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jackl
10-24-2007, 09:21 AM
I know it recycles some exhaust. but my question is, if my EGR is bad, does it affect the engine performance/economy?

I've taken it off and cleaned it and the block port, but still have the SES light. And I'm wondering how much of a priority replacing it is for me.

froggy81500
10-24-2007, 09:29 AM
If the EGR is opening at the wrong times or is stuck open, you'll have drivability issues, like sputtering and stalling. If the EGR is not opening when it should, combustion chamber temperatures rise and you start getting pinging (spark knock) and likewise the pcm retards the ignition timing. Either way, it will affect fuel economy and performance to some extent.

97coupe
10-24-2007, 09:39 AM
EGR does a few things, first it dilutes the incoming mixture and reduces the amount of oxygen in and which lowers peak combustion temps to reduce NOx emissions. Second, it also slows flame speed and reduces tendency to knock making it tolerate lower octane fuel a bit better. The "problem" is that these both tend to reduce engine efficiency (power and MPG). BTW, EGR is not applied at WOT or at idle and speeds just above idle nor at high vacuum at higher RPMs. (very low manifold pressure) As to how important it is depends on how you drive and the fuel you use since its absence while cause the PCM to retard spark even more at times with low octane fuel and it could have some effect on MPG and performance. Being that it is getting cooler outside this time of year, octane needs drop and it may not be a issue for you. Personally, if you do not have a E-check to pass I would not worry about it much and I would just drive it until you have the desire or funds to fix it.

alleninpa
10-24-2007, 10:19 AM
I know it recycles some exhaust. but my question is, if my EGR is bad, does it affect the engine performance/economy?

I've taken it off and cleaned it and the block port, but still have the SES light. And I'm wondering how much of a priority replacing it is for me.


If you have the funds, replace it with a new one. Don't get a junk yard one, as they all seem to fail at some point. Also, with the SES light on, you don't know if you have another code show up that could be more important. You are going to need to replace it sooner or later anyway. I played the clean it game for 6 months, only to replace it anyway, at the cost of 6 months of lowered fuel economy.
6 months at 30 mpg versus 33 mpg at 1000 miles per month = $54 extra in gas....:hmm:

eljefino
10-24-2007, 11:57 AM
A properly working one will make the car run bog-free. Worst bogging from a dud EGR for me, stick shift driving, is when leaving a red light at mid throttle. Bogs out from 1k to 3k RPM then takes off. Irritating. Possibly less so with an automatic. The bog is from the spark retard.

oseberg
10-24-2007, 12:48 PM
A properly working one will make the car run bog-free. Worst bogging from a dud EGR for me, stick shift driving, is when leaving a red light at mid throttle. Bogs out from 1k to 3k RPM then takes off. Irritating. Possibly less so with an automatic. The bog is from the spark retard.

Actually the bog is from when the EGR is stuck open. A stuck open EGR will also cause a rough idle. The bog is caused due to less power caused by less fresh air and fuel entering the engine because it's being replaced by innert exhaust gases.

Also, because it's normally only supposed to be open at high idle low power requirements (constant speed driving on the freeway for example) when you have a properly functioning EGR, there is no effect of the EGR when you're at full throttle. Therefor the EGR does not effect overall power or pinging under high load full throttle conditions which happen to be the time when pinging is most likely.

At low throttle constant speed driving we really don't care how much horsepower our engines have, so we might as well do whatever we can to increase fuel economy and decrease emissions as much as possible at those times. The EGR is a device that performs this task.

However, at low throttle constant speed driving, the EGR will cause your engine to run cooler, so that when you do punch it, your cylinder, head, and piston are cooler. With a stuck closed EGR, all of these will be hotter leading to a higher probability of pinging when you do punch it.

97coupe
10-24-2007, 01:23 PM
Actually the bog is from when the EGR is stuck open. A stuck open EGR will also cause a rough idle. The bog is caused due to less power caused by less fresh air and fuel entering the engine because it's being replaced by innert exhaust gases.

Correct when it is stuck shut there is no bog


Also, because it's normally only supposed to be open at high idle low power requirements (constant speed driving on the freeway for example) when you have a properly functioning EGR, there is no effect of the EGR when you're at full throttle. Therefor the EGR does not effect overall power or pinging under high load full throttle conditions which happen to be the time when pinging is most likely.

You do not have this quite right. It is basically open at medium power requirement when there is a higher manifold pressure (lower vacuum and usually in the 5 to 10 inch range) when the induction of exhaust gasses can be better mixed tolerated.


At low throttle constant speed driving we really don't care how much horsepower our engines have, so we might as well do whatever we can to increase fuel economy and decrease emissions as much as possible at those times. The EGR is a device that performs this task.

