Poor gas mileage?
Thread Starter
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Cross-Country
Joined: Nov 2023
Posts: 98
Likes: 3
Poor gas mileage?
I have a 1985 f150 302 manual, it has a carb and the motor is not stock. It originally had a i6 but then got swapped to a 302, when the guy swapped the motor he uses the duraspaek ii ignition systems, everything is hooked up as should be and everything works. The 302 has an auto lite 2150 on it, the motor was bored to a 306 and had a slightly bigger cam in it, nothing serious. It has 3.27 gears in the rear with a 4 speed with od manual, I don’t drive the truck that hard, and I still get around 9 mpg. Would anyone know what could be the cause or could give some tips to help?
it has a brand new gas tank and sending unit in it, I have also completely replace the ignition system with a new distributor, factory motocraft ignition control module, and new ignition coil, I never take the rpm higher than 4, normally stay around 2-2.5k.
any help would be greatly appreciated
it has a brand new gas tank and sending unit in it, I have also completely replace the ignition system with a new distributor, factory motocraft ignition control module, and new ignition coil, I never take the rpm higher than 4, normally stay around 2-2.5k.
any help would be greatly appreciated
Lead Driver
Joined: Apr 2002
Posts: 5,316
Likes: 1,243
Do you smell gas, as if I you have a fuel leak or as if you are running rich. Is the choke operating properly? Is the engine winding out in the power bands when shifting up building speed, running strong? Or is performance weak? 9 mpg is too low for a properly tuned engine.
Thread Starter
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Cross-Country
Joined: Nov 2023
Posts: 98
Likes: 3
Do you smell gas, as if I you have a fuel leak or as if you are running rich. Is the choke operating properly? Is the engine winding out in the power bands when shifting up building speed, running strong? Or is performance weak? 9 mpg is too low for a properly tuned engine.
Lead Driver
Joined: Apr 2002
Posts: 5,316
Likes: 1,243
Last tune up was? Spark plugs look clean and hot burning? Wires?
My experience with the 302 is limited but my neighbor had a carbed 302. It ran like a Swiss watch. A good match for a 150. He said he was getting 15 mpg. He wasn't racing it around. Proper tune up is a first step. Your carb may be adjusted rich. How's the exhaust system?
My experience with the 302 is limited but my neighbor had a carbed 302. It ran like a Swiss watch. A good match for a 150. He said he was getting 15 mpg. He wasn't racing it around. Proper tune up is a first step. Your carb may be adjusted rich. How's the exhaust system?
Tuned
Joined: Oct 2020
Posts: 341
Likes: 99
It could be your rear end gearing.
From research I have been able to do. It appears that when ford put 302's and a overdrive transmission they installed either a 3.55 or a 4.10 rear end.
It seems that the 302 likes a little higher rpm than the 300. Makes sense as far as the torque curves goes.
I changes from a 3 speed to a m5od, I got poor fuel milage much like yours.
When I ran across the differences in rear end gearing. I installed a 3.55 and now I run a average of about 15mpg
Although my motor is a bit more modified than yours i think.
From research I have been able to do. It appears that when ford put 302's and a overdrive transmission they installed either a 3.55 or a 4.10 rear end.
It seems that the 302 likes a little higher rpm than the 300. Makes sense as far as the torque curves goes.
I changes from a 3 speed to a m5od, I got poor fuel milage much like yours.
When I ran across the differences in rear end gearing. I installed a 3.55 and now I run a average of about 15mpg
Although my motor is a bit more modified than yours i think.
Thread Starter
|
Cross-Country
Joined: Nov 2023
Posts: 98
Likes: 3
It could be your rear end gearing.
From research I have been able to do. It appears that when ford put 302's and a overdrive transmission they installed either a 3.55 or a 4.10 rear end.
It seems that the 302 likes a little higher rpm than the 300. Makes sense as far as the torque curves goes.
I changes from a 3 speed to a m5od, I got poor fuel milage much like yours.
