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quickbird144 07-30-2012 04:14 PM

Charcoal canister affect gas mileage?
All stock 87 GTA with Bosch III injectors and flowmaster exhaust, no cat.

I've removed the smog pump and had temporarily removed the charcoal canister.

Would this have any large affect on gas mileage? It also seems since I recently put the exhaust on, my mpg dropped a bit?

Put in 12 gallons, didn't completely fill her up, 2 days ago. So far have put 100 miles, all city, no highway, and I'm a hair under half a tank.

This is where the gauge was when I did 12 gallons:

soarestransam19 07-30-2012 08:16 PM

Re: Charcoal canister affect gas mileage?
Tire pressure plays a big roll in mpg make sure they are to factory specs, another is fuel filter will help mpg, also how is plugs an wires have u done a new distributor cap an rotor, also is the torque converter locking up around 35 mph it should drop rpm's in overdrive this is a common problem i had with my 89 gta 350 tpi. Replaced the lock up switch an it was good to go check these common things

merc3065 07-30-2012 09:12 PM

Re: Charcoal canister affect gas mileage?
also, heavy on the foot will affect gas mileage. Everyone who's driving loves to hear that awesome rumble of the engine when they have a new exhaust system on.

The charcoal canister is just an emissions thing. It sucks up the gas vapors and when you take your foot off the gas on deceleration a solenoid will open and the engine vacuum will suck in the gas vapors. That's all it does. Just make sure the vacuum line is properly capped off or it will affect gas mileage.

quickbird144 07-30-2012 09:18 PM

Re: Charcoal canister affect gas mileage?
Gonna check tire pressure and see where those are. Wires, plugs, cap, rotor, all that was done about 4k miles ago. Did air filter today, fuel filter is also new but that may need to be changed as well. My rpms drop when it hits overdrive. Just curious, what are your guys rpm around 60-70mph?

I'm rarely ever heavy on the foot, believe it or not. Most of the time I do the limit and with easy take offs.

My canister is completely out and all lines are capped except for the fuel vent line. I have no pressure, ever, when I release my fuel cap. And also I've been smelling fuel around the rear left tire on hotter days. Fairly strong.

Should you ever smell gas like that?

merc3065 07-30-2012 09:21 PM

Re: Charcoal canister affect gas mileage?
Yep, the fuel smell is normal.

There's a white pressure valve that is hooked up to the tank that will bleed off excess pressure when it gets too high.

RPM for my 350 and the 700R4 in overdrive is around 2200-2400rpm

quickbird144 07-30-2012 09:25 PM

Re: Charcoal canister affect gas mileage?
Ah. Well I guess that's right. As well as the RPM.

Well how about this merc. My 87 5.7 TPI canister vac lines were routed for a 86 5.0 TPI. And while it was all hooked up, I had a bad idle surge all the time. So I unhooked it all. I found the diagram for the 87 5.7 TPI. Is it worth hooking the canister back up? I'd imagine it'd work like it's supposed to then.

quickbird144 07-30-2012 09:26 PM

Re: Charcoal canister affect gas mileage?
Here are the two diagrams btw.

YumikoGTA 07-31-2012 11:51 PM

Re: Charcoal canister affect gas mileage?
I would say that your mpg is average for this type of car. Because you do not mention how much fuel was in your GTA before you added the twelve gallons. If it was almost empty when you put in this fuel, after one hundred miles of city driving, for it to be just below the half way point is fine.

On my GTA, which is also a 1987 5.7 liters, if I fill up the fuel tank completely and then drive it on the highway only, I can actually drive between sixty and eighty miles before the fuel gauge starts to move. Once it starts to move, it moves very quickly. Also if my GTA is almost empty, if I put in only two or three gallons of fuel, the gauge really moves up alot from just those few gallons.

From my memory, the 1987 5.7 liter with automatic transmissions was supposed to get about eighteen miles per gallon in the city and that was when it was brand new.

It also depends if you have your air conditioning on, how many people are in the car, if there are a lot of hills in the city, and how much traffic there is where you are constantly pressing the gas pedal off and on but not going far.


Fred91GTA 08-01-2012 01:29 PM

Re: Charcoal canister affect gas mileage?
The reason for the engine not running right is probably because either the canister you have is dead or it's the wrong one. There are two types of solenoids for the charcoal canisters. Normally open and Normally closed. Obviously, if you're using the wrong one, then the CC is purging when it shouldn't be because the solenoid would be doing the exact opposite of what the ECM is commanding. Get a replacement canister for your car and it should fix that.

It's interesting though that it was wired up like an 86 when it's an 87. Hmm....

merc3065 08-01-2012 09:44 PM

Re: Charcoal canister affect gas mileage?
Charcoal canister is disconnected from original post.

You don't have to hook it back up unless you want to be emissions compliant. Extra weight, new canister from GM runs about 200 bucks anyhow...

quickbird144 08-01-2012 10:14 PM

Re: Charcoal canister affect gas mileage?

