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I have a 1991 ford bronco xlt with a 302 5.0l ( single gas tank, shorty headers, straight piped. ) and i just recently did a tune-up ( replaced the spark plugs, wires, ignition coil, distributor cap and rotor. ) and before i did it, it was misfiring and getting 3.3 mpg ( 100 miles to 30 gallons. ) and now after i did the tune up its getting around 0.7 mpg ( 20 miles to 30 gallons. ) and when i start it / drive it, it smells like its dumping gas through the exhaust. Where should i start to figure this out? i don't want to dump a bunch of money into it and it still have bad gas mileage. ( only check engine light i have is Thermactor air bypass solenoid circuit failure code 552, which i had before the tune up. )
 

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aka: kemicalburns
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sounds like your Fuel pressure regulator has failed or you have some injectors that are stuck open. Is the o2 sensor hooked up? have you checked for vacuum leaks? Pull the vacuum line off the FPR and see if fuel comes out. then rent/borrow a fuel pressure gage and see what is occurring there.
 

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Discussion Starter #3
sounds like your Fuel pressure regulator has failed or you have some injectors that are stuck open. Is the o2 sensor hooked up? have you checked for vacuum leaks? Pull the vacuum line off the FPR and see if fuel comes out. then rent/borrow a fuel pressure gage and see what is occurring there.
there's no fuel in the vacuum line off the FPR and the o2 sensor is connected but ill check for vacuum leaks and the fuel pressure
 

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Ever pull codes to explain the initial horrible fuel mileage? Something is definitely up and instead of chasing parts, let the 'puter tell you something- most part stores will scan you for free....
 

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Yo Reb,
As broncosarge77 advised, or Self Test for Diagnostic Trouble Codes by my pal, BroncoJoe19 @ Code Reader.....

Also, any Black Smoke from tailpipe?
Black exhaust smoke is an indication of rich fuel condition. These are possible causes:
Fuel Injectors: A leaking or dripping fuel injector will cause a rich fuel condition.
Fuel Pressure Regulator: A stuck closed fuel pressure regulator will cause a rich fuel condition.
Fuel Return: A restricted fuel return line will cause a rich fuel condition. "
By ASE Study Guide
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Discussion Starter #7
Yo Reb,
As broncosarge77 advised, or Self Test for Diagnostic Trouble Codes by my pal, BroncoJoe19 @ Code Reader.....

Also, any Black Smoke from tailpipe?
Black exhaust smoke is an indication of rich fuel condition. These are possible causes:
Fuel Injectors: A leaking or dripping fuel injector will cause a rich fuel condition.
Fuel Pressure Regulator: A stuck closed fuel pressure regulator will cause a rich fuel condition.
Fuel Return: A restricted fuel return line will cause a rich fuel condition. "
By ASE Study Guide
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the only code i have is: code 552 ( Thermactor air bypass solenoid circuit failure ), there's no black smoke but the inside of the exhaust tip is black, and lately the trucks been idling at 1k rpm even when fully warmed up. I haven't had a chance to check for a vacuum leak yet because of the weather.
 

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Yo RebelLensPhotos,
Well now, as you are aware DTC 552 isnt the MPG issue but is addressed way below.

One more low mpg perp could be, it's common for Fuel Pressure Regulator (FPR) diaphragm to fail, allowing high pressure gas to leak into it's vacuum line, which then dumps straight into the intake plenum. This can flood the engine, and if allowed to continue, it can wash the rings out & damage the bearings. To check it, simply disconnect the vacuum line with the engine idling & inspect for the presence or any aroma of gas.
It may take a few seconds for the FPR to fill with gas and begin spitting it out the FPR's engine vacuum nipple.
Also;
Check Timing
Try Sixlitre Tune-Up @ ignition upgrade and timing bump (no 56K!).
Restricted air inlet or dirty air filter causes excessively rich fuel/air mixture
Worn spark plugs cause inefficient combustion, wasted fuel
Worn O2 sensor unable to detect and adjust air/fuel mixture without a DTC.
Dirty or substandard engine oil Increases internal engine friction.
Loose gas cap allows fuel to evaporate

⊙⊙⊙

552 - Secondary air injection bypass (AIRB) circuit failure KOEO indicates voltage output for secondary air injection solenoid did not change when activated.
CIRCUIT TEST KC - SECONDARY AIR INJECTION SOLENOIDS Thermactor Air Diverter (TAD) AIRD & Thermactor Air Diverter (TAD) AIRD
Diagnostic Aids
PCM uses secondary air injection solenoids to control Air Injection By-pass (AIRB) and Air Injection Diverter (AIRD) valves. AIRB and AIRD are used to direct air to either engine or exhaust system.
Perform this test when instructed by QUICK TEST or if directed by other test procedures. This test is only intended to diagnose:
• AIRB/AIRD solenoid valve assembly.
• Wiring harness circuits (AIRB, AIRD and VPWR).
Vacuum supply. CHECK ALL HOSES for leaks
• Powertrain Control Module (PCM).

To prevent replacement of good components, be aware the following non-EEC related areas may be at fault:
• Secondary air injection system (belt, pump, valve or reservoir).
• Blocked or restricted secondary air passages in engine.
• AIRB/AIRD circuit(s) shorted or open.
• Fault in solenoid or PCM.
Battery must be fully charged
Disconnect both solenoid connectors. Turn ignition on. Measure voltage between VPWR terminal (RED wire) of wiring harness connector and battery ground for both solenoids. If either voltage reading is less than 10.5 volts, repair wiring harness open circuit, and repeat QUICK TEST. If both readings are 10.5 volts or more, go to step 9).

9)Measure Solenoid Resistance :
Turn ignition off. Leave solenoid connectors disconnected. Measure resistance of both solenoids. If either reading is not 50-100 ohms, replace solenoid assembly and repeat QUICK TEST. If both readings are 50-100 ohms, go to step 10).
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I wouldn't do this part until you are certain that the TAB/TAD are ok and all vacuum lines are good.
10) Check Circuit Continuity:
When breakout box is mentioned, go to the EEC pin# instead.
Leave ignition off. Disconnect PCM 60-pin connector. Inspect terminals, and repair if damaged. Install EEC-IV Breakout Box, leaving PCM disconnected. Measure resistance between AIRB test pin and AIRB terminal at wiring harness connector. Measure resistance between AIRD test pin and AIRD terminal at wiring harness connector. If either reading is 5 ohms or more, repair open circuit and repeat QUICK TEST. If both readings are less than 5 ohms, go to step 11).

11) Check For Short To Ground :
Leave ignition off and solenoids disconnected. Measure resistance between test pin No. 51 and test pins No. 40, 46 and 60 at breakout box. Measure resistance between test pin No. 11 and test pins No. 40, 46 and 60. If any reading is less than 10,000 ohms, repair short to ground and repeat QUICK TEST. If all readings are 10,000 ohms or more, go to step 12).

12) Check For Short To Power Circuit :
Turn ignition off. Measure resistance between test pin No. 51 and test pins No. 37 and 57 at break-out box. Measure resistance between test pin No. 11 and test pins No. 37 and 57 at breakout box. If any resistance is less than 10,000 ohms, repair short to power and repeat QUICK TEST. If code is repeated, replace PCM. If all resistances are 10,000 ohms or more, replace PCM and repeat QUICK TEST


EEC IV 60 Pin Connector Pin Diagram by Ryan M

Legend by Ryan M at Ford Fuel Injection
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