EGR can reduce NOx (the only real reason there is a EGR valve) but it usually has a negative impact on economy because reducing combustions temps reduces efficiency (work is captured from hot expanding gasses) so EGR is not a plus for good economy (other than when it stuck shut)


However, at low throttle constant speed driving, the EGR will cause your engine to run cooler, so that when you do punch it, your cylinder, head, and piston are cooler. With a stuck closed EGR, all of these will be hotter leading to a higher probability of pinging when you do punch it.

You kinda have this backwards. Intake air help cool the engine, particularly the valves and intake ports and hot EGR gasses reduce this cooling. Also more fuel has to be consumed because with reduced efficiency due to reduced expansion efficiency more heat is produced. As far as pinging, the blending of EGR gasses masks true octane needs of engine while reducing its thermal efficiency (the amount of power extracted for amount of fuel consumed) Myself I say screw the EGR and use better fuel and get better efficiency too.

oseberg
10-24-2007, 02:16 PM
You do not have this quite right. It is basically open at medium power requirement when there is a higher manifold pressure (lower vacuum and usually in the 5 to 10 inch range) when the induction of exhaust gasses can be better mixed tolerated.

Yeah, that's what I meant by high idle low throttle freeway cruising.

EGR can reduce NOx (the only real reason there is a EGR valve) but it usually has a negative impact on economy because reducing combustions temps reduces efficiency (work is captured from hot expanding gasses) so EGR is not a plus for good economy (other than when it stuck shut)

Well, yes, I didn't actually mean to say that the EGR improves fuel economy. What I meant was that during freeway cruising is the best time to think about emissions and fuel economy because you don't really care about your horsepower output and ability until you actually press the petal to the floor.

You kinda have this backwards. Intake air help cool the engine, particularly the valves and intake ports and hot EGR gasses reduce this cooling. Also more fuel has to be consumed because with reduced efficiency due to reduced expansion efficiency more heat is produced. As far as pinging, the blending of EGR gasses masks true octane needs of engine while reducing its thermal efficiency (the amount of power extracted for amount of fuel consumed) Myself I say screw the EGR and use better fuel and get better efficiency too.

Actually, I believe you have it backwards.

If you recirculate exhaust gases through the intake, then there is less air and fuel in there to burn. Also, fuel economy is improved due to reduced pumping.

Pumping is when you shut off the air supply forcing the engine (since it is a pump) to pump all the air out of the intake manifold creating a vacuum. The engine fighting this vacuum is a huge waste of energy. Allowing exhaust gases into the intake manifold at this time can eliminate this vacuum thus reducing the loss of energy due to pumping.

Also, since you're diluting the air fuel mixture, you actually have less air and fuel burning than you would have had to burn in order to generate the power to generate the vacuum thus producing less burnt fuel thus generating less heat.

The heat generated is directly proportional to the amount of fuel burned. Less fuel burned equals less heat generated. Less heat means cooler exhaust gases getting sucked into the intake manifold.

The whole point of the EGR is to reduce the combustion temperatures. I believe that whoever designed this thing knew what they were designing and was therefor successful in reducing the combustion temperatures by installing an EGR, and that's why it's installed on so many engines.

Higher combustion temperatures generate more NOX. Lower combustion temperatures generates less NOX. That's why they installed an EGR on all these engines to reduce NOX.

Another thing, my EGR failed. First it was stuck open, and my engine barely ran. Then I cleaned it. Now it's stuck closed. I'm also running 91 octane right now instead of 87. With 91 octane + stuck closed EGR I'm getting just about the same fuel economy as with 87 octane + properly functioning EGR. So I would say that a stuck closed EGR does not improve fuel economy. And if 91 octane will improve fuel economy (I'm not sure about this yet) then a stuck closed EGR reduces fuel economy.

97coupe
10-24-2007, 05:33 PM
Well, yes, I didn't actually mean to say that the EGR improves fuel economy. What I meant was that during freeway cruising is the best time to think about emissions and fuel economy because you don't really care about your horsepower output and ability until you actually press the petal to the floor.

Actually I do care as I like nice crisp throttle response which EGR usage does not give


Actually, I believe you have it backwards.

If you recirculate exhaust gases through the intake, then there is less air and fuel in there to burn. Also, fuel economy is improved due to reduced pumping.

You have it backwards. There is no free lunch here. You need to burn fuel to heat and expand gasses to drive piston down to turn crank to produce power to move vehicle down the road. No magic is going to change this HP requirement to do this work (power vehicle) Any gains from lower pumping losses would be offset by reduced efficiency resulting in a higher fuel fuel to net same power at crankshaft.