Originally Posted by merc3065 (Post 434470)
Charcoal canister is disconnected from original post.

You don't have to hook it back up unless you want to be emissions compliant. Extra weight, new canister from GM runs about 200 bucks anyhow...

So it won't put a dent in my mpg?

Just got new tires today as my old ones were dry rotted and causing nasty vibrations all the time. More evident when I was coming to a stop. Rides 100 times smoother and vibrations are gone except for my shot engine mounts.

merc3065 08-01-2012 10:19 PM

Re: Charcoal canister affect gas mileage?
It wont affect MPG unless there's a vac leak.

I'm fighting with vibration problems with my 10 year old truck and wondering if it might be engine mounts...

quickbird144 08-01-2012 10:38 PM

Re: Charcoal canister affect gas mileage?
Interesting. So then, would this be considered a vacuum leak? The line I circled, and you can see the vac line right next to it. Do I need to plug this off? Or perhaps run that line and have it go up under the TB, but block the canister side off?

And yeah, I have a slight vibration at idle but can't feel anything now when cruising like I used to. Was under the car awhile ago and they are long long gone. Just need to build up the courage to replace them.

YumikoGTA 08-01-2012 11:13 PM

Re: Charcoal canister affect gas mileage?
That thing is connected to the egr valve with a vacuum hose and the other vacuum hose connects to the plenum. That thing is the egr selonoid. It operates with vacuum lines. If you do not have the egr valve anymore, then I do not know what you would need that component for. To have it disconnected like that, I would say it is considered to be a vacuum leak because it should be hooked up to either the egr valve or the plenum with vacuum hoses. Have you ever tried to close the opening while the engine was running to see if doing so changes anything with how the engine is running or if it makes any difference in performances or mpg?

It looks like the other vacuum line is next to it in the photograph to the right of it.

Also if you do have to change the motor mounts, that is a lot of work because most likely you also have to remove the front springs and lower control arms so you are able to reach the nuts and washers that are on the inside of the cross member component that bolts the motor mounts down. Good luck.


quickbird144 08-01-2012 11:20 PM

Re: Charcoal canister affect gas mileage?
Thanks Yumiko. I'm aware of what it was and it's operation, just wasn't sure if it was causing a leak or not. One side is in fact connected to the EGR valve which I still have. As you stated, the other runs up under the throttle body and connects there, and also splits off and goes to the canister.

I have not tried to close the opening to see what happens, but with it open, I get no SES light and no funky idle issues. If I've been running the past 2 months with a vacuum leak and the car running as good as it is now, I'd be surprised.

YumikoGTA 08-01-2012 11:26 PM

Re: Charcoal canister affect gas mileage?
So have you closed the opening that is located on the plenum that was connecting the vacuum lines to the egr selonoid and the charcoal canister? Or is it just left open? That would be considered a vacuum leak I guess.


quickbird144 08-01-2012 11:30 PM

Re: Charcoal canister affect gas mileage?
Well, that line I circled is all that's "disconnected" I guess is the proper word. Everything is connected on the other ends, it's just unplugged right there. So yeah, guess that would be a vac leak, just never thought of it that way.

Question. if I were to plug that opening off and idle goes all weird, what would be the issue then? I suspect my idle problems were from a faulty purge valve on the canister, but haven't picked up another to see if it works. Cause as soon as I plug it in, there goes my idle.

quickbird144 08-01-2012 11:43 PM

Re: Charcoal canister affect gas mileage?
So went out and checked it. As I put my finger over that open hole, nothing was happening. Hooked the line back in and took the open lines from the canister and plugged those off, idle got weird.

So I was puzzled for a minute. Then realized that the vacuum was coming from the port under the throttle body! So with that line disconnected how it was in the pic, I've had a vacuum leak.

Seems my option now is either just run a straight line from the EGR solenoid to the TB port, or just plug off the TB port and leave everything as it is.

YumikoGTA 08-02-2012 12:06 AM

Re: Charcoal canister affect gas mileage?
Yes that is what I am saying about where it is connected from under the plenum. If you do not have that plugged in or closed, you will have a vacuum leak from under the plenum.

I do not think that you should connect the egr valve directly to the plenum with a vacuum hose, because that would mean that the egr valve is always getting vacuum pressure. I do not think it is supposed to always get vacuum pressure, only when the computer sends the message to the egr selonoid. I am not completely sure if that is correct, so many other members who have more experience with it will know for sure. Good luck.


merc3065 08-02-2012 08:23 AM

Re: Charcoal canister affect gas mileage?
You've got it correct Yumiko. The vac line for the EGR should be going to the EGR solenoid. Computer wants EGR, pulses solenoid, allowing vac to hit the EGR to make the flapper move.

And yeah...vac leak heh good thing you found it!

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