6-7 MPGs--Help!
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6-7 MPGs--Help!
Okay--I did a rebuild last year on my 390. I've been getting it to run better and better over the last few months. So it runs pretty well (small exhaust leak to fix still), but I did the whole 'top it off-drive a while-top it off--difference in miles divided by gallons put in' and I'm getting about 7 MPGs.
That won't do.
What can I do about this? What can I adjust/tweak/etc?
I've already done the usual: timing, carb adjustments, tire pressure, idle adjustment...
That won't do. What can I do about this? What can I adjust/tweak/etc?
I've already done the usual: timing, carb adjustments, tire pressure, idle adjustment...
Post Fiend
Joined: Apr 2008
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From: Grants Pass, Oregon
Not my area of expertise, but i can only suggest that you go into much more detail of what you did, and whats in or on the engine.........Carb, intake, exhaust, cam, ignition, etc......Then gearing, tire size, For that matter are you satisfied that your odometer is accurate, because if it isnt, you literally dont know what kinda mileage your getting......Detail is KING, the more info you can provide, the easier it will be for one of the true experts here to diagnose the problem, and come up with a solution.....Good Luck!
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12 at vacuum T and 12 at carb.
At iggy: Tore down engine but did not remove cam or crank--all rings still working well--crank bearings good, new timing gears and chain, cleaned the hell out of the engine, new gaskets for entire engine, stock intake, 2150 2 barrel motorcraft, stock exhaust manifolds to Y exhaust single muffler. All vacuum lines are connected according to a diagram for my year and engine size (390). Removed the smog set-up except for the EGR valve (I did block of the EGR--sorry FMC--cause I needed the spacer for the carb.
PO mangled the harness, so I switched to a 1-wire HEI dizzy. New ignition switch as well.
Changed trans filter/fluid. Thinking I may need to adjust the modulator...
I'm sure there is more; it took me about 7 months to do everything.
At iggy: Tore down engine but did not remove cam or crank--all rings still working well--crank bearings good, new timing gears and chain, cleaned the hell out of the engine, new gaskets for entire engine, stock intake, 2150 2 barrel motorcraft, stock exhaust manifolds to Y exhaust single muffler. All vacuum lines are connected according to a diagram for my year and engine size (390). Removed the smog set-up except for the EGR valve (I did block of the EGR--sorry FMC--cause I needed the spacer for the carb.
PO mangled the harness, so I switched to a 1-wire HEI dizzy. New ignition switch as well.
Changed trans filter/fluid. Thinking I may need to adjust the modulator...
I'm sure there is more; it took me about 7 months to do everything.
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
Assuming that's inches of Mercury, that's way too low. You should be seeing upwards of 17 to 19 inches. Shut the truck off, turn both mixture screws in until they seat (gently), then back each one out about one and a half turns. Start the truck up, and slowly back them out until the needle reading is maximized (both screws should be at roughly the same position). The idle speed should climb as vacuum goes up, so you'll need to bring the idle speed back down too. It's a bit of a trial and error process. If the needle doesn't respond, you can't keep it steady, or it can't get high enough, then something else is wrong (such as a vacuum leak). Chances are, however, if it runs well, it should be fairly responsive and just needs to be tuned.
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MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
It's hard to say, since the actual problem has yet to be identified. This is just a start. Nonetheless, it's unreasonable to expect an engine to experience sufficient economy with low vacuum at idle. Improved vacuum certainly can't decrease mileage.
Low vacuum can correspond to wasted fuel, or some other problem that requires extra fuel to compensate. The engine spends a lot of time idling; fuel consumed here is part of the denominator in your mileage calculation. How the engine performs at idle also stages how well the carburetor transitions out of the idle circuit.
An engine is essentially a glorified air pump - higher negative pressure corresponds to increased efficiency (more air moved for less fuel consumed). Note that most of the "economy meters" of the day were really just vacuum gauges.
Low vacuum can correspond to wasted fuel, or some other problem that requires extra fuel to compensate. The engine spends a lot of time idling; fuel consumed here is part of the denominator in your mileage calculation. How the engine performs at idle also stages how well the carburetor transitions out of the idle circuit.
An engine is essentially a glorified air pump - higher negative pressure corresponds to increased efficiency (more air moved for less fuel consumed). Note that most of the "economy meters" of the day were really just vacuum gauges.
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Well I backed out those carb adjustment screws, slowly, till they were almost out, in fact the drivers side screw did come out, and the vacuum gauge only went up to 14.
Any ideas? The truck runs pretty good, so I wouldn't suspect a vacuum leak, but I guess I couldn't rule it out.
I've seen people blow cigarette smoke into an engine to look for vac leaks--good idea?
Any ideas? The truck runs pretty good, so I wouldn't suspect a vacuum leak, but I guess I couldn't rule it out.
I've seen people blow cigarette smoke into an engine to look for vac leaks--good idea?
Fleet Owner
Joined: Dec 2009
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From: NorCal
Well I backed out those carb adjustment screws, slowly, till they were almost out, in fact the drivers side screw did come out, and the vacuum gauge only went up to 14.
Any ideas? The truck runs pretty good, so I wouldn't suspect a vacuum leak, but I guess I couldn't rule it out.
I've seen people blow cigarette smoke into an engine to look for vac leaks--good idea?
Any ideas? The truck runs pretty good, so I wouldn't suspect a vacuum leak, but I guess I couldn't rule it out.
I've seen people blow cigarette smoke into an engine to look for vac leaks--good idea?
Otherwise, use a hose attached to a propane bottle (ummm.. unlit!!!) and crack the valve. Wave the hose end at potential vacuum leak spots. The RPMs will increase if ya find them.
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I saw the smoke approach used with the engine off.
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
Not sure how that would work, as the engine would need to be running so as to draw in the fumes. In general, however, cigarettes and engines don't mix. An unlit propane torch or a can of carburetor cleaner work pretty well.
Low but steady vacuum could also be indicative of improper timing or low compression. Maybe rule out a vacuum leak for now if there are no driveability issues. Have you done a dry compression test since the rebuild?
Low but steady vacuum could also be indicative of improper timing or low compression. Maybe rule out a vacuum leak for now if there are no driveability issues. Have you done a dry compression test since the rebuild?