When I ran across the differences in rear end gearing. I installed a 3.55 and now I run a average of about 15mpg
Although my motor is a bit more modified than yours i think.
From research I have been able to do. It appears that when ford put 302's and a overdrive transmission they installed either a 3.55 or a 4.10 rear end.
It seems that the 302 likes a little higher rpm than the 300. Makes sense as far as the torque curves goes.
I changes from a 3 speed to a m5od, I got poor fuel milage much like yours.
When I ran across the differences in rear end gearing. I installed a 3.55 and now I run a average of about 15mpg
Although my motor is a bit more modified than yours i think.
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Thread Starter
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Cross-Country
Joined: Nov 2023
Posts: 98
Likes: 3
Last tune up was? Spark plugs look clean and hot burning? Wires?
My experience with the 302 is limited but my neighbor had a carbed 302. It ran like a Swiss watch. A good match for a 150. He said he was getting 15 mpg. He wasn't racing it around. Proper tune up is a first step. Your carb may be adjusted rich. How's the exhaust system?
My experience with the 302 is limited but my neighbor had a carbed 302. It ran like a Swiss watch. A good match for a 150. He said he was getting 15 mpg. He wasn't racing it around. Proper tune up is a first step. Your carb may be adjusted rich. How's the exhaust system?
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FTE Legend
Joined: Nov 2015
Posts: 30,912
Likes: 4,123
From: Angier, NC
Do you have a tach in the truck and if so what is the RPM @ MPH in what gear?
Also a AFR (air fuel ratio) gauge helps so you know just how it is burning the fuel.
To see if a higher RPM from a gear change will work you can try running a tank of fuel and not use over drive.
You may want to do a little math to see what the final ratios are in the transmission with the different rear gear ratios to see if the 3.55 will put the 302 in the sweet spot and the need to go with say 3.73 gear?
I have a 300 six manual granny 4sp with an over drive unit and a 2.75 rear gear.
I can tell you at 45 MPH and in over drive it runs richer then in 4th gear based on the AFR gauge.
I can also tell you I got the same MPG, avg 15, with and with out over drive.
The only thing it did is lower the RPM so I have less engine wear and if pulling a heavy load can split all the gears.
Dave ----
Also a AFR (air fuel ratio) gauge helps so you know just how it is burning the fuel.
To see if a higher RPM from a gear change will work you can try running a tank of fuel and not use over drive.
You may want to do a little math to see what the final ratios are in the transmission with the different rear gear ratios to see if the 3.55 will put the 302 in the sweet spot and the need to go with say 3.73 gear?
I have a 300 six manual granny 4sp with an over drive unit and a 2.75 rear gear.
I can tell you at 45 MPH and in over drive it runs richer then in 4th gear based on the AFR gauge.
I can also tell you I got the same MPG, avg 15, with and with out over drive.
The only thing it did is lower the RPM so I have less engine wear and if pulling a heavy load can split all the gears.
Dave ----
FTE Legend
Joined: Nov 2015
Posts: 30,912
Likes: 4,123
From: Angier, NC
Oh the other thing how sure are you the miles on the odom are right?
Running a different size tire then factory will throw the miles off.
Also how many tanks of fuel did you run to get the 9 MPG?
I know mine is on the money based on a GPS I have used to make sure it was good.
Dave ----
Running a different size tire then factory will throw the miles off.
Also how many tanks of fuel did you run to get the 9 MPG?
I know mine is on the money based on a GPS I have used to make sure it was good.
Dave ----
Moderator
Joined: Jan 2001
Posts: 56,984
Likes: 2,738
From: Virginia
I do not see any problem with anything you described with the truck so far. So that leaves timing.
1st thing to check is the vacuum advance. Pull the rubber hose off the distributor, and see if you have vacuum on it at idle. Also if you have vacuum on it at idle, does the engine slow down when you pull the line off? If it does, that means the vacuum advance unit is good. If you have vacuum on the vacuum line all the time, and the engine speed doesn't change when you pull it on and off, something is wrong with the vacuum advance. It's a big contributor to fuel mileage.
