Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Poor Fuel Mileage on 4.3 V6 with Quadrajet 17056250
don3194:
Well, when I get chance I'll check the gaskets and vacuum acessories. The only things that are hooked up directly to the carburetor are the distributor and the the vacuum cannister. I will recheck it, but those two items I did not have hooked up and still had the problem. I know my brake booster is going bad but I do not believe it is the problem. I will not eliminate it as I have not rechecked it since I noticed the high idle.
I also did verify my throttle blades do completely seal.
bry593:
Another thing to look for is intake manifold gaskets. If the gasket is leaking to the outside of the engine, it is much easier to find. Usually, you can start the engine and let it idle. Then pour some water on the intake manifold, along the valve cover and wait to see if the idle slows. If it doesn't, try the other side. I like this method because it is not flammable, nor does it dissolve your engine paint.
Internal vacuum leaks are more tricky to flush out. I've actually plugged the valve covers and put a vacuum gauge on the dipstick tube. Using this method, you should see pressure build as piston ring blow-by slowly pressurizes the crankcase. If you see vacuum, you definitely got a problem. If you see no change, then your engine is not well sealed.
Cliff Ruggles:
Another way to find a leaking intake gasket is to let the engine idle and one by one pull a spark plug wire from the cap. If it's leaking on one runner, which happens on occasion you will not get nearly as much engine miss-fire/RPM drop when that plug wire is removed at idle speed.
Until I outfitted my current 455 with the large port Felpro "blue" gaskets (steel core) for the Super Duty 455's I had 3 different sets of aftermarket large port gaskets fail. Each time they started leaking at one intake runner. It took a few years for each failure, but it was difficult to diagnose other than a slight miss-fire at idle speed.
I found the problem by gently removing one plug wire at a time from the distributor cap and listening for a miss-fire or pretty decent engine speed change. When I got to the cylinder with the leaky gasket the change in RPM was very minimal.........Cliff
don3194:
Well, I sure hope its not my intake gasket. I did just do those about 4 months ago. I haven't had a chance to get out and do any more diagnostic work, but I think tomorrow I can eliminate more possibilities.
don3194:
Idle still seems high after removing every vacuum operated item and capping the ports.
I do have a question though, when performing the tip-in procedure, should the power piston be seating as far down as the APT screw lets it?
I found that when I revved the engine up to 2000 rpm's, the power piston still seemed to be in an upper position. To adjust the APT I had to push it down and seat it through the vent so the APT adjuster tool didn't get caught on the pin sticking out of the power piston.
If it also helps I've driven it around a decent amount and found I get 11.5 mpg city and maybe 12 on the Highway. Those two numbers being so close is suspicious to me.
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