'72 U code Supreme...replaced my fuel pump because the old one started leaking around the seam where the upper and lower halves join together. Checked for leaks and didn't see any...
Drove about 60 miles yesterday and burned 1/2 tank. That's about 6 mpg. Before the swap, I was getting about 12 mpg.. how does that happen?

 

Unless it is pushing gas under (to much fuel pressure) the needle it usually doesn't. You might another fuel/ millage check and see if you get the same results. Does it smoke or show richness.... Tedd

 

Pull the dipstick to see if an internal leak sent fuel into the oil pan.

 

I had a problem with my '68 for 7 years where I was getting 8-9mpg, the car would not idle correctly, and at idle the carburetor was being choked with raw fuel. After rebuilding the carburetor several times, checking everything out with it and my spare carburetor, a mechanic friend of mine suggested I check the fuel pressure from the pump to the carb. It checked out at 10 pounds. I thought the fuel pump either worked or it didn't work and it was working so I didn't give it a second thought. Apparently, it was working too good! The book says the fuel pressure to the carb should be 5-7psi. A friend of mine who knows quadrajets inside and out said a quadrajet will handle a max of 7.5psi. So, I bought and installed a new fuel pump and, after 7 years of trying to figure this thing out, the problem went away! My new fuel pump is at 5.5psi and the gas mileage is around 13mpg.

Check the fuel psi between the pump and the carburetor. That might be your problem, that your new fuel pump is working too well.

Randy C.

 

All good suggestions. Car runs like a champ whether at idle or at 65 down the highway. Brand new Holley 770 Street Avenger as of May this year. No rich smell or black smoke out of the pipes.

My other thought was I made an adjustment on my parking brake around the same time I replaced the fuel pump. It doesn't drag when I'm driving and the rear wheels are not even warm after driving, but could the adjustment be off just enough to cause the decrease in mileage? Long shot, but I dunno where else to turn...

 

You're 100% positive you ordered and installed the correct fuel pump for your engine? Did you match part numbers from the old fuel pump to the new fuel pump? Cross-referenced the part number on the leaking (replaced) fuel pump with a known OEM fuel pump? Is it a re-manufactured fuel pump or brand new fuel pump?

With the vehicle in neutral, does the car roll up or down a hill EASILY? The car should roll EASILY in neutral. In fact, you should be able to push your car by hand on a level driveway, pavement, etc.

 

Brand New AC Delco fuel pump specifically for the U code 455. With the parking brake set and in drive or reverse, car doesn't move with my foot off the brakes.

 

I bought a new fuel pump that put out too much pressure. I returned it and got a replacement that also put out too much pressure. So my point is do not assume the fuel pump is OK just because it's new - check the pressure to be sure it's working as it should.

 

Unless I'm missing something, what does setting the parking brake w/ the car in drive/reverse and your foot off the brake have to do w/ adjusting the parking brake cable correctly? I'm at a loss for understanding what this method suggests. You set the parking brake cable as outlined in the CSM? Put it in neutral and determine if the car ROLLS. It should roll easily with no parking brake pedal. If it does not roll EASILY in NEUTRAL, then the parking brake cable is set too tight.

 

Unless the pump is leaking into the engine, I don’t see how replacing a fuel pump can kill economy that bad, and still have the car run normally. Any chance you cracked a hard fuel line, causing a fuel leak?

 

I get ya, but what other symptoms should I notice other than bad mileage?

 

If I had a leak that bad, I would have a strong fuel smell, correct? Also, I would see it running out of the line, wouldn't I? Besides, I'm pretty sure I didn't crack a line.

 

Sorry for the confusion Norm. When I made my adjustment, I didn't use the CSM. I set it just tight enough so that when I put it in drive or reverse, it wouldn't roll with my foot off the brake pedal. I didn't check it in neutral cuz I figured once it didn't roll in drive or reverse, I was good.

Are you suggesting that maybe it is the parking brake adjustment that's causing my crappy fuel mileage even though I have no drag or heat buildup on the rear brakes?

 

I would not adjust a parking brake as you did. If anything, I would adjust the parking brake so that the car barely stopped while in neutral with very little parking brake pedal travel - certainly not in drive/reverse. You realize the variable(s) here? Let's say your idle RPM is set to 1500 RPM, or 1100 RPM, or 1000 RPM. How much 'travel' of the parking brake cable occurs when you apply the parking brake with the car in 'gear' at 1500 RPM, 1100 RPM, 1000 RPM, 500 RPM? The higher the RPM the more parking brake will be required to hold the car in gear in your method. And, this method is a blind-man's approach. The CSM clearly spells out how to adjust the parking brake. Some 'wing-nut' didn't even have the cable routed through one of the parking brake 'guides' when I purchased my car. I had to completely loosen the parking brake cable, route it correctly, and then make the adjustment to the 'equalizer'. Hopefully you're making your adjustment to the parking brake cable at the 'equalizer' as identified in the CSM.

I have absolutely no idea if your issue w/ poor gas mileage is related to your parking cable adjustment. You must be suspicious though, since you asked the question. Read the CSM is my suggestion. Not much sense in owning one if you don't use it.

