EGR vs Gas mileage

The Dark Side of Will · Tue Sep 02, 2008 11:33 am

The EGR on my Formula has been non-functional since I got it on the road. The tube broke in half. Rather than spend $100 on a new tube and induce all the stress and frustration of installing it, I had a free block-off plate made that's cake to install with the tube gone.

However, one of the benefits of EGR is that is improves gas mileage by reducing pumping losses in the engine.

Does anyone have any practical experience with how much it improves gas mileage? Is this a 1-2 mpg effect, or a 0.2 mpg effect? Right now I get ~28 mpg in pure highway driving in the 75-80 speed range. It would be sweet if I could crack 30.

p8ntman442 · Post by **p8ntman442** » Tue Sep 02, 2008 4:27 pm

I hit 30 mpg highway without it. No experiance with it. Try calibrating your right leg.

Nashco · Post by **Nashco** » Tue Sep 02, 2008 4:43 pm

Bump up tire pressure (40+ psi and put on your skinniest tires), reduce (or zero out) toe, use the cruise, and depending on your paint quality you can get decent gains drafting off big rigs (lots of rock chips!). These are all free as well and might help your fuel mileage if you're not already doing it.

I haven't done a direct A-B test, but from my experience when mine was busted I'd expect a mpg or so reduction. Nothing major, but a lot harder to hit the highs you can get with it working properly. If money is tight, you *can* braze them to fix them, but it won't hold up forever (20k miles or so was the going rate on my repairs. I've got an EGR tube in great shape on my Formula I'd trade for block off plates and some bits and pieces I need (exterior trim mostly) if you were interested in a trade.

Bryce

CincinnatiFiero · Post by **CincinnatiFiero** » Tue Sep 02, 2008 7:22 pm

With no egr and no cat my 88GT 5speed broke 30 mpg on 70mph highway trips.

Atilla the Fun · Post by **Atilla the Fun** » Wed Sep 03, 2008 11:49 am

back to back testing on a '78 camaro 350, with 700R4 swapped in, saw a difference of 2 mpg. fell from 25 to 23 by disconnecting it, and retuning didn't help. I was using stock manifolds and cam. I had 9.8:1 from a head swap. And a dual exhaust system, no cat. I sold the old, free, Holley Street Dominator I intended to install and ordered a Performer EGR.

Jinxmutt · Post by **Jinxmutt** » Wed Sep 03, 2008 3:37 pm

I have a factory EGR tube i'll send you with the heat wrap still in place for the price of shipping. If you're interested.

whipped · Post by **whipped** » Wed Sep 03, 2008 9:16 pm

From what I've read on an automotive engineering forum, an EGR will give you a maximum gain of 8%. I've never tried fixing the EGR of a car it didn't work, and never disconnected the EGR of a car that it did... so I've done no back to back comparisons, sorry.

lucky · Post by **lucky** » Thu Sep 04, 2008 12:05 am

With my EGR connected, but the passage in the plenum clogged, the car got better milage, but threw an "EGR valve position error" when maintained above 75 on the highway for more than 10 minutes. (86v6 4spd) After I cleared the passage (1/2" socket extension & hammer, then 12ga bore brush in a powerdrill), the car ran much better & no code, but frequency off fillups seemed to increase. My understanding of why is that the O2 sensor is telling the ECM it's running rich because of no EGR flow, so the ECM leans out the fuel curve to compensate. (I'm not gonna quote actual #'s here, I'm sure most of you remember that thread)

Atilla the Fun · Post by **Atilla the Fun** » Thu Sep 04, 2008 11:17 am

where did they get 8%? It helps more on a big 350 than a small 173, no doubt. It helps more on a carbureted engine than a computer-controlled one, partially dependent on tuning. Mostly, It's about the Quadrajet I was using, and most GM carb'ed V8s used year after year. With more gear, it helped mileage less, but it was for emissions control. a 3.73:1-geared '77.5-79 Z28 sees less impact on mileage, especially without overdrive. You can't run 2800 rpm on the idle circuit.
But this is NOT a Camaro forum. And the thread starter is not running a V8Archie swap. So my experiences are irrelevant.

