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Old 10-08-2005, 08:40 PM
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11-12MPG!!! what the heck?!?!?

my 82 grand prix is eating gas like crazy, last filll up i got 11-12mpg. its got a 79 rebuilt chevy 350 4bbl stock besides edlebrock rv cam, performer intake, and 305 4bbl 416 heads(quote - doublevision:The compression ratio with a .041 thick gasket will come in around 10:1), and a brand new 4bbl q-jet. my timing is set to 11 base, and 10 on the adjustable vacum canister using ported on the carb. the transmission is a TH350 with 2.73 rear gears, exhaust is stock manifolds with dual exhaust and glass packs of some sort. i am running premuim fuel.

since the engine only has 500miles on it i am driving it very lightly, easy on the throttle. cant imagine the mpg if i drove it very agressivly.

is this normal for a car like this or have i built the motor with wrong parts to get any kind of mileage? is my carb needing adjustment or somthing?

i am planning a 8-10 hour round trip with this car in a week but its gona cost me a cuple hundred bucks for gas! getting mileage of a 1 tone pulling a trailer up a hill in the wind! well maybe not but you get my drift

please help me out

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Old 10-08-2005, 08:59 PM
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Sounds way out of line....I would expect at least 18-20.......maybe more. What is your timing set at, is the advance working properly?
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Old 10-08-2005, 09:04 PM
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10 base and 11 on the vacume, i have a adjustable vacume brand new accel one, yeah it works.

the motor came from my paressianne and on the 4 hour trip home with it, she got 27mpg on the highway cruisn the speed limit like always.
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Old 10-08-2005, 09:32 PM
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What is your total timing and when is it all in by? How are your plugs looking?
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Old 10-08-2005, 09:43 PM
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pugs not sure but will check tonight, total timing im not sure ethier, i can do that tommarow after work, to late to rev up the car now, nieghboors would come shoot me lol.

thanks for the replys
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Old 10-08-2005, 10:20 PM
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ok i got a picture of the 1 plug, they all look about the same, the threads are black oily and the end parts are tanish a pretty light coloring.
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Old 10-08-2005, 11:54 PM
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i was looking at the fuel lines and my new fuel pump has a ruber hose as a inlet, metal line as the outlet to the carb and a third hookup for somthing that is caped off with a little rubber plug and pressing the end of it sprays gas out! the plug is starting to get eatn up from gas and almost falling off.

what is this hookup supposed to be for? for some reason i felt the need to take some plyers and colapse the little metal hookup line thinking it would stop any leaking and it didnt. put a new piece of rubber hose with a bolt in the end to plug it but the hose is way to big and isnt sealed it off.

should i go buy a new feul pump again just make sure i get one without that third hookup or what would be a proper way to plug it off?
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Old 10-09-2005, 05:43 AM
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Check to see that you're getting full vacuum advance at part throttle (road load). I had a 265 cube '56 Chevy with the stock dual quad setup and no vacuum advance (factory setup). Was running a 4.11 rear. Best mileage was 12 mpg on the Ohio Turnpike. Yeah, that's a pretty low rear gear, but that's a pretty small engine, too.
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Old 10-09-2005, 05:44 AM
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That small outlet is a rerturn line to the tank, that some cars had. If not needed, it can be plugged off. Use a piece of proper sized neoprene hose with a bolt and 2 clamps...........or a proper plug. All gas lines should have neoprene on the rubber sections. Regular rubber hose will rot off in no time.

BTW, the plugs look pretty normal, I would replace them anyways.

Advance your timing a few degrees at a time. If it starts pinging going up a hill under acceleration, back it off a bit...........That should give you good mileage.

Adjust your mixture screws on the cab for highest vacuum..........
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Old 10-09-2005, 06:12 AM
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Make sure your engine is getting up into the 195 degrees coolant temp operating range. A cold motor will drink the gas like a wino at open bar.

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Old 10-09-2005, 07:00 AM
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Have you a tach installed..a slipping trans can raise havoc with MPG..also if there is any way to check to see if the jetting on the carb is on the money that would be good..

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Old 10-09-2005, 07:35 AM
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During break in it will use more fuel but mileage will improve later on, even at that 11-12 with your set up does sound WAY out of line. The fuel pump line could be part of it, get that taken care of and see what happens. What is it pulling for vacuum at an idle?

I noticed you said the motor came out of your Parieienne but it only has 500 miles on it. Was it getting 27MPG in this configuration in the Parisienne or before all the mods? Is this the transmission from the Parisienne too? My Monte Carlo was getting 12-13, turns out a lot of my poor mileage was a slipping transmission. It wasn't bad enough to be real noticeable driving it until it finally let go but I'm sure with a fresh tranny the current setup could get at least 16-17. I'm having a 200-4R built this Winter to replace it, swapping in a 3.73 posi and hoping for 18-19 with the current 350.
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Old 10-09-2005, 01:06 PM
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Quote:
Originally Posted by montea
my 82 grand prix is eating gas like crazy, last filll up i got 11-12mpg. its got a 79 rebuilt chevy 350 4bbl stock besides edlebrock rv cam, performer intake, and 305 4bbl 416 heads compression 10:1, and a brand new 4bbl q-jet. my timing is set to 11 base, and 10 on the adjustable vacum canister using ported on the carb. the transmission is a TH350 with 2.73 rear gears, exhaust is stock manifolds with dual exhaust and glass packs of some sort. i am running premuim fuel.

since the engine only has 500miles on it
please help me out
You probably have several things.

Fix the gas leak.

Excessive fuel pressure due to blocked off return line.

Carb calibration. first two items must be fixed first.

All G bodies from 79 up had lockup torque converters. Is yours operating? Mine cost 2 mpg. It should feel like another shift.. like shifting into overdrive, and I could easily hear/feel the difference in rpm below 50 mph. At what speed is the lockup occurring? I changed mine from 52 to 30 mph.

Distributor advance curve and the vacuum advance can, each must be calibrated to the other. As a last effort, sometimes manifold vacuum to the canister will help.

I don't think you will ever get over 18 mpg running empty on the highway at 65 mph.
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Old 10-09-2005, 08:12 PM
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-motor runs around 190 degrees

-yeah i have a tach installed and i ever since ive driven this car i feel like the transmission is slipping, it was rebuilt from a GM mechanic friend i known awhile back, not totaly shure what he did to it as he did it for exhchange of a few parts. when im just cruisn and i give it gas the rpms raise to 2,000 rpm range then it feels like its grabing, but it could possibly be the way its supposed to be because this is the first auto ive driven besides a integra probaly 4 years ago and thats a 4 banger fwd jap can.

-how do i see how much vacume it has? im guessing i need to buy some sort of vacum guage?

-the motor got 27 on the highway in the paressiane and it was all original stock, no mods. the transmission was also a TH350 but i never used it in my car, i used the original one from the Grand prix.

-on my gas cap i drilled a small hole because the return line was blocked off as i removed the charcoal canister. so the fuel pressure shouldnt be to high.

- i never knew anything about the converter locking up, i dont think i have ever felt that, next time i go on the highway i will see. when the transmission shifts gears its very light and barley feel it what so ever unless im excelerating fast. whats the way of changing it to lock up faster?
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Old 10-09-2005, 08:18 PM
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Quote:
Originally Posted by Hippie
During break in it will use more fuel but mileage will improve later on, even at that 11-12 with your set up does sound WAY out of line. The fuel pump line could be part of it, get that taken care of and see what happens. What is it pulling for vacuum at an idle?
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thats what I thought too.
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