Increasing EGR flow for better mileage

[MetroMPG]

If I recall, the Metro problem is a coked up EGR passage that affects the middle cylinder. The result can be a burned exhaust valve (since EGR reduces combustion temperatures, without it, things get too toasty).

[Christ]

Originally Posted by MetroMPG

If I recall, the Metro problem is a coked up EGR passage that affects the middle cylinder. The result can be a burned exhaust valve (since EGR reduces combustion temperatures, without it, things get too toasty).

That seems to be what they say, that I've heard anyway.

Daox - For the care, cleaning, and maintenance of your system, short and straight as possible. There's nothing fluid, and unless you're going to filter it through water or some other fluid medium, or add in filters/screens, etc... just pipe it all in there, and maintain it accordingly. You'll be checking on it all the time anyway, I'm sure, so you'll get a feel for how long it takes before it needs to be cleaned out.

EGR cleaning should be a part of vehicle maintenance for anyone that does alot of cruising at steady speeds. It seems that those vehicles see coked EGR's the most, at least to me.

[tasdrouille]

As Christ pointed out, the MAP sensor will not sense it. I removed this part from my post.

EGR works as long as the pressure in the exhaust manifold is higher than the pressure inside the intake manifold. It is possible to have significant EGR levels at WOT and even at positive intake manifold pressure. In fact, diesel engines always operate EGR with positive intake pressure.

[Christ]

Originally Posted by tasdrouille

As Christ pointed out, the MAP sensor will not sense it. I removed this part from my post.

EGR works as long as the pressure in the exhaust manifold is higher than the pressure inside the intake manifold. It is possible to have significant EGR levels at WOT and even at positive intake manifold pressure. In fact, diesel engines always operate EGR with positive intake pressure.

Without sounding pompous about it - doesn't an engine, by design, always have higher pressure in the exhaust than in the intake?

Barring a few very special situations, I can't think of a time that for it to be running properly, it doesn't have more pressure coming out than going in.

I don't fully understand EGR, so that's not a rhetorical question, and I don't mean it to come off that way.

[tasdrouille]

I just wanted to express my view as to whether EGR is drawn by vacuum or forced by pressure. My comment was merely academic as we were talking about the same net effect anyway.

[Daox]

Originally Posted by tasdrouille

As Christ pointed out, the MAP sensor will not sense it. I removed this part from my post.

EGR works as long as the pressure in the exhaust manifold is higher than the pressure inside the intake manifold. It is possible to have significant EGR levels at WOT and even at positive intake manifold pressure. In fact, diesel engines always operate EGR with positive intake pressure.

I'm not understanding why this is. I'm cruising along at part throttle, so I have vacuum in the intake manifold. Then, I open up the EGR valve a bit. This is going to reduce the vacuum. The MAP sensor should see that.

At full throttle I could definitely imagine seeing positive pressure in the manifold, but I don't see that happening any other time than that. I guess I was also under the assumption that exhaust pressure was always higher than intake pressure as well.

[tasdrouille]

Won't your MAP think more air just went in and the ECU would adjust the fueling accordingly? Your o2 sensor would pick it up though, setting the record straight trimming the fuel, but maybe that would throw a MAP fault code.

Here's an interesting article on EGR control strategies.

[Christ]

Originally Posted by Daox

I'm not understanding why this is. I'm cruising along at part throttle, so I have vacuum in the intake manifold. Then, I open up the EGR valve a bit. This is going to reduce the vacuum. The MAP sensor should see that.

At full throttle I could definitely imagine seeing positive pressure in the manifold, but I don't see that happening any other time than that. I guess I was also under the assumption that exhaust pressure was always higher than intake pressure as well.

It's a thought... I'm not totally sure about it.

I don't really think that adding EGR is going to reduce the vacuum, because it's just adding mass from an area other than the throttle body. In other words, the engine is still drawing the same vacuum on the manifold, but the EGR is basically a second intake source. While you would show less flow through the throttle body, the vacuum figure should be the same regardless of EGR flow. (Some fluctuation might be noted due to expansion of heated gasses, etc.. )

[Gasoline Fumes]

Originally Posted by Christ

It's a thought... I'm not totally sure about it.

I don't really think that adding EGR is going to reduce the vacuum, because it's just adding mass from an area other than the throttle body. In other words, the engine is still drawing the same vacuum on the manifold, but the EGR is basically a second intake source. While you would show less flow through the throttle body, the vacuum figure should be the same regardless of EGR flow. (Some fluctuation might be noted due to expansion of heated gasses, etc.. )

I'd expect a reduction in vacuum with EGR. Since EGR reduces power, you'll have to open the throttle further to maintain the same speed. And the EGR flow you're letting into the intake manifold is higher pressure than the ambient pressure entering the throttle body, so that alone should decrease vacuum even if throttle position remains the same.

Also, the ECU in Suzukis (as well as other cars) uses the MAP sensor to monitor EGR function. If the ECU doesn't see the expected change in MAP when the EGR valve is told to open, you get a CEL. But from what I've read, it's a very small change in vacuum. But if you're increasing EGR flow, that could change.

[MetroMPG]

Well, another option might be feeding the exhaust into the intake tract, upstream of the throttle.

Though that might mess things up because of the intake air temp (IAT) sensor ...