 

You have to realize the "wheels" are NOT going to heat up. So, if you're evaluating this based upon the heat of the wheels, that's a long shot. You'd almost have to see your rotor turn bright red before the wheel(s) would heat up. If anything is going to heat up, it's going to be the rotor(s) not the wheel(s). There's a very small amount of wheel metal interfacing with the rotors - held on by lug nuts.

 

My family was visiting relatives in Wisconsin during the winter - snow & ice on the roads - I was 15 years old w/ a learner's permit. Dad said I could/should start the car to let it heat up before we left our relatives. I thought it might be nice to pull the car forward about 20-30 feet closer to the sidewalk leading up to the house. The problem was (3-on-the-tree stick shift) I couldn't get the car to move forward - I figured it was the ice or snow under the car. So, I rocked & rocked forwards and backwards to get that car UN-stuck - but to no avail - it wouldn't budge. I'll bet I sat their popping the clutch in that car for at least ten minutes trying to get it to move - thoroughly convinced it was stuck in the ice/snow. Then, mysteriously, I began to smell this foul order like rubber was burning. Well, that foul order lingered for quite some time - long enough so when our family got in the car to leave, Dad asked - what the hell is that smell? Yeah, he wasn't too happy about me burning up his clutch while the parking brake was fully engaged. Oh ****.

 

Hey Norm, I see now where you're confused, so let me start again:
When I adjusted the parking brake, the car was not running and in park. Once I made my adjustment, I started it up, applied the parking brake and shifted into reverse. The parking brake held, so I shifted into drive, and the parking brake held in drive also. Since it held in drive and reverse, I figured I was good and called it a day. (And yep, I made the adjustment at the equalizer.)
As far as the heat buildup, I don't smell burning brakes either while driving or after I park, nor do I notice any issues with the rear drums getting hot either. I only mentioned the part about the parking brake being the possible cause of my decrease in fuel mileage because that's the only other variable in this equation.

 

As has been previously mentioned consider measuring the fuel pressure between the fuel pump & the carburetor - I believe it shouldn't exceed what - 7.0 psi for the new AC Delco fuel pump you purchased?
Double-check your math?

 

New fuel pump pressure max pressure is 6.5 psi...right where it should be.

 

Did you MEASURE that psi with a gauge?

 

The fuel pump has nothing to do with fuel economy. All it does is provide fuel to the carburetor when the demand is there determined by the float level. If your not leaking, then double check your math.

 

Drove 58 miles and burned 1/2 tank. Tank is 20 gallons. That means I burned 10 gallons.. that's about 6 mpg...

 

are you going by the gauge or actual gallons needed to fill back up from 1/2 tank.

 

Going by the guage...it's been pretty accurate up to this point.

 

Is there a gas shortage in your area? Seen any suspicious people walking away from your car/house w/ a gas can & piece of tubing?

 

I see maybe 4 likely problems-
1) You have a crankcase full of gas. Check oil for volume/smell
2) You Have an overflowing carb w/manifold drowned in gas (which you don’t))
3) You coincidently developed a gauge problem. Go to gas station & confirm what tank takes.
4) E Brake adjusted too tight (drum/wheel would be hot if so tight you noticed a big mileage drop)
5) You have been targeted by the supreme deity or punks that have created a mysterious fuel disappearance event.

 

You left out #(6). On Saturday you started the car to test the emergency brake cable w/ the car in gear, got a phone call went in the house, decided to take a nap, woke up 2 hours later and found the car still running. You can burn a lot of gas sitting at idle for two hours.

 

LOL..Norm's got jokes...

 

Checked the oil for fuel..all good.

 

Start all over again with a new test, betcha you get a different result... Tedd

 

^x2^

Fill the tank up to the very top/brim. Take it on a 25 mile ride (or however many miles). Upon return fill the tank up to the very top/brim. Divide the amount of gas purchased into the number of miles driven. This removes any fuzziness which might be associated with the gas gauge. Assuming the odometer is working correctly as designed you'll get a more meaningful number using this method.

You may or may not have witnessed how many gas gauges on many various types of vehicles are not necessarily proportional - I have over the many years of driving. That is to say, notice how on some/many vehicles you fill up the tank, and the needle barely even moves say for the first 50-75 miles (or whatever), then the needle starts to slowly count down towards empty, then when you're about 1/4 tank remaining (as viewed on the gas gauge), the needle moves rapidly to empty in a course of 5-10 miles. Perhaps the gas gauge was simply in one of these areas known as "The Twilight Zone".

 

Ok, so here's my numbers:

I filled it up last Wednesday, odo reading was 102,755. Just filled it up again this morning, odo reading is 102,851 and it took 10.4 gallons.
102,851-102,755=96 miles driven since last fill-up.
96÷10.4=9.2 mpg...prob right where I need to be, agreed? Oh, and BTW, the gauge showed just under 1/2 when I filled up today, so it seems it's pretty accurate.

I'll run the experiment one more time, but for now I'm going to put this to rest. I'll post another set of numbers once I get them.

Once again, thanks to all who gave their advice... this forum rocks!😎

Happy Oldsmobiling?
Dave

 

Dave, maybe pull a spark plug, from what you're saying I would guess they would be pretty black sooty. This certainly is a head scratcher.