The bronco in my sig is getting bad mileage and I was wondering if it needs an o2 sensor If it does I am S.O.L. being there is not a blace for one on the exaust pipes. the bronco has a 300inline with a feedback carb and a TFI dizzy

 

if it was efi you would need an o2 sensor. I would perform a tune up, check the timing and possibly rebuild the carb. how many miles on motor?

 

140,000 miles

 

Ford Truck 300 CID 6-cylinder, 1984-1985, feedback, carbureted

This pickup/Bronco application typically suffers from abuse, neglected service and a poorly designed manifold and feedback system. The vehicle will drive tolerably in open loop all the time, suffering from poor driveability and fuel economy (typically in the 8-10 mpg range). Check the mixture control solenoid on the carburetor for a varying duty cycle. A fixed 50 percent duty cycle is the open-loop default value. Check the oxygen sensor for switching (frequency) and amplitude. Verify powers and grounds to the ECU (computer - located inside, above the accelerator pedal). The ECU and 02 sensor grounds are on the firewall in the engine compartment low on the driver's side. Other critical inputs to the computer are the coolant temperature sensor and manifold absolute pressure (MAP) sensor (frequency-based), which both ground ahead and above the battery on the right inner fender well.

Manifold leaks are very common (both intake and exhaust). Check all manifold, carburetor mounting and manifold preheat bolts and gaskets for tightness and leaks (flowing propane around gasket areas makes this fairly simple). The exhaust manifold is a relatively thin casting and can be badly warped, particularly if the vehicle has been heavily loaded, used for towing or driven through deep water. A leaking exhaust manifold will dump raw oxygen on the O2 sensor and can make the feedback system attempt to go rich.

Carburetor throttle shaft wear is an area for particular attention. Some of the carburetor screws have a propensity for vibrating loose and having the carb literally disassemble itself in normal use. When rebuilding the carburetor pay careful attention to the reassembly of the accelerator pump components -- the washer goes on top of the diaphragm, not below it. This problem has been seen on several remanufactured carburetors as supplied from the rebuilder. Rejetting should be considered a last resort.

Air injection switching (upstream, downstream), exhaust system integrity, and catalyst function are areas of common problems on these trucks as well.

 

If it's a feedback carb, it will need an o2 sensor as well. The computer relies on the O2 sensor readings, so it can adjust the fuel mixture at the feedback solenoid (Fuel Mixture solenoid) acurately.

If it doesn't have a working o2 sensor, the truck will always be in open loop mode, and performance and mileage will suffer.


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If you have never changed the O2 sensor in 140,000 miles it is time to change it out. They are concidered a tune up item, and some books I've read say change the O2 sensor every 100,000 miles or so.

Hope this helps...

 

well it did help some, I just got the truck and it does not have an o2 sensor on it but I was wondering if it needed one because it does have an feedback carb. No I did not put 140,000 with out replacing the o2sensor