The Dark Side of Will · Thu Sep 04, 2008 11:36 am

lucky80 wrote:With my EGR connected, but the passage in the plenum clogged, the car got better milage, but threw an "EGR valve position error" when maintained above 75 on the highway for more than 10 minutes. (86v6 4spd) After I cleared the passage (1/2" socket extension & hammer, then 12ga bore brush in a powerdrill), the car ran much better & no code, but frequency off fillups seemed to increase. My understanding of why is that the O2 sensor is telling the ECM it's running rich because of no EGR flow, so the ECM leans out the fuel curve to compensate. (I'm not gonna quote actual #'s here, I'm sure most of you remember that thread)

Do you have actual mileage numbers like odometer vs. # gallons?

When the ECM turns on EGR, it reduces fuel injected by a pre-programmed amount (in addition to altering timing). This should not affect the O2 reading because the amount of fuel reduction is compensated by the inert gas added by the EGR (which should be around stoich unless there's another problem in the engine).

Now I can see a *limited* benefit by leaning the mixture via the EGR program but adding cold air instead of hot EGR gas, but I'll think about that more later.

Falcon4 · Post by **Falcon4** » Fri Sep 12, 2008 11:58 am

This is an odd topic indeed... I notice a *decrease* in efficiency when I have my EGR working. For quite a while I had the vacuum hose unplugged and capped. After watching "an inconvenient truth" (no really), I figured it was worth the mileage tradeoff and plugged it back in. I haven't seen 18MPG (on the high end) since... 17 is the highest, 16.5 is average-ish. But then again, I never get the opportunity to do a lot of highway driving. EGR also causes an annoying amount of hesitation when it's cold... push on the gas, get a split second burst of acceleration, die for another split second, then accelerate like normal... I still haven't been able to track down why that happens, but it's much reduced when the EGR isn't there.

Strange that it's generally accepted that EGR improves mileage...

Atilla the Fun · Post by **Atilla the Fun** » Fri Sep 12, 2008 2:58 pm

sounds like some part of your system isn't quite right. On another note, have you read this entire thread? on some vehicles, EGR does improve highway mileage, thus the widespread belief has merit.

whipped · Post by **whipped** » Fri Sep 12, 2008 5:12 pm

Atilla the Fun wrote:where did they get 8%?

Calculated figure based on gasoline lean burn limits and reduced pumping losses. YMMV

Atilla the Fun · Post by **Atilla the Fun** » Fri Sep 12, 2008 7:22 pm

then it might be valid for EFI engines that always cruise at an air:fuel ratio of 14.7:1, but most carbureted cars ran slightly richer, thus more help from EGR.

Blue Shift · Post by **Blue Shift** » Sat Sep 13, 2008 12:47 pm

Will, you can go to Home Despot or OSH or some hardware store and pick up a ~12" gas hookup tube for a dryer or stove or the like - it's made out of stainless and is designed to survive a fire, meaning that it'll live at high enough temperature to be used as an EGR hose.

I just took the plenum off and tapped the hole the tube flange bolts to, and threaded one end in, then made an adaptor from the flange I scavenged from the broken tube, welded a fitting on, and threaded on the other end of the hose. 8 bucks and available at any hardware store when I made it, and it even looks sorta close to factory if you do it right.

Another idea in the pursuit of being a knuckle dragging typical cheap fiero owner! :la:

Oh, also do be aware that I had a stumble and hesitation issue at ~2500 RPM - which I traced down to a gasket with a restrictor plate/hole assembly in it that you need to install... I never realized it was there and forgot to.

Starlite528 · Post by **Starlite528** » Sun Sep 14, 2008 12:21 am

Blue Shift wrote: Another idea in the pursuit of being a knuckle dragging typical cheap fiero owner! :la:

Oh, also do be aware that I had a stumble and hesitation issue at ~2500 RPM - which I traced down to a gasket with a restrictor plate/hole assembly in it that you need to install... I never realized it was there and forgot to.

