Re: BAD gas mileage.

by ss2323ss Mon May 09, 2011 10:39 pm

Michigan must have tough winters !! We are lucky here , some of our best cruising is in the winter.... no bugs or tar !!! lol

by 74MonteCarlo Mon May 09, 2011 10:43 pm

Very rough.

by ss2323ss Mon May 09, 2011 10:46 pm

The wind and dirt is the bad part here!! Keep em inside on those days !!!

by ant7377 Tue May 10, 2011 10:23 pm

Take the plugs out oand look at them closely. Maybe you have a crack in one of the porcelin's .Might have a skip you just cant feel. Try some new ones.

by TWBouska AKA Wooderson Wed May 11, 2011 12:15 am

lagunainnewark wrote:How much gas is in the tank? My Laguna seems to go for ever until it hits 3/8 of a tank, then it drops like a stone. Wire wound fuel senders aren't linear, full to 3/4 may not represent the same amount of fuel as 1/2 to 1/4. The best way is compare from a full tank and track the mileage. Good Luck, Ron

funny you say that, mine stays at a half tank forever, then next thing i know its on E

by texan01 Wed May 11, 2011 12:50 pm

My '77 with a freshly rebuilt 2 barrel, somewhat optimized timing, and box stock 150,000 mile 305 and all its factory emission controls working including the original catalytic converter nets me about 12-14mpg doing commuter duty with the A/C on. Highway with the A/C on and 70mph speeds it gets about 17mpg.

It dropped like a rock for a while and turned out to be a leaky carb, stripped threads, and a bad fuel pump that would pump gas into the crankcase.

My gas gauge is funny, sometimes it works, but most of the time lately it's been sitting with the needle horizontal on full, then drops to E when it gets down to 5 gallons left in the tank.

by pila Wed May 11, 2011 10:20 pm

Texan01, you might want to check the ground wire that comes from the tank sender unit, and is grounded to the frame. The ground at the frame may need to be removed, cleaned up etc, and a new screw to fasten it to the frame usually helps. I use stainless hardware whenever I can for grounds..........

by texan01 Thu May 12, 2011 8:28 am

I would also suggest checking your catalytic converter but I doubt you've got one on yours. Mine is starting to slag down I think because I took it on a 200 mile trip last night and it got a stellar 13mpg and was having a hard time climbing hills.

Pila, I've checked the ground, it's in the sending unit itself when I unplug the wire and check it with a voltmeter it reads zero. Just need to drop the tank and replace it. Nice thing about southern cars, everything unbolts just like it was new.

by 74MonteCarlo Thu May 12, 2011 9:32 am

No cat.

Those didn't start until 75.

I have full dual stainless exhaust.

by bigredlaguna Thu May 12, 2011 9:45 am

The fuel gauge on any G3 I've owned has swung wildly at any level except completely full or completely empty. Just the difference between accelerating and braking (conservative driving-not street racer) swings the needle 1/4 tank.

That is why I don't really trust miles per tank/1/2 tank/1/4 tank claims. The grade of the ground is enough to make the needle show a different reading.

The only way to truly judge the fuel consumption is to fill the tank, drive and record miles, fill tank at the same pump and let it shut off the same way you did before, then do the math.

Other than that, I am inclined to think that your carb might be putting extra fuel into your engine. If it's a 2 bbl, they are one of the easiest to rebuild.

by 74MonteCarlo Thu May 12, 2011 9:48 am

It does swing a lot - always did. That's why I use parked in my level garage, as a true gauge.

by SeeTen Fri May 13, 2011 6:51 pm

Have you checked the choke to make sure it is opening fully when the engine gets up to temp?

by 74MonteCarlo Fri May 20, 2011 6:15 pm

Gotten many tips to smell the oil for gasoline.

Did that, and all is well.

Drove 27 miles today, and burned about 1/8 tank. I threw on my spare air cleaner lid upside down(so I didn't mess up the nice new paint on my good one). So, that seemed to maybe help a little. I still don't think it's where it used to be scratch

by ant7377 Fri May 20, 2011 9:13 pm

More I think about it that really is a lot of gas. maybe the motor just needs to be rebuilt . its got kinda low mileage on it right maybe its just old run a compression check on it id still throw in a new set of plugs too.

by bigredlaguna Sat May 21, 2011 2:54 am

One other thing I forgot to mention, it may be possible that the air filter is too restrictive or clogged up. I remember from times past that it was just the filter causing poor fuel economy.

Carbureted engines are more sensitive to this because of the nature of how it works. Might as well try a new filter before doing something major, because sometimes it really is the simple things that cause the biggest problem.

by 74MonteCarlo Sat May 21, 2011 9:02 am

It's already brand new.

by 74MonteCarlo Sat May 21, 2011 9:17 am

With the lid flipped, I did get a squawk out of the rear at half throttle from a stop...didn't even try to! Ha. Very Happy

by 74Malibu383 Mon May 23, 2011 6:30 pm

What about your vacuum advance? If they're not operating properly, or hooked up incorrectly, they can cause bad mileage.

by 74MonteCarlo Mon May 23, 2011 6:32 pm

I'll have to check that. Thanks Bryan.

by texan01 Wed May 25, 2011 8:56 am

My vacuum advance had died, I put a new one on mine last night, will have to see if that improves it. I had one distributor that had the mechanical advance sieze up.

I had to install a California emissions part, since the Federal part was special order. Oh well it gives a bit more advance.

Check the timing with a timing light and watch the mark when you unplug the vacuum line.

by pila Wed May 25, 2011 10:18 am

Temp of the coolant can effect mileage also. The best mileage is usually at design temp, like 195, with a bit more power when temp is around 160 or maybe lower. That's the purpose of cool-cans on the fuel lines before the carb. We used to use dry ice in those at the drag strip. Don't know if cool cans are still used (?) I don't get out much.......
Ignition timing at as advanced as it can be without spark knock would be optimum for mileage of course, which is why knock sensors are used in later ECM controlled engines, to keep timing advanced without being too much.

by bigredlaguna Thu May 26, 2011 12:50 am

I'm not sure of that pila, the only thing regulating fuel consumption on a carbureted engine is the carburetor. Your statement about coolant temp holds true for factory style computer controlled engines.

by 74MonteCarlo Mon Jun 13, 2011 2:38 pm

Drove about 20 miles yesterday...burned another 1/8th Embarassed

I let her idle to OT today and the gas needle went down. Just from idling.

It was dead on 1/2...now it's a little under.

-sigh-

I'm getting tired of this crap. I don't even want to drive it anymore.

At least I have my Plymouth.

by bigredlaguna Mon Jun 13, 2011 3:15 pm

I would suggest rebuilding the carburetor. You might have an air bleed orifice partially blocked. If it is a Rochester 2bbl the only thing simpler is a lawn mower carb.

You might first try squirting carb cleaner through any holes you see that are above the venturi and see if that helps.

by texan01 Mon Jun 13, 2011 7:08 pm

Have you pulled the fuel pump off to see if it's leaking past the diaphram?

Mine had that happen and mileage went to pot, but no visible leaks outside. I figured 20 bucks and 10 minutes worth of work to replace something that might be 30 years old would be good insurance. Turned out that it was a major fuel leak, and explained why it hardly ever used oil despite the valve stem seals being shot. It was pumping it into the crankcase. Performance never changed, though mileage went up about 3mpg.

fixed the vacuum advance and recurved the mechanical advance on mine the other day and it gets better mileage than my 95 Explorer with it's EFI V6. (20mpg vs 19) Check that on the timing light, and pull the vacuum advance and see if the timing moves when you unplug the vacuum line.

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