EGR valve modulator
09-04-2018, 04:39 PM
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EGR valve modulator
I have an 9/82 build 4x4 SR5. The current EGR valve modulator is part # 25690-35010 California spec. It has P and Q port only. The part is discontinued of course. Can I replace it with 25870-35040 which has PQR ports and plug up the R port? Any help would be greatly appreciated.
11-14-2018, 10:27 PM
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I have an 9/82 build 4x4 SR5. The current EGR valve modulator is part # 25690-35010 California spec. It has P and Q port only. The part is discontinued of course. Can I replace it with 25870-35040 which has PQR ports and plug up the R port? Any help would be greatly appreciated.
Thanks
09-17-2019, 08:38 PM
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Lol I’m now dealing with this as well. They have the three ports for sale on EBay for $15 so I might go ahead and try it. I’ve been looking for hours online for an answer to this and nothing has come up.
09-18-2019, 07:25 AM
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After researching this I have an answer for you all. Yes, you can use the later Modulator (Part # 25870-35040) & cap off the "R" vacuum port, and it will work just fine. Of course this will all be dependent on your actual EGR Valve being functional, if you have a good vacuum source signal, and whether or not the exhaust gas port that the bottom port of the Modulator hooks up to is clogged with carbon or not. That bottom port allows the Modulator to "sense" how much exhaust pressure the engine is making at any given time. Here's the explanation.
As I said, the bottom port on the Modulator is for exhaust pressure. The "P" & "R" ports hook up to vacuum source (more on the "R" port later) & the "Q" port hooks up to the EGR Valve.
The EGR system reduces oxides of nitrogen, which is a pollutant (greenhouse gas). This is accomplished by recirculating some of the exhaust gases through the EGR Valve to the intake manifold, slightly leaning out the air/fuel mixture and therefore lowering peak combustion temperatures. A hotter burn produces more oxides of nitrogen. This lowers the oxides of nitrogen levels at the tailpipe.
The EGR vacuum modulator controls the EGR valve by modulating the vacuum signal with an atmospheric bleed. This bleed is controlled by the amount of exhaust pressure acting on the bottom of the EGR vacuum modulator (diaphragm). The exhaust pressure changes as engine load changes (low load & high load conditions).
Since recirculation of exhaust gas is undesirable at low rpm or idle (engine would run too lean & possibly cause overheating & damage), the system limits itself by sensing the exhaust flow. Under low load conditions, such as idle, low speed driving or part throttle cruise, the exhaust pressure is low. In these conditions, the diaphragm in the modulator is pushed down by spring force and the modulator valve opens to allow outside air into the vacuum passage. The vacuum in the line is reduced, the EGR valve does not open as far (or at all), and the amount of recirculation is reduced (or stopped).
Under high load conditions or high rpm driving, the exhaust pressure is increased. This pushes the modulator diaphragm upwards and closes the bleed valve. A full vacuum signal is transmitted to the EGR valve; it opens completely and allows full recirculation. The slight reduction in combustion temperature (and therefore power) is not noticed at highway speeds or under hard acceleration.
Here is where the "R" vacuum port comes into play. Some vehicles also control the EGR with a thermally controlled vacuum switching valve (VSV), also sometimes known as a Thermo Vacuum Switching Valve (TVSV). The VSV is controlled by a thermal sensor that monitors coolant temperature & allows the Emissions Control Computer (ECC) to further control the EGR under certain conditions. The ECC will electrically close the VSV if the engine is not warmed up and the throttle valve is in the idle position or if the engine is under very hard acceleration. A system using the TVSV works almost the same way, but isn't electrical. An engine that is not up to full operating temperature requires a richer fuel mixture so allowing the EGR to dump exhaust gas into the intake & lean out the air/fuel mixture is not desirable. In a "cold" engine this would lead to a pretty fair amount of unburned fuel coming out the exhaust pipe. Aside from these conditions, this EGR system (late models) operates in accordance with the normal vacuum modulator function.