Pumping is when you shut off the air supply forcing the engine (since it is a pump) to pump all the air out of the intake manifold creating a vacuum. The engine fighting this vacuum is a huge waste of energy. Allowing exhaust gases into the intake manifold at this time can eliminate this vacuum thus reducing the loss of energy due to pumping.

It is not nearly the energy waste you suggest and again, allowing exhaust gas to flow reduces efficiency and no EGR flows during closed throttle or times of high vacuum and low power output.



Also, since you're diluting the air fuel mixture, you actually have less air and fuel burning than you would have had to burn in order to generate the power to generate the vacuum thus producing less burnt fuel thus generating less heat.

Where are you getting this??? Fuel is needs to power the engine and using exhaust with mixture means reduced efficiency and therefore more heat must be burned to make same power which means more heat.


The heat generated is directly proportional to the amount of fuel burned. Less fuel burned equals less heat generated. Less heat means cooler exhaust gases getting sucked into the intake manifold.

Less fuel mean less power is developed and the drive requirement of car are not changed but EGR as it still take "X" amount of HP to power it and with reduced efficiency, more fuel is burned to do same work.


The whole point of the EGR is to reduce the combustion temperatures. I believe that whoever designed this thing knew what they were designing and was therefor successful in reducing the combustion temperatures by installing an EGR, and that's why it's installed on so many engines.

The ONLY reason it is there is to reduce NOx, not because it make engine more efficient for the amount of fuel it uses. Funny thing is EGR while reducing NOx, increases CO2 emissions per mile because more fuel is burned due to efficiency losses.


Higher combustion temperatures generate more NOX. Lower combustion temperatures generates less NOX. That's why they installed an EGR on all these engines to reduce NOX.

Again lower peak temps mean less energy extracted from mixture and high fuel usage for same output.


Another thing, my EGR failed. First it was stuck open, and my engine barely ran. Then I cleaned it. Now it's stuck closed. I'm also running 91 octane right now instead of 87. With 91 octane + stuck closed EGR I'm getting just about the same fuel economy as with 87 octane + properly functioning EGR. So I would say that a stuck closed EGR does not improve fuel economy. And if 91 octane will improve fuel economy (I'm not sure about this yet) then a stuck closed EGR reduces fuel economy.

Somehow I doubt this as stated.

oseberg
10-24-2007, 06:19 PM
You have it backwards.

Are you sure?

http://en.wikipedia.org/wiki/Egr

"The addition of inert exhaust gas into the intake system means that for a given power output, the throttle plate must be opened further, resulting in increased inlet manifold pressure and reduced throttling losses."

In other words, less pumping == more efficiency.

"Lowered peak combustion temperatures not only reduces NOx formation, it also reduces the loss of thermal energy to combustion chamber surfaces, leaving more available for conversion to mechanical work during the expansion stroke."

"The lower peak temperatures result in more of the released energy remaining as sensible energy near TDC, rather than being bound up (early in the expansion stroke) in the dissociation of combustion products."

"A lean intake charge has a higher specific heat ratio than an EGR mixture. A reduction of specific heat ratio reduces the amount of energy that can be extracted by the piston."

I think maybe you should read up a little, then come back and argue with me when you've learned a bit more than you currently believe that you know about this subject.

fdryer
10-24-2007, 07:19 PM
I know it recycles some exhaust. but my question is, if my EGR is bad, does it affect the engine performance/economy?

I've taken it off and cleaned it and the block port, but still have the SES light. And I'm wondering how much of a priority replacing it is for me.

Unless you can retrieve the SES trouble codes you're guessing.

In as few words as possible despite the bickering leading away from the question, our engines are now strictly emissions controlled by agreed mandates to reduce pollution. To accomplish this feat, manage performance and return reasonable fuel economy would be to compromise in every area to meet overall design requirements. A faulty EGR valve can only fault in one way related to loss of engine performance and fuel economy; the egr valve staying open more than necessary will cause engine stumbling, hesitation, and stalling tendencies. Read the many EGR valve related problems posted. The other way for the egr valve to fault is to stay closed (rarely) that affects combustion temperatures, raising oxides of nitrogen. A component of smog. Leaving the egr valve (broken or disconnected) this way contributes to more emissions of NOx that our catcons can't completely neutralize, emitting more pollution, possible long term damage to the engine from too high combustion chamber temperatures that weren't designed to run this way but giving back a bit more in fuel economy and performance that you may/may not feel by the seat of your pants. Some with egr valves blocked off in an emissions-free state swear to an increase in power and it may be so due to the simple fact that no exhaust enters and combines with the fresh air/fuel mixtures to dilute/lessen the advantage of a normal a/f charge. As was mentioned at the beginning, its a compromise all around. There has to be a give and take in engine/emissions management and the way its set up there isn't anyone with the combined knowledge of electrical/mechanical/chemical/electronic engineering in the automotive field to come up with a better plan that affects the entire car line than what's been designed by every car manufacturer. Of course there are the tweakers amongst us that can wring out the most mileage from a gallon of gas but there's a downside to everything and the downside to high mpg's is balky warm-up performance, very conservative throttle management, and constant awareness of deliberate conservative driving techniques to wring out the best fuel mileage possible, to the detriment of everyone else on the road. The one good upside is passing emissions. There's no such thing as having your cake and eating it as in high performance and maximum fuel economy and anyone thinking so is living in a fool's paradise. If any or all of this post confuses you then just simply forget what you've just read.

