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Old 01-20-2011, 08:24 AM
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EFI to carb conversion

I have an 89 Econoline with a 300/6 that only gets 15mpg. I'd like to improve upon that figure. I've read that there are plenty of mods that can be done to the carb versions of this engine to get into the 20+ range. In fact, the older I6s I've had got around 22 mpg without any mods. So, I am thinking that maybe an EFI to carb conversion is the way to go. I do not know if this is even possible. If so, I have created a list of what I think I would need. Any help would be appreciated.

Different intake and exhaust manifold
Carb with breather
Old style distributor
Ignition contol box
Ignition coil
Various connectors

The van needs new exhaust anyway, so I've even thought about eliminating the CAT. I figure I can get a lot of this stuff from a junkyard. Well, except for the carb. I just assume get a new one. What say ye?
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Old 01-20-2011, 08:56 AM
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I think it would be hard to beat the EFI for fuel economy IF the EFI is working correctly and all sensors are working correctly. How many miles does it have on it? Have you ever changed the O2 sensor?

It is very common for fuel economy to fall off on EFI engines as the O2 sensor degrades. They are typically good for about 60K miles. My Town & Country, for example, always got me about 21 MPG to and from work for years on end. Then I noticed that fuel economy was falling off. It got down to 16 MPG. At this stage, the engine was still running fine and I did not notice any loss of performance. I replaced the O2 sensors (it has 2) and the mileage instantly jumped back up to 21 MPG.
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Old 01-20-2011, 09:27 AM
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Well, I've read that 15 mpg is pretty much what these EFI I6s get. Even the MPG stats at fueleconomy.gov put the mpg at 15. It's total crapola because I know these I6s are capable of much better fuel economy. There just doesn't seem to be much that can be done with a computer controlled system to make any improvements.
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Old 01-20-2011, 10:05 AM
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There are plenty who get up in the 20s with their EFI 4.9s. Although, with a full sized van, it's hard to say if that's possible simply due to the size. Dunno.
But I agree, try a simple tuneup, check the computer codes to see if there's anything that may be failing, replace the O2 sensor, and see how it goes.

If not, you're on the right track. Although you don't need to replace the exhaust manifolds. Replacing the stock carb exhaust manifold with the ones from an EFI engine is a very popular performance upgrade.

You'd also need to address the fuel, since the EFI pump is MUCH higher psi than the carb can handle. A fuel pressure regulator would be necessary.
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Old 01-20-2011, 10:16 AM
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Thanks. I wondered if I would have to do anything about the fuel pressure. I an guessing that a regulator would fit inline somewhere and have some sort of dial on it for adjustments?
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Old 01-20-2011, 10:18 AM
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I believe so. I've never installed one, but I've heard it's not hard. The EFI is around 40+ psi, and the carb needs around 6 - 8 or so.
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Old 01-20-2011, 10:36 AM
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how much does your truck weigh? what are your rear gears? when is the last time you had a major tune up? do you have any vacum leaks? these are the questions I would adderess first. you have an awful lot of things to change to put a carb on.depending on your trans, you may even have to change that, because once you remove the computer, it can't shift.
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Old 01-20-2011, 10:42 AM
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The van had a tune-up done last week, including plugs, wires, cap and rotor, and fuel filter. It's running the way it should with no codes. If it purred any better I'd name it Leo (as in Leo the lion). It has the 3.08 rear end and the C6 tranny. Don't know for sure about what it weighs, but it's your basic E-150 van conversion that started life as a panel van.

You say the three speed C-6 won't shift without the computer? That would be a project killer for me. I don't mind changing stuff out, but I am not willing to replace everything. I'd be better off looking for an older van on Ebay.
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Old 01-20-2011, 10:49 AM
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A C6 is one of the most durable and dependable automatics ever made, but its never been known for giving good MPG.
If you have 30" tires, you're turning around 2600 RPMs at 75mph, which is pretty high RPMs for the 300. High RPMs, no overdrive, an automatic with no locking torque converter, and a full sized heavy van probably isn't the best recipe for the greatest MPG.

I'd still try replacing the O2 sensor if its never been done. As stated above, they can really drop mileage without a performance loss or a sensor code. My brother's 4.0 inline 6 in his jeep dropped from 20 to 12 as his O2 sensor went out.
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1984 Ford Bronco. 300I6 Offenhauser C Intake Holley 600 4bbl, 31" BFG A/T, NP435, 3.55 rear 8.8" EFI Manifolds. 2.5" high flow cat/muffler.
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Old 01-20-2011, 11:20 AM
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Yeah, I will probly get a new O2 sensor. I know he didn't change that unless he didn't charge me for it. Fat chance of a mechanic ever doing that. LOL!
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Old 01-20-2011, 11:30 AM
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you may want to think about an aod trans. they are not electrically controlled, and with your rear gears, could put you over the 20 mpg you desire. the thing is, once you spend the money on the upgrade, will it work out that you did any more than break even? all of my 300's, through all the years weren't exactly great gas mileage champs, but man, they last.one truck I have has over 500,000 miles,and the efi motor has never been opened up. this truck is a 4x4 f 350, and gives me 11-12 mpg. I guess it paid for the bad economy in the long run, by running longer than any of the v8 trucks I have had.
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Old 01-20-2011, 11:46 AM
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A locking torque converter might help. Consult with an AT performance tech on whether a lower stall speed and/or towing upgrade in the transmission might help.
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Old 01-20-2011, 11:46 AM
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