Continuous EGR

[Taylor95]

Soon I will be in the process of upgrading my intake manifold and exhaust headers on my 1997 Jeep Cherokee. I noticed that my new intake manifold has an extra hose attachment that I do not need. It is for a very small hose (maybe 1/4" OD). It would be very easy for me to add a continuous EGR system this way.

I would like to hear from people who know more about EGR: Would this be beneficial or could I run into some problems? Would carbon buildup become an issue?

[Ecky]

Carbon does build up, and a catch of some sort isn't a bad idea.

Generally speaking, EGR is normally run at low to moderate loads, but not at idle or full load. At part throttle, EGR can decrease pumping losses, but it can defeat idle strategies without access to tuning the engine - you'll have to see how it idles. At full load you lose horsepower, which is undesirable for many drivers.

Hot EGR and cold EGR have different benefits and drawbacks.

Is your Jeep's EFI system MAF or MAP based?

[Taylor95]

My Jeep has a MAP sensor.

So I guess it really is not that easy. At a minimum I would also need a valve to open at specific times- something that is probably too hard for me to do (and to be worth it).

[Ecky]

I've considered adding EGR to my engine. Unfortunately, MAP based systems are probably the most "thrown off" by EGR, as they have no way to differentiate between exhaust gases and fresh air. Adding EGR can decrease manifold vacuum, so the computer behaves as if it's running at higher load, by adding fuel and pulling timing. Generally speaking, you want the opposite to happen. A few precent EGR and it probably does net good, but the gain would be similarly small.

[cRiPpLe_rOoStEr]

If your engine doesn't have EGR, it's preferable to avoid adding one. On a sidenote, what would be your goals if you eventually manage do add EGR to it?

[Taylor95]

EGR is supposed to net some benefits by leaning out the fuel mixture and decreasing emissions.

Unfortunately, it seems to properly do this would add an unnecessary degree of complexity relative to the (small) gains. I agree that it probably is not a good idea.

[serialk11r]

If you can ensure the EGR isn't excessive, you only need to adjust the spark timing on a MAF based ECU. However the benefit is very small if the EGR flow is small, so it's best to have it properly metered.

[cRiPpLe_rOoStEr]

Originally Posted by Taylor95

EGR is supposed to net some benefits by leaning out the fuel mixture and decreasing emissions.

It doesn't really lean out the fuel mixture, it displaces part of the fresh air in a way that decreases the amount of oxygen and nitrogen which would otherwise react and turn into NOx.