97coupe
10-25-2007, 08:56 AM
Are you sure?

http://en.wikipedia.org/wiki/Egr

"The addition of inert exhaust gas into the intake system means that for a given power output, the throttle plate must be opened further, resulting in increased inlet manifold pressure and reduced throttling losses."

In other words, less pumping == more efficiency.

"Lowered peak combustion temperatures not only reduces NOx formation, it also reduces the loss of thermal energy to combustion chamber surfaces, leaving more available for conversion to mechanical work during the expansion stroke."

"The lower peak temperatures result in more of the released energy remaining as sensible energy near TDC, rather than being bound up (early in the expansion stroke) in the dissociation of combustion products."

"A lean intake charge has a higher specific heat ratio than an EGR mixture. A reduction of specific heat ratio reduces the amount of energy that can be extracted by the piston."

I think maybe you should read up a little, then come back and argue with me when you've learned a bit more than you currently believe that you know about this subject.



You still have it wrong, I studied IC engine design many years ago. A IC engine extracts its energy from hot expanding gasses. The hotter the gas the more force they expand with and the more energy that is extracted from them (the gas cools as it expands and reduces energy recovery) The main reason diesel are so efficient is because they have very high peak temps and a very high CR which results in a higher expansion ratio with which to capture the energy from it. There is no mystery here. There is also a bit more energy in its fuel but the it the peak temperatures and pressure and the expansion ratio that makes it all happen. It would be possible to boost gas engine MPG 10 to 15% or more fleet wide "IF" they would take 87 octane off market (and maybe 89 to) and offer one grade of 92 or more octane and build engines with a higher CR (11 to 1 or more) to extract more energy and efficiency from it. This will never happen because the average consumer feels that fuel octane has no bearing on efficiency and would burn 84 if they had it a few cents cheaper. Detroit's spark retard to control knock has them in ignorant bliss. You lower pressures and temps to reduce NOx's, not to improve efficiency. BTW, you could lower CR to 6 to 1 to reduce your pumping losses that you are so worried about (they are really not a concern here) and better use your 87 octane but then your efficiency would drop with lower CR but then you could feel better about pumping losses.

eRic 02sc2
10-25-2007, 09:55 PM
I know it recycles some exhaust. but my question is, if my EGR is bad, does it affect the engine performance/economy?

I've taken it off and cleaned it and the block port, but still have the SES light. And I'm wondering how much of a priority replacing it is for me.

The Exhaust Gas Recirculation (EGR) valve is used to allow a controlled amount of exhaust gas to be recirculated into the intake system. This limits peak flame temperature in the combustion chamber so the engine produces less NOx (oxides of nitrogen).

A negative backpressure EGR valve is used to control the amount of exhaust gas which is recirculated. Intake manifold vacuum is supplied directly to the top of the diaphragm to pull open the normally closed valve. Exhaust backpressure pushing against the valve keeps the diaphragm pushed against the bleed hole. When the rpm is high but the throttle is closed, exhaust backpressure becomes negative and the diaphragm is pulled down just enough to uncover the bleed hole. The vacuum on top of the diaphragm leaks off and the valve slowly closes.

The EGR vacuum is controlled by the PCM through a solenoid valve. The PCM energizes or de-energizes the solenoid by providing or withholding ground at the appropriate times. When the solenoid is energized, it prevents vacuum from reaching the EGR valve by venting it to the atmosphere. Once the proper conditions have been met, the PCM removes the ground, thus de-energizing the solenoid and allowing vacuum to open the EGR valve. The valve is open only when the throttle is open more than 4 percent and coolant temperature is above 104F (40C) for the DOHC engine with automatic transaxle, or 122F (50C) for all others.

froggy81500
10-26-2007, 09:02 AM
There are both negative and positive backpressure EGR valves. And the latest, for the last decade maybe, electronic EGR's that incoporate a solenoid and pintle location sensor for more accurate control.