the VSV strictly when cold. Why is it needed when cold?
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Well at cold the engine is still in open loop where fuel trim is set by a lookup table and that is a sort of rote best guess. The rings may not have sealed the pistons to the cylinder walls yet. I think at this point the cooling effect of exhaust gas in the combustion chamber may cause the engine to stop or run rough. I think that it is a case where every effort is made to keep the newly started engine going until it can warm up and set the fuel trim by the observation of the O2 sensors. If the desired effect of EGR is to reduce harmful emissions then it seems that the loss of EGR at startup is out weighed by the desire to get into closed loop quickly.
 
I haven't noticed any difference in cold start with the VSV closed all the time and it's been 15 below zero here this week. Interesting.
 
Parsnip said:
That is a different VSV.

Actually, idle is max vacuum while increased RPM reduces vacuum.
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Manifold vacuum is.

However EGR doesn't run off manifold vacuum. The 'sense' lines on top of the modulator run off ported vacuum which is a vacuum port that is next to the throttle blade. When the throttle is closed, the edge of the throttle blade closes the vacuum port = no vacuum.

Partially open throttle increases vacuum to the modulator, and 1/2 open or more decreases vacuum.

The egr vsv cancels the egr action during cold engine operation to increase engine smoothness. It works by bleeding off the vacuum signal to the egr valve to prevent it from opening. Most other Toyotas of that vintage do it the other way, by interrupting the vacuum line.

By blocking the vacuum tee on the top of the egr valve on the drivers side, you eliminate the vsv. You can do this with a vacuum cap, or with a golf tee on the end of the short hose running to the intake manifold, thru a pipe to the underside where the vsv is hidden.
 
86tuning said:
The egr vsv cancels the egr action during cold engine operation to increase engine smoothness. .
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Any speculation as to why I'm seeing no difference in cold engine operation without a VSV? Is the difference strictly in cold start emissions as opposed to cold start performance?
 
Nice little chart from the FSM.

EGR VSV Chart.webp

EGR VSV Chart.webp
 
Ahhhhh--very helpful. So I'm losing all those options to the right of the 'ON' column which includes an open EGR in some conditions. Since I passed the emissions test with the VCV off, it must mean that the single combination of events which turns on the EGR with the VCV off, isn't enough to raise NOx levels to the point of failure.
 
No, you have it backward. ON means the valve is closed to stop vacuum from escaping. OFF would be the options you are losing.
 
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