Haha, knuckle dragging, :withstupid:

I've noticed my car running worse and worse lately. I probably need to do a tune-up. I've had the same stumbling, I believe, ever since I replaced the engine. How on Earth are you people getting 28-30mpg? I'm lucky if I hit 20! I'm going to go ahead and get the new style EGR tube from Fierostore, along with several plastic door molding clips, and maybe the 'formula' door decals because I'm eventually going to have have the car painted.

Atilla the Fun · Post by **Atilla the Fun** » Mon Sep 15, 2008 10:28 am

maybe the real question is, why aren't you getting around 30 mpg? I know from my own experiences that replacing the exhaust gas oxygen sensor, if it needs replaces, can be 7 mpg. In my case, from 21 back up to 28.
Next is driving style. Also, clean air filter, good spark plugs and wires, good cap and rotor if applicable, tires at the max. pressure on the tire sidewall, not the doorjamb sticker, excess junk removed from trunk, and best of all, install a vacuum gauge and always drive for highest vacuum at lowest rpm. If your car has a 5-speed, you may find that in some instances, you're better off to shift down a gear rather than almost-lugging the engine.
And while you're at it, how much idle vacuum do you have? And for that matter, which engine? Does it have a performance cam in it? and IF you have the '84 4-speed, it was offered with a choice of "econo" or "sport" ratios, the latter of which won't get 30 mpg. I tried. 3000 rpm at 65 mph, even with the 2.5, just won't sip fuel.

Atilla the Fun · Post by **Atilla the Fun** » Mon Sep 15, 2008 10:31 am

also, there's the initial timing, if yours has a distributor. Are your injectors clean? Is the TPS adjusted correctly? Check every sensor. Particularly the barometric pressure sensor. Also, when was your last 4-wheel alignment? how are your bushings? have you hit any curbs or potholes since your last alignment?

Starlite528 · Post by **Starlite528** » Mon Sep 15, 2008 5:00 pm

Why aren't I getting 30? I've never gotten that good in this car, ever. I've not gotten much better than 20, tops, even after replacing the whole engine. It's the 2.8 v6, in the 88 Formula (auto). Stock.

I also notice in the last few months, it idles higher than normal, around 1500. The O2 sensor is less than a year old.last 4 wheel alignment was a long time ago, and I need new shocks and tires too (dunno about the springs.)

It was running really crappy the other day. Today I put new plugs and wires in, so we'll see how it runs later on. Taking the decklid off sure makes life easier!

I've never had a vacuum gauge, so I have no idea what the car runs at.

Here's a pic of the old plugs; yes there's an oil leak around the rocker arm cover, and I'm not going to do anything about it until I have a new EGR tube, and maybe injectors (original, I'm sure) to replace all at the same time.)

Series8217 · Post by **Series8217** » Mon Sep 15, 2008 6:27 pm

I really don't think anybody with a 2.8 is getting 30 mpg average besides pure highway driving with a manual transmission. Even then it seems kinda suspect, but if they're running in tip-top shape I could see it. You guys with autos are out of luck. Swap to a manual. If you just want gas mileage, put in an Isuzu 5-speed/2.5L drivetrain.

I got 30 mpg combined city/highway with my duke Fiero running in tip top shape. I've been on a lot of Fiero runs and cruises and never had to fill up nearly as much as the 2.8 folks when I was running the duke. The best I've had with the duke was 38 mpg on the interstate. The worst was 24 on a canyon road driving it like a race course.

I calculated the mpg by resetting my trip counter every time I filled up my gas tank. When I filled up I would write down the trip counter mileage and divide by the number of gallons it took to fill up the tank until the auto shut-off on the pump. I always filled up until the auto shut-off. The mpg varied by 2 mpg or so depending on where I filled up (probably a different auto shut off setting on the pump). As far as I'm concerned there is no other accurate way to measure your gas mileage without measuring the actual amount of fuel in the tank.

The best I've had with the 3.4 DOHC so far is ~29 mpg on the interstate. I have only been getting 24 mpg lately but haven't had time to figure out why. I think something is dragging or I have some poor wheel bearings..

Note that some people have inaccurate odometers/trip counters due to non-stock tire sizes. This will mess up the gas mileage calculation.