Lots more good stuff to digest. If I may, I'd like to mention the importance of knowing which type of valve is installed when troubleshooting. I remember hearing how this was tripping up a lot of mechanics at the time.

The sealed style is probably what most people are familiar with, and not just with EGR applications. Consider a vacuum advance canister on a distributor, for example. If you pulled a vacuum with a handheld vacuum pump such as a MityVac, the diaphragm would move and stay there when you stopped pumping. Only when you bled off the vacuum would the diaphragm return to the (spring-loaded) relaxed position.

Contrast that with the air bleed type. The diaphragm chamber basically has a calibrated leak. If you pulled a vacuum with a MityVac, you'd have to keep pumping and pumping to overcome this leakage. If you weren't familiar with the design and thought it was a sealed design, you'd swear it had failed. Apparently this became something of a problem when these vehicles were new, and lots of perfectly good air bleed EGR valves were getting replaced.

 

Can I pick your brain about emissions testing for NOX? If an EGR system was inoperative, either due to a component failure or having been deactivated, would this show up during a typical emissions test? I'm talking about if the EGR valve failed closed, because if it stuck open that typically causes rough idle which would be obvious, even if the root cause wasn't.

I just took my truck through emissions testing yesterday. This state-run (Oregon) facility only tested the exhaust for HC, CO, and CO/CO2 ratio. Am I correct to state that most emissions testing facilities have no means to measure NOX? In other words, it's more of a laboratory-grade measurement and too expensive to test at the consumer level for poor guys like us just trying to renew their registration. Does that sound accurate?

 

Most vehicles emissions tests do not include testing for NOx, in England however NOx is part of the yearly MOT but they use an actual rolling road for their testing.
So it would be region to region.

If you have properly set up and tuned engine you can beat emissions CO/ HC testing even without any emissions devices.

CO or Carbon Monoxide results from partially burned fuel, fuel that has combusted, but not completely high CO levels always result from an overly rich mix.
HC or Hydro Carbons is fuel that did not burn at all.

CO2 or carbon dioxide is what you get when fuel burns completely.

The CO/CO2 ratio gives you an indication of the efficiency of the combustion.

 

Ford generally used air bleed types. you can quickly identify an air bleed EGR as they will have a felt ring on the underside of the diaphragm
See below pic (Late 70's Ford)

Whereas the sealed one's won't and will just have the rubber diaphragm
See below pic (Mid 70's 460)


Air Bleed EGR



Sealed type

 

The description of how the bleed type EGR works is not quite correct. If you pull a vacuum on a bleed type unit, you will get nothing, ever. You will think it's broken and order another one, only to find out the brand new one does the same thing, been there, done that.

What makes it work is exhaust backpressure. The factory exhaust system has some backpressure to it, stock they all do. The more engine rpms, the more backpressure you will have. Inside the EGR valve they have two diaphragms, one of them is metal. The metal one is exposed to the exhaust system. The metal one also controls the vacuum leak. With the engine off and no backpressure, the valve will do nothing when a test vacuum is applied, it will all leak through the felt seal around the diameter of the valve underneath. If you start the engine and rev it, that will create backpressure which will start pressing on the metal diaphragm, which will start making the leak smaller which will start opening the valve.

Before I read the book on how this thing worked, I got the smart idea to close the metal diaphragm all the time. I had carefully taken a grinder and had opened up one of these valves and was playing around with it. I found by closing the vacuum leak off, essentially making it a sealed unit, the engine would stall anytime I revved the engine.

That lesson made me realize adding this exhaust gas is a delicate procedure, they are feeding the engine just enough that it still runs alright. My problem was I had added headers and dual exhaust, and now even when hooking the EGR up I still had pinging. After all this experimenting I realized I had reduced the backpressure so much, that the EGR was not working like it should.

This also made me realize why CARB and the feds will not allow you to legally change the exhaust from the cat to the front, and why all these "cat back" systems are so popular and legal. Essentially, the exhaust system is part of the emissions system. You monkey with it, and it messes up the emissions of the engine.

 

Did you use a hand pump unit or a vac pump with continuous pressure? I assume it takes a minimum pressure to move the needle on a vac pressure gauge and you can't get that much on a hand powered unit.

 

Try it, you will never see it move.

 

That is not true as it depends on the EGR's calibration and the potentially the type air bleed EGR valve

There are 2 types of air bleed EGR valves, positive back pressure and negative pressure.

Positive backpressure EGR valves require exhaust pressure to close the air bleed orifice negative backpressure EGR valves do not.

Negative pressure EGR valves use the negative pressure behind each exhaust pulse to close the air bleed and allow the EGR to operate. Negative backpressure EGR valves are not dependent on exhaust back pressure to operate and will still function even on straight pipes.
Ford and GM both used positive and negative pressure EGR valves.

Electronically controlled EGR valves typically are of the sealed type and rely on a solenoid/s to control the flow of vacuum to the EGR.

The EGR valve has gone the way of the dodo much like the thermactor on modern gasoline engines do being able to increase valve overlap in certain conditions with variable valve timing. EGR's are starting to be eliminated even on diesel engines, replaced with a single module after treatment and revised cam timing.. We are getting to the point where engines are able to control the combustion process precisely enough that the only emissions systems required are exhaust after treatment.

 

But they never used this negative pressure EGR's in these era trucks correct? If they did, what was the application? I have never ran across one or seen it described in any of the books I have. The book I have for this era trucks describes the pure vacuum type, and the exhaust backpressure type.

On most all of these trucks we work on, I challenge anyone to go out to their truck that does not have a computer, and with the truck off take the vacuum line off the EGR valve, put a vacuum pump of any kind on it, and see if they can get it to move. I am not sure, I guess maybe if you put a large enough vacuum pump on it, the volume of flow could overwhelm the felt filter and create some vacuum to start moving it. I doubt it though, the vacuum port on the EGR is not large enough to support enough flow to do that.

 

And some positive pressure air bleed EGR's will activate with a constant source of just vacuum they just will not fully open and may even need the initial vacuum to open the valve enough to initiate the flow of exhaust back pressure to close the air bypass and allow the valve to open fully. If you open the exhaust up you may even be able to compensate by opening up the inlet metering orifice on the EGR for vacuum. This is best done in increments of 0.005" or less (they typically have orifices of 0.020" or 0.030") and will decrease the amount of back pressure needed to open the valve...

Positive and negative backpressure type EGR valves were both used again depends on calibration. Both positive and negative are often referred to as exhaust backpressure type EGR's which is correct as both rely on exhaust pressure to active just one is positive and the other negative.
This in part why there can be literally dozens of EGR valves for each engine in the same application.

Positive Back pressure EGR Valve






Negative pressure EGR Valve

 

Gotcha, you only tried a hand pump. The other posts answered my other questions.

 

Try it with any kind of vacuum pump you can find, even hook it to the intake manifiold of another car and rev the other car, I bet any backpressure type EGR will never move.

 

But you haven't tried any of those other methods have you? So unless someone has, we won't know what it doesn't until we try it. But for sure they test as if they are bad if you use a hand pump?

 

I have had one of these valves apart. I carefully took a grinder and ground away at the outside diameter till it split in two. I then took it apart and saw how it was put together. I then experimented with it. I had machine shop equipment available at the time, and made some miniature c-clamp type clamps to go around the perimeter to put it back together after I took it apart. This is when I had a lot of free time where I worked, and a lot of nice equipment available to me.

If that metal diaphragm in there never gets pushed up to start to seal the vacuum chamber, it's never going to move. This is only the type of valve that has the felt around the outside bottom of the valve.

 

What's the highest vacuum you pulled on it?