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1988 Ford Bronco 351w, c6, auto locking hubs, manual transfer case, 3inch lift, 33s
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Discussion Starter #1
I have a 1988, 351 running on 33s and consistently getting 7mpg. I have searched and searched, but I'm a noob and cant seem to find things I can directly apply to myself. I bought this because a bronco was always my dream vehicle, knowing they didn't have great mpg, I was not expecting 7... it was totaled, so I got it cheap, replaced side panels and hood and whatnot, to make it driveable, then noticed the drinking problem. I did a tune up, got a k&n air filter, removed the cat. And recently replaced the FPR because they are cheap. I don't know the history of the motor and what has been done to it, it's the c6 tranny, which definitely isn't helping. I was doing reading and saw that leaky injectors can be a common issue, and could smell fuel if I got on it. Soooooo doing not quite enough research, I went to summit racing and spent a pretty penny on 24lbs/hr injectors, not realizing that they weren't the stock 19lbs/hr. I have the truck torn apart, should I send them back and get 19? Or should i get a tune with the 24? Which would help mpg most? Nothing seems to be improving the gas mileage.

Anything is appreciated! Thanks
 

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yeah, 7 is pretty bad, Im getting between 7 and 12 in my 86 on 32s, running a carb. 351W that has been rebuilt, but I don't know how mild to wild it was built....it is a dog though.

I would start with checking everything on the motor, compression, cylinder leak down, vacuum leaks, check timing, plug gaps, exhaust leaks. if the motor is stock, I would stick with stock size injectors.

check for drag in the driveline, all u-joints, trans and t-case, rear diff. make sure brakes aren't dragging. check front hubs after a drive, make sure they are not hot. make sure the front driveshaft isn't spinning while in 2WD.
 

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The speed-density systems like you have aren’t easily tuneable. I would stick with stock injectors.

Your O2 sensor is another fueling component that can get lazy.

Many other sensors go into the fuel delivery tables, but if they are in the neighborhood of correct, a well-functioning O2 sensor should help compensate.

I wouldn’t expect much more than 10 even in a good state of tune. Unsprung weight is hell on MPGs and a C6 lacks gearing and is a power hog. Poor MPGs are part of the Bronco experience.
 

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2006 Lincoln Navigator
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Send the 24 lb. injectors back, you need 19s. 33" tires and a C6 is not helping you any. How are you calculating your MPGs? Has the speedometer driven gear been changed to correct for the taller tires?
 
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1982 FSB 302, c6, NP208 manual, manual hubs, manual windows, 3.00 gears LSD rear, 235/75/15 HK ATM
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410 Posts
A lock up type torque converter goes a LONG way to improving your fuel mileage with a c6. Check your gearing. If you are running anything under 3:55 such as 3:31 or 3:00 you are over geared and running a larger load than needed. If you have manual hubs, make sure they are unlocked when not in use. I would imagine you could have the duty cycle of your injectors reduced. This would be worthwhile if you planned on doing a little forced induction or n2o down the road, but not if you plan on leaving it stock. If you plan to leave it stock it would be easier and cheaper to buy the correct injectors. If you have steel wheels a set of lighter alloys can increase your gas mileage pretty significantly too.
 

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The larger tires create more rotational mass and this lowers fuel economy. The C6 doesn't help fuel economy. My advice if its running good, drive it and enjoy it.
 

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first things first - get stock injectors. and like @rla2005 said, make sure your speedo is accurate.

secondly - i would suggest making sure your engine is working at peak performance. a leak down and compression test would be good to complete. that'll confirm the integrity of the piston rings, head gaskets, etc.

you can validate the gearing of the differential by counting rotations of the tires and driveshaft. you'll need to look up thee procedure there. i don't have it memorized.

after you make sure your engine is good and performing correctly, and you have the correct parts on it, the speedo cable has the correct gear, and you know your gearing, you should be able to get a good accurate idea of your mileage.

also, you could check your timing to make sure it's properly set.
 

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Yo Jay,
Welcome!
Check Engine Light (CEL) comes on when the electronic engine control system is not working properly. The check engine warning indicator comes on briefly when the ignition switch lock cylinder is turned to ON, and should turn off when the engine starts. If the check engine warning indicator does not come on when the ignition switch lock cylinder is turned to ON or if it comes on while the vehicle is moving, the system is malfunctioning
If the CEL does not light up at all when starting it; then suspect that bulb is burnt-out or loose, socket was damaged by PO or shop, etc. or someone removed it, which does happen, unfortunately.

