Fuel leak or is this really my mpg?

SPANGLTK

MEMBER
FORD F-250

Ford, f250 1982, 140,000 on the body and 90 on the rebuilt engine. (351 Windsor)

I purchased this truck to get me across the country. When the owner said poor gas mileage I kind of expected that but I wasn't expecting 5.8 MPG!

I took it to my mechanic and he looked at the carburator (2 barrel Holly). It wouldn't respond to him tunning it he took a further look and said it was packed with dust/sand. He rebuilt it and gave it back.

this time it had 5.7 MPG.

Figured out that the carburator wasn't the original one for the engine. He rebuilt it with a kit and thought since it was meant for a different engine that the Float / Power Valve was the wrong size and thinks he put the right one in. I do to because it doesn't drive rich any more. No more strong fumes or black smoke coming out and it drives great.

5.8 MPG

Could this be an internal fuel leak? Or is this all I'm going to get. I can't spend much more money on repairs and I'm leaving in a week. Any Ideas or advice?

ps not sure if this has to do with the fuel problem but when it idled it wouldn't be constant. Sometimes it would rev and other times it would idle so low it would die. It did this up until I changed the fuel filter or it was after float/power valve

4 Replies

INDYUKE

MECHANIC
416 POSTS

Tell us a little about your truck? Dually rear wheels? Scale weight? Tranny type? Rear end gear ratio? Even if you should have a monster of a truck, I'd think you'd get at least 10-12mpg with a 351.

You say 90 miles on the rebuilt, or 90,000? If only 90, then you will certainly get somewhat poor fuel economy until it gets broken in. Let us know.

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Sunday, August 26th, 2007 AT 8:43 PM

SPANGLTK

MEMBER
3 POSTS

No dually wheels, it's pretty basic the only after market thing done to it is that they put a larger size tire on it. The engine has around 90,000 miles on it.

I put the spark plugs in myself (autolites) and I learned last night about gaging. I hadn't gauged them, could that be the problem?

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Monday, August 27th, 2007 AT 10:31 AM

INDYUKE

MECHANIC
416 POSTS

Hmmm. If you have the auto C6 tranny with say 4.10 gears, then that would bring your fuel economy way down.

Double check your engine health. Manifold pressure at no load should be around 19 to 20Hg with no bouncing of the guage. That will clue you in if you have a vac leak, some burnt valves, incorrect timing or incorrectly adjusted carb.

Yes, your spark plugs should be gapped correctly. They often don't come from the factory to the correct specs, though that wouldn't create a large drop in fuel economy. Check ignition timing. Point gap and dwell too, though that will show more as a sputtering misfire. I hope this helps.

Sometimes I take an vehicle to the emissions station and have them run it on the dyno to see if it's burning cleanly. You have to pay them, and if the truck is old enough it's not required for state licensing where i'm at, but I sometimes do it just the same so I know.

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Tuesday, August 28th, 2007 AT 3:29 AM

SPANGLTK

MEMBER
3 POSTS

Well thanks for the tips. I'll check the gap on the spark plugs and see how that goes. It has been at the shop for a month before I got it, and he was rebuilding the carb, adjusting the float and power valve. It runs way better now and no longer runs rich. Which was the confusing thing for me. It was throwing out a lot of black smoke before but now it runs great. But there is still no difference in the MPG.

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Tuesday, August 28th, 2007 AT 9:53 AM