Horrible MPG

[Anonymous]

I just figured out what the MPG of our '78 Monarch with 250 was from
fill-up to fill-up. 12.7 MPG!! I know that these cars should be able
to get at least 20, even just with around-town driving.

So, I am wondering what might be wrong? The car seems to run well. I
installed new plugs last year, and a new air filter, fuel filter, cap
and rotor this year. Oil was recently changed. Could the stock
catalytic converter be partially plugged, and choking the engine?
With today's gas prices, I'd like to remedy this problem sooner rather
than later. Thanks for any help.

Dave

 

[hasa68mustang]

I seemed to have gotten about the same I got 11 in town and 16 on the highway... I added the cam, springs, and timing chain.. maybe Ill get 30 mpg now! maybe we can find something out...

 

[XFlow_Fairlane]

around town that 250 is gonna drink a bit of gas (I am assuming it has an auto trans) are you running AC?

 

[Anonymous]

I removed the AC belt since the system wasn't working properly anyway.

My '69 GTO with a built up 400 gets about the same mileage as this! Surely a 250 with a 1bbl carb should be sipping fuel a bit more slowly, no? I believe I have heard a lot of people say they can get 20+ from this engine in this car.

Yes, it is an auto trans.

 

[XFlow_Fairlane]

see I am getting 27mpg (or was until I parked it) BUT I was running a 5 speed (read OVERDRIVE) lean carb jetting and weak timing and driving ONLY highway miles. I think the best you might do is about 20 with intown driving only with that 250. how are your plugs? idle set high or low? good aircleaner and fuel filter?

 

[Guest]

Just in case....

Count the gallons on the reading on the gas pump, not the dollars.

I knew someone who was counting the dollars of the pump, and gas was about $2.00/gal. He ended up with half the miles per gallon, cause he was counting the money that he put in versus the miles driven, instead of the gallons that the pump read vs. miles driven at the end of a fill up.

We both just laughed.

Just info, whether prevelent or not.

 

[Anonymous]

You can tap a pressure gauge into the exhaust manifold to see if the cat is plugged.

 

[MustangSix]

After 27 years it would be a fair bet that it's not in the best of shape. Even if it were, cat technology has come a long way since then. The newest cats have very little restriction and there are almost no gains to be had from removing them. If you need a cat to remain legal, a replacement is a little pricey, but will be a better performer.

Keep in mind that 1978 was the abyss of performance and economy. The old vacuum controlled emissions control devices, mechanical fuel delivery, low compression, and less than optimal cam timing all came together to make for weak AND thirsty engines.

You might want to tune the car up a bit by inspecting the EGR, inspecting the distributor, and adjusting the timeing.

Pull the distributor, disassemble, and inspect the advance mechanisms. They should not be worn, the holes and bushings in the bob weights should be free and there should be decent lube on everything. The springs should not be stretched or broken. Inspect the vacuum advance to ensure the diaphragm is working and not leaking. Add platinum tipped plugs (gapped at .050-.060), a good cap and rotor, and good plug wires.

When you reassemble everything, advance the timing from stock specs. Most of these engines like a lot of timing, so 12-14 degrees BTDC at idle should be a good setting.

Check the air cleaner to ensure that the hot air flap is operational. Use a good air filter.

Check tire pressures and set them to the maximum recommended pressure (NOT max pressure), usually 30-32 psi. Get a good alignment.

A well tuned Granada can pull down milieage in the mid to upper 20's without too much effort. Yours should be able to do the same.

 

[CZLN6]

Howdy Dave And All:

While you didn't specify what carb your '78 250 has I'm going to assume it's a smog version of the Carter YF. If it is it may limit your adjustibility ranges.

In addition to Jack's great advice let me add-
*Run a can of fuel system cleaner through the tank and spray down the inside and outside of the carb with carb cleaner.
*Set the Automatic choke for the leanest possible setting. If you choke is working properly this will also lessen the choke/cold start idle speed and length of time. If your engine uses much choke, you might need to adjust the choke linkage to lower cold idle speed.
*Set the curb idle to the lowest possible setting.
*Adjust the low speed air screw to the highest possible vacuum reading.
*Adjust the accelerator pump rod to the least possible setting that will still allow smooth acceleration.

The biggest variable is fuel economy is the driver. Take some time to review your driving habits and consider changing what you do and how you do it when driving. For instance, I have a habit of starting the engine and then putting on my seat belt. That few seconds of idling the engine is not necessary and a waste. I'm working on changing that habit.

Adios, David

 

[Anonymous]

Use a good air filter.

That means stay AWAY from FRAM!!

*Set the Automatic choke for the leanest possible setting. If you choke is working properly this will also lessen the choke/cold start idle speed and length of time. If your engine uses much choke, you might need to adjust the choke linkage to lower cold idle speed.

Make sure the vacuum powered choke pull off is working or the choke will stay closed during warm up.

 

[Anonymous]

Well, you guys have given me a great list of items to check. I'm not sure exactly where I will start. Do you think it would be possible for me to swap out the stock cat for an aftermarket one on my own? Or would it be a nightmare? I thought maybe I could just cut the old one out, then install the new one with two small pieces of exhaust tubing in each end, and use some stainless band clamps and exhaust sealer to eliminate leaks? Simple job or big ordeal?

Carbs are kind of foreign to me, as are vacuum and mechanical advance mechanisms. I'll try to pick one thing at a time to check and play with and see if we can get these abysmal numbers up some.

 

[Anonymous]

Simple job or big ordeal?.

Simple job. Check pressure first, though. No sense in replacing what doesn't need to be.

Check your engine's compression.

 

[MustangSix]

Hard part is just cutting the old one loose. It helps to have a few tools available, and if you have an experienced car buddy, bribe him with a brewski or two to help (or at least watch). Sometimes even simple tasks like cutting a pipe might involve more than two hands.

 

[CZLN6]

Howdy back Dave;

How about starting with the free and easy items and then moving on to more expensive and difficult.

Free and easy-
*Air up the tire pressure.
*dejunk and unclutter to reduce weight.
*clean and regap the plugs to .050".
*adjust the carb idle and choke settings to the lowest possible settings.
*check and clean EGR valve.
*make a thorough inspection for vacuum leaks, gas leaks, broken or cracked hoses.

a little more complicated and some cost-
*set the initial timing at 15 degrees. (Cost of a timing light)
*Spray down the inside and outside of the carb with carb cleaner. Put a can of gas system cleaner into the gas tank. (cost of spray cleaner and can)
*replace air cleaner and fuel filter. (cost of filters)
*set the low speed air screw to the highest vacuum reading (cost of a vacuum guage)
*install a fresh 190 degree thermostat.
*switch to a lighter grade oil or synthetic at your next oil change.

A car this old, with an original cat, is sure to be in need in this department. Rather than messing with pressure taps fore and aft of the cat, consider simply removing it and replacing it with a temporary "test" pipe. If the test pipe makes a difference do what you need to, as for your locales requirements, for a cat replacement.

Don't forget to work on your economy driving technique.

Are you enjoying the journey?

Adios, David