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Hi Folks,

I am back from my vacation from Far East and back to my car project. This is my everyday car.

Car is running good. But MPGs have dropped after EGR removal. It was around 12-13 (city driving) before EGR removal. It is now 9-10. I only use 91 octane. Very short drive (4 miles each way to work)....so it is not bad...but still significant drop.

P.S. EGR was removed a while back. I was pre-occupied w/ over heating of engine (which is solved thanks to folks on this site) Thought it may be related. But it is not.

SHOULD I put EGR back on (Pros and Cons). OR THERE MAY BE ANOTHER AVEs to investigate.

ALSO:- I smell gas when the engine shows >180F temp. Generally runs below that.

351C...Q engine w/ A/C
New Spark plugs
ok well, removing a EGR can cause a decrease in MPG

here is the way it works

The EGR valve controls a small passageway between the intake and exhaust manifolds. When the valve opens, intake vacuum draws exhaust through the valve. This dilutes the incoming air/fuel mixture and has a quenching effect on combustion temperatures which keeps NOX within acceptable limits. As an added benefit, it also reduces the engine's octane requirements which lessens the danger of detonation (spark knock).

basically what that means is for each operation of the engine with a EGR it requires less fuel to do the same amount of work.

Now from a performance standpoint people remove the EGR because it drops performance, that is the trade off performance verse efficiency.

for a car you drive on the street you want to aim towards efficiency most bang for the buck, for the track you want the most go for the pedal.

on a dyno what you will find is removing the EGR makes a colder intake of fuel mixture making more power, also the fuel is denser in the mixture.
EGR by nature leans the mix but cools the temps in the combustion chamber, but you can then run more timing with less fuel.
there are many pluses and minus.

the decision is what do you want the engine to do? your chief complaint is my MPG dropped. if that is all then you can reinstall the EGR and try it out for a while,, if you feel the car did lose power you could then advance timing or adjust the vacuum advance to get a little bit back.

you could also readjust the carb to give more gas and then run more advance but then the MPG would drop off again.

more fuel and to some extent air will always make more power at the loss of fuel economy.

My car started out with all emissions devices removed. it ran like garbage on the street and my fuel economy was so bad that inside the car you would be tearing up and crying from the stench of fuel keeping the engine going. when i rebuilt it i decided to slowly add emissions devices back onto the car and see what the effect was. each device i put back on the car made an improvement, so i'm the type of guy that wants to put the original equipment back on the car because it usually runs better and i will see over 80,000 miles of service from it. Had i been a drag or street racing person then things would of been very different.
It's an 8:1 motor... run 87 octane.

1973 Mach 1 Q code 351 4V, 9A paint, standard interior, 3.50 rear, C6 trans.
I had no idea about all that EGR stuff! Very informative.

The most useless emissions device I've encountered has been the thermactor smog pump. There's really just no need for this dumb device unless you're running cats - they seize up all the time and cause major problems. I have, however, retained the evaporative canister feature on every vehicle because it really helps reduce the fuel odors and fumes in the garage when parked and I believe it also helps preserve fuel from evaporation that would otherwise take place. The EGR system, due to its complexity and general lack of compatibility with high performance parts, I usually eliminate on fuel injected performance vehicles, but if maximizing fuel economy were the goal I might consider re-enabling it after reading this information.

1971 Mustang fastback: 10.3:1 C90E 408W hydroller - CDAN4 EEC-V w/EDIS8, girdled, lowered and caged
It's possible the vacuum diaphragm was leaking, causing a lean condition. If the carburetor were adjusted for the lean condition it would be too rich now. I would try going down a size or 2 on the jets. I agree with Totalled, if you have stock compression run 87.

“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
just note,,, the octane ratings have changed major over the years, what was regular gas of the 1970s is not the same as today.
Ethanol is suppose to increase octane but weird things can happen. basically you have to listen to what the motor can tolerate.
if you are pinging on 87 then up the octane till it stops. or you have to detune the engine reduce timing to compensate for 87 even on a low compression engine.

i built my engine low compression to run on 87 gas,,,, state switched to ethanol 2 years later, engine would PING insane,, had to go to 93 since i could not find 97 and detune engine.

the thermactor smog pump is the big POOP emissions device that can be tore off the engine with vengeance, The Smog pump was a Kludge to get around California emissions regulations.

but many people don't understand
Vacuum advance
spark delay
vacuum delay
and other band aids that have come out over the years before computers took over engine management.

fuel injection is a whole new ball game but dealing with carburetors you have to remember it gives the wrong fuel air mixture at every throttle blade angle lol you have to fudge it to get it working in tolerance.

but you can tune the carb if you want to lean it out. you can jet down, adjust accelerator pump shot and duration, lean the mixture at idle. even use a vacuum gauge and tune the power valve. really a small change has a big effect.

the choke has big effect also.... with the weather changing colder you want to adjust it as well. shorter in summer longer in winter.
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