Try a Self Test for Diagnostic Trouble Codes (DTC)s by my pal, BroncoJoe19
Code Reader..... - 80-96 Ford Bronco - Ford Bronco Zone Early Bronco Classic FullSize Broncos
A helper is good to assist in reading Codes; best is to take a cell fone vid and replay it.
Some basics;
Visual Check
1.Inspect the air cleaner and inlet ducting.
2.Check all engine vacuum hoses for damage, leaks, cracks, blockage, proper routing, etc.
3.Check EEC system wiring harness for proper connections, bent or broken pins, corrosion, loose wires, proper routing, etc.
4.Check the Powertrain Control Module (PCM), sensors and actuators for physical damage; IAC, TPS I see was replaced, etc.5.Check the engine coolant for proper level and mixture.
6.Check the transmission fluid level and quality. See E4OD Fluid Condition Check Below)
7.Make all necessary repairs before continuing
8. Check headlights

The engine temperature must be greater than 50° F for the Key On Engine Off (KOEO) Self-Test and greater than 180° F for the Key On Engine Running (KOER) Self-Test.
Run it around to heat the engine up and shift thru all gears including Reverse.

Make sure A/C is off and transmission is in Park (automatic); or in Neutral for a Manual & release clutch.
Turn off all accessories; radio, lights, A/C, heater, blower, fans, etc. (close driver's door)
Then turn off engine and wait 10 seconds.
Do KOEO test First

You should see 2 digit code (s) if any
Post Code(s) here according to:
KOEO
&
KOER
***

Following applies to California, etc.
CALIF REFORMULATED GAS (CARFG) - SERVICE TIP
TSB: 96-10-7
very minimal mpg loss.

FUEL ECONOMY - EXPECTATIONS VS ACTUAL
99-26-9 for 91 F250 Revised


***
Fuel Saving Tipso
I have always factored in any cost to improve MPG in the overall $ "savings".
Free:
Reduce weight of the Bronco and tools, accessories and ... passengers; driver's get a pass; also, grille guards, side steps, etc.
Ensure that brakes are not dragging
Driving style/ light foot - Foot on Pedal too much while accelerating and high speed driving over speed limit
proper tire pressure - Under inflated tires Increase rolling resistance 1-2 mpg;
Dirty air filter Causes excessively rich fuel/air mixture 2.0 mpg;
Worn spark plugs Cause inefficient combustion, wasted fuel 2.0 mpg;
Worn O2 sensor Unable to detect and adjust air/fuel mixture 3 mpg (not it your year);
Dirty or substandard engine oil Increases internal engine friction 0.4 mpg;
Loose gas cap Allows fuel to evaporate 2.0 mpg
Potential loss in fuel economy if all of the above were neglected 11.4 mpg
Add them all up and you get yaris mpg. Lol

Check Timing
Try Sixlitre Tune-Up @ ignition upgrade and timing bump (no 56K)

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, any 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."

Or;
Blue/Gray Smoke:
PCV System: A stuck closed PCV valve will cause excessive crankcase pressure resulting in blue/gray smoke.
Oil burning in the combustion chamber. Pull a few spark plugs & ck iaw Spark Plug Diagnostic Chart
spark plug fault diagnosis
Oil Deposits Symptoms: Oily coating caused by poor oil control. Oil is leaking past worn valve guides or piston rings into the combustion chamber. Causes hard starting and misfiring.
by ASE Study Guide

& more Items that Cost $:
alignment
tune-up. plugs - pull and check/re-gap, replace if nec, same for plug wires (check for insulation cracks, and resistance (more on this later), distributor cap and rotor
brake parts - always $ WELL SPENT
I use Ford Motorcraft parts via AMAZON; they are VG at delvy and esp pricing; I bought a $150.00 DPFE Sensor (not incl. sales tax) for $50.00 with free shipping and no state sales tax; same for the IAC sensor.
Some buy from a Ford pn on-line or local parts stores that list the Ford PN, but the ads show the PN as a reference only and the supplier may be a company off-shore. other than a Ford supplier.
***

High Quality Motorcraft TFI Ignition Coil Attributes
Source: by SeattleFSB (Seattle FSB) @ MSD 8227 coil problems
Many Bronco owners shop for an ignition coil by looking for the highest voltage available. But I venture to say that there is much more to look for in achieving both a quality ignition system and saving money in the long run.
For clarification, it takes approximately 10-14,000 volts to initiate the spark across the OEM spark plug gap. After the initial arc, the voltage required to sustain the arc is much less and drops off significantly. So while you may have a 48,000v coil you can't actually get that across the plug. The extra power becomes reserve voltage which compensates for worn plugs, increasing resistance in wires and carbon fouling. This increased stress can require an additional 1-5000 volts.
Fact is a higher voltage coil does not work any better, it just lasts longer due to having a higher reserve reducing heat. You cannot push more than 20,000 volts across a spark plug without bad things happening. If you were to try you would see arcing down the side of the plug, across carbon buildups at the electrode end and out any weak points in the wire insulation and connections.
The bottom line is the ideal coil output required for normal applications is about 30,000 volts. So no, your coil does not need to be 48,000v for proper ignition. The benefit would be in having enough reserve to compensate for high resistance due to a worn or altered ignition system.
This is why the Sixlitre Tune recommends a 48,000v coil and larger spark plug wires – to compensate for a substantial increase in resistance from larger