Why do we have an EGR valve?

90 4.0 Renix.

From another thread I asked about low idle, it was suggested that I check my EGR valve. The plunger does go in and out, but the engine will stumble & die. I plugged off the EGR valve and all the associated ports, and the thing runs fine. I intend to replace the EGR when I can get around to it, but what am I missing engine function wise, with it out of the system?

can anybody tell me where and what this egr valve looks like? Mine's an 87 4.0

The Exhaust Gas Recirculation (EGR) Valve is an integral part of a vehicle's emission control system or EGR System. It controls the engine's emission of nitrous oxides by reducing combustion temperature.

The EGR valve first appeared in automobiles in 1972. EGR valves basically reduce emissions by sending some of the exhaust gas through the intake manifold back into the cylinders. Because exhaust gas most often doesn’t burn, it stays and takes up space in the combustion chamber and lowers the temperature there. Older vehicles used to have mechanical engine EGR valves, but the newly manufactured vehicle use electronic EGR valves.

Most EGR valves look something like this.

it is right under the intake manifold. and it recycles exaust gas's and puts them in the intake to reduce engine temperature. and over all reducing emissions. like NOx

whoops what his post said ^^^.

....and if it doesn't work at all, harmful detonation (knock) under load is likely.

Jim Mesthene said:

....and if it doesn't work at all, harmful detonation (knock) under load is likely.

Click to expand...

So I better fix it sooner rather than later, huh?

The usually run fine without them, but you will likely have to have it functioning for emission testing.
Another way to understand there function is, the end results are just about the same as using a higher octane fuel or retarding the timing a bit. They bleed a little exhaust gas into the mix, to slow down combustion a bit. Which results in more complete combustion and at a slightly lower temperature.

The EGR valve pictured should give you a good idea for a "basic" valve - sometimes, they also have a "vacuum transducer" attached to them, which looks like a copy of the diaphragm case (yellowish part) only plastic. You should find it sticking out the side of the intake manifold plenum, about halfway back. There is a metal tube under it that goes to the exhaust manifold.

The purpose of the EGR (Exhaust Gas Recirculation) valve is to reduce NOx emissions by slowing combustion/reducing temperatures by allowing a metered amount of exhaust gas into the intake stream at cruise. That's the oficial story - for the AMC242, I think the purpose of the thing is just to crap up the intake with carbon. I say this because my 87 had a non-functional EGR valve - but it passed the "sniffer" test just fine - and failed the "functional" test (which I didn't authorise, but they did anyhow.)

On higher-compression engines, I can see the utility - but on low-compression engines (like ours,) the EGR doesn't really have an effect on NOx, but a functional EGR will actually increase HC and CO emissions! I've got "before and after" reports indicating this... This is why I have a hard time with the idea of the thing - I'd sooner be quit of it, but the California Air Police won't let me (despite test results.)

Testing the EGR valve -
Draw a vacuum (using a hand pump, engine OFF) to ~10" and hold for 30 seconds. The vacuum should hold - this is done to check for leaks at the diaphragm.

Repeat, with engine IDLING. The engine should stall out due to the intake stream contamination - if it doesn't, you've got blocked passages, or a blocked valve (which happens - there's a lot of carbon in there.) If you have to remove/replace the EGR valve, the gasket is OEMR only, but I've also used RTV Copper with good results.

5-90