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Terrible Gas Mileage


mx71

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Ok, so my truck I bought has 173,000 miles on it. It seems pretty well maintained, but it's also likely the rings aren't great, and the valves could be out of spec. Other than those two things, anybody have any idea why a '85 4x4 z24 with properly aired 235/75r15s and well working brakes would get around 5-6 mpg? I haven't found any fuel leaks.

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Does your truck have the electronic feedback carb?  Because the mileage tends to suck on the 4X4 with out it, or if it's not working.

 

I never got over 14-17 mpg out of my 4X4 WITH the feedback carb.

 

That system HAS to work right to get decent mileage. 

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Look at the electrical connector at the right rear of the carb. If round and six wires you have the ECC carb and you will have an O2 sensor in the exhaust manifold. If you ahve the regular carb it will be a three wire connector no O2 sensor. Let me know which.

 

 

Either carb could have a stick choke that doesn't shut off, or the electric choke heater not working. With engine warmed up just remove the top of the air filter and look. The choke flap should be vertical. Is it? For a properly working choke heater the alternator must be working and charging.

 

If choke is open, start the engine and take another look. Is there gas dripping or wetness down inside? Carb could be flooding from float stuck or set wrong or needle valve stuck open with dirt.

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Nope, just the good old Hitachi. I typically get about 20 point something if I do a lot of local driving, about 21 to 22 if I spend most of the time hanging out around 55-60. Most of the freeways around the Twin Cities are 55 and 60, with a few exceptions. The book says I should be getting 21 on the highway and 19 in town, so I am not sure why my mileage is on the high side. Mine is also a California emissions version, and I almost always shift around 4000 when accelerating in 2nd and 3rd.   :confused:

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It has a motorcraft carb, which was identified as a Holley 5200 variant, which apparently is a licensed version of the Weber 32/36. So in a way, I have a really old shitty version of the Weber. There's only one wire coming out of it and it wasn't hooked up until recently. I assumed it was the choke so I hooked it up to the choke circuit on the 6 pin round plug (it does have an o2 sensor in the exhaust manifold). No idea if the choke works or not, but it seems to start easier since I hooked it up.

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I had it hooked up to the blue wire on the six pin connector, but tested it with the ignition on and it wasn't getting any power. I hooked it up to the red/yellow wire next to it which was getting 12 volts. Is that the fuel cutoff wire?

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Red is usually the idle cut solenoid.

 

Connecting the wire to the choke heater only turns it off after 5-10 min. It won't help you start as its cold and should be on anyway. Having a properly shutting off choke would clean the plugs and maybne make starting easier.

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Blue............................. auto choke heater

 

Yellow/Black (stripe) &

White............................ air ratio solenoid.

 

White/Green (stripe)..... Throttle switch... tells ECU if above idle.

 

Red &

Black.............................. idle cut solenoid

 

 

You only need the Blue for the electric choke heater. I doubt the Holly has an idle cut solenoid but if so then you just connect the Red wire. The solenoid will grounded return through the carb.

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Auto choke heater is run from a relay right on side of glove box on the upper part of the kick pad just out of sight. It needs a charging alternator to work or it may have been removed as not needed. It's a little blue or brown plastic cube about 1" square. The black fuel pump relay is also there but it's the size of a pack of Camels. Any ignition switched supply will work for the choke heater..

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