EGR valve forced open at high boost?
Jul 13, 2009, 04:48 PM
#1
EGR valve forced open at high boost?
A few days ago, I found that one of the hoses going to the EGR valve on my car had popped off. It probably popped off more than a year ago, but I never noticed because I had disabled the EGR test. After reattaching the hose, I got to looking at the hose routing diagram in the FSM and thinking about the air pressure and spring pressure that control the EGR valve. Air pressure is supplied to both sides of the diaphram to control the valve. When the EGR valve is commanded to open under light load, there is near-atmospheric pressure under the diaphram and vacuum above the diaphram sucking the valve open. Under boost when the valve is always commanded to stay shut, the air pressure above and below the diaphram is equal, and the only thing keeping the EGR valve shut is the spring in the valve.
I wonder how much exhaust pressure it would take to overpower that spring. I'm thinking that with a stock turbo at high boost, the exhaust pressure could be very high due to the turbo being well out of its efficiency range. I wonder if the pressure could be high enough to overcome the spring. The most thorough way to avoid the possibility would be to remove the EGR valve. The other option would be to pull the air pressure line off the bottom of the EGR valve diaphram. This would cause the pressure holding the valve shut to increase as boost increases. I may have to try this second option.
I wonder how much exhaust pressure it would take to overpower that spring. I'm thinking that with a stock turbo at high boost, the exhaust pressure could be very high due to the turbo being well out of its efficiency range. I wonder if the pressure could be high enough to overcome the spring. The most thorough way to avoid the possibility would be to remove the EGR valve. The other option would be to pull the air pressure line off the bottom of the EGR valve diaphram. This would cause the pressure holding the valve shut to increase as boost increases. I may have to try this second option.
Jul 13, 2009, 08:30 PM
#3
Although the absolute value of the exhaust gas pressure may be higher than the boost pressure, the actual force also depends on the effective area of diaphragm, port, hose, etc. Therefore, I "guess" EGR might be stay closed even at higher boost.
But I guess it is a good idea to get the block plate as Appauldd suggested.
BTW, I am a big fan of your insightful posts, mrfred.
But I guess it is a good idea to get the block plate as Appauldd suggested.
BTW, I am a big fan of your insightful posts, mrfred.
Jul 13, 2009, 08:42 PM
#4
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I'm a much bigger proponent of removing it entirely. Or if you need to visually keep it. Physically block it with a gasket that does not allow exhaust gas to ever touch the EGR valve. You can do that with a pop can.
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Jul 13, 2009, 10:21 PM
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Be careful when you take off your EGR....you can re-use the stock gasket. Put it between the block plate and the intake manifold. The EGR won't need a gasket as it is not part of the exhaust system any longer.
If you don't plan on keeping the EGR in the car, you will need some shorter bolts for the block plate.
I've had mine blocked for several months now with really no down side. I did notice a tad bit longer to warm the car....but that is about it.
If you don't plan on keeping the EGR in the car, you will need some shorter bolts for the block plate.
I've had mine blocked for several months now with really no down side. I did notice a tad bit longer to warm the car....but that is about it.
Jul 14, 2009, 01:08 AM
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I got it off & on a bone stock car, without removing anything else.
Use a touch of rtv on the surface of the plate. Just because there is exhaust on one side, doesn't mean you can't leak boost out of the other. It is connected to the intake manifold, and without some care when you put it back on, or put a block off plate on. It can leak.
Let the rtv tack up before trying to install it. You'll be reaching through a lot of crap to get down there... so the potential to goober a bunch of sh*t up with rtv exists.
Use a touch of rtv on the surface of the plate. Just because there is exhaust on one side, doesn't mean you can't leak boost out of the other. It is connected to the intake manifold, and without some care when you put it back on, or put a block off plate on. It can leak.
Let the rtv tack up before trying to install it. You'll be reaching through a lot of crap to get down there... so the potential to goober a bunch of sh*t up with rtv exists.