If you do not have vacuum on the distributor at idle, rev the engine while checking the line with your thumb. If you suddenly have vacuum when you rev the engine, that means the vacuum is on a ported source, which I personally like. But to check the vacuum advance in this situation, find a capped off vacuum port that has suction on it at idle. Move the distributor line to there while the engine is idling, the engine should speed up. If it does, that means the vacuum advance unit is good. If not, it has a problem.
1st thing to check is the vacuum advance. Pull the rubber hose off the distributor, and see if you have vacuum on it at idle. Also if you have vacuum on it at idle, does the engine slow down when you pull the line off? If it does, that means the vacuum advance unit is good. If you have vacuum on the vacuum line all the time, and the engine speed doesn't change when you pull it on and off, something is wrong with the vacuum advance. It's a big contributor to fuel mileage.
If you do not have vacuum on the distributor at idle, rev the engine while checking the line with your thumb. If you suddenly have vacuum when you rev the engine, that means the vacuum is on a ported source, which I personally like. But to check the vacuum advance in this situation, find a capped off vacuum port that has suction on it at idle. Move the distributor line to there while the engine is idling, the engine should speed up. If it does, that means the vacuum advance unit is good. If not, it has a problem.
Mountain Pass
Joined: Jan 2019
Posts: 183
Likes: 19
From: Columbus Ohio
If you do not have vacuum on the distributor at idle, rev the engine while checking the line with your thumb. If you suddenly have vacuum when you rev the engine, that means the vacuum is on a ported source, which I personally like. But to check the vacuum advance in this situation, find a capped off vacuum port that has suction on it at idle. Move the distributor line to there while the engine is idling, the engine should speed up. If it does, that means the vacuum advance unit is good. If not, it has a problem.
When I bought my Holley, the instructions said "THIS PORT IS FOR VACUUM ADVANCE" and it is constant vacuum.
FTE Legend
Joined: Nov 2015
Posts: 30,912
Likes: 4,123
From: Angier, NC
If it is a ported port and the idle speed is to high then it can have vacuum at idle.
As for what type of vacuum to run it is up to the person what they want to run.
Some say running manifold vacuum will advance the timing at idle and raise the idle speed,
You then adjust the idle speed back down but now when you shut the motor off the timing goes back even more for easier start up when hot.
Me I have always run ported vacuum.
Dave ----
Mountain Pass
Joined: Oct 2019
Posts: 130
Likes: 31
Richie the wrench 🔧
I have a 1985 f150 302 manual, it has a carb and the motor is not stock. It originally had a i6 but then got swapped to a 302, when the guy swapped the motor he uses the duraspaek ii ignition systems, everything is hooked up as should be and everything works. The 302 has an auto lite 2150 on it, the motor was bored to a 306 and had a slightly bigger cam in it, nothing serious. It has 3.27 gears in the rear with a 4 speed with od manual, I don’t drive the truck that hard, and I still get around 9 mpg. Would anyone know what could be the cause or could give some tips to help?
it has a brand new gas tank and sending unit in it, I have also completely replace the ignition system with a new distributor, factory motocraft ignition control module, and new ignition coil, I never take the rpm higher than 4, normally stay around 2-2.5k.
any help would be greatly appreciated
it has a brand new gas tank and sending unit in it, I have also completely replace the ignition system with a new distributor, factory motocraft ignition control module, and new ignition coil, I never take the rpm higher than 4, normally stay around 2-2.5k.
any help would be greatly appreciated
Tuned
Joined: Jan 2022
Posts: 416
Likes: 44
From: Edmonton
Interesting, this is the first I've stumbled across ported vs constant vacuum and now I need to go give myself 3 days of headaches trying it out. I do need to find a vacuum that isn't constant first though.