So basically with the three port EGR Vacuum Modulator, it will function like the 2 port type as long as you cap off the "R" port to prevent any vacuum leaks at the Modulator. And as long as you have a good exhaust pressure signal, good vacuum signal and the EGR Valve itself is working properly. The "R" port was added on later models as a way to reduce emission further as emissions standards got more strict during the 80's. The ECC sometimes also controlled how much fuel a EFI Throttle Body put out, to more closely match engine conditions (engine temp, load, etc), which helped reduce how much unburned hydrocarbons went into the atmosphere (pollution reduction). The "R" port was basically a "band-aid" for vehicles with a carburetor, as by design carb's can't be as precise as EFI with computer control.
Bottom line: go with the 3 port Modulator & cap off the "R" port & it should work fine. Just make sure you use a good quality silicone rubber cap on the port; I've seen the cheap plastic one fall apart in just a few months & you'll end up with a vacuum leak you'll have to chase down later.
As I said, the bottom port on the Modulator is for exhaust pressure. The "P" & "R" ports hook up to vacuum source (more on the "R" port later) & the "Q" port hooks up to the EGR Valve.
The EGR system reduces oxides of nitrogen, which is a pollutant (greenhouse gas). This is accomplished by recirculating some of the exhaust gases through the EGR Valve to the intake manifold, slightly leaning out the air/fuel mixture and therefore lowering peak combustion temperatures. A hotter burn produces more oxides of nitrogen. This lowers the oxides of nitrogen levels at the tailpipe.
The EGR vacuum modulator controls the EGR valve by modulating the vacuum signal with an atmospheric bleed. This bleed is controlled by the amount of exhaust pressure acting on the bottom of the EGR vacuum modulator (diaphragm). The exhaust pressure changes as engine load changes (low load & high load conditions).
Since recirculation of exhaust gas is undesirable at low rpm or idle (engine would run too lean & possibly cause overheating & damage), the system limits itself by sensing the exhaust flow. Under low load conditions, such as idle, low speed driving or part throttle cruise, the exhaust pressure is low. In these conditions, the diaphragm in the modulator is pushed down by spring force and the modulator valve opens to allow outside air into the vacuum passage. The vacuum in the line is reduced, the EGR valve does not open as far (or at all), and the amount of recirculation is reduced (or stopped).
Under high load conditions or high rpm driving, the exhaust pressure is increased. This pushes the modulator diaphragm upwards and closes the bleed valve. A full vacuum signal is transmitted to the EGR valve; it opens completely and allows full recirculation. The slight reduction in combustion temperature (and therefore power) is not noticed at highway speeds or under hard acceleration.
Here is where the "R" vacuum port comes into play. Some vehicles also control the EGR with a thermally controlled vacuum switching valve (VSV), also sometimes known as a Thermo Vacuum Switching Valve (TVSV). The VSV is controlled by a thermal sensor that monitors coolant temperature & allows the Emissions Control Computer (ECC) to further control the EGR under certain conditions. The ECC will electrically close the VSV if the engine is not warmed up and the throttle valve is in the idle position or if the engine is under very hard acceleration. A system using the TVSV works almost the same way, but isn't electrical. An engine that is not up to full operating temperature requires a richer fuel mixture so allowing the EGR to dump exhaust gas into the intake & lean out the air/fuel mixture is not desirable. In a "cold" engine this would lead to a pretty fair amount of unburned fuel coming out the exhaust pipe. Aside from these conditions, this EGR system (late models) operates in accordance with the normal vacuum modulator function.
So basically with the three port EGR Vacuum Modulator, it will function like the 2 port type as long as you cap off the "R" port to prevent any vacuum leaks at the Modulator. And as long as you have a good exhaust pressure signal, good vacuum signal and the EGR Valve itself is working properly. The "R" port was added on later models as a way to reduce emission further as emissions standards got more strict during the 80's. The ECC sometimes also controlled how much fuel a EFI Throttle Body put out, to more closely match engine conditions (engine temp, load, etc), which helped reduce how much unburned hydrocarbons went into the atmosphere (pollution reduction). The "R" port was basically a "band-aid" for vehicles with a carburetor, as by design carb's can't be as precise as EFI with computer control.
Bottom line: go with the 3 port Modulator & cap off the "R" port & it should work fine. Just make sure you use a good quality silicone rubber cap on the port; I've seen the cheap plastic one fall apart in just a few months & you'll end up with a vacuum leak you'll have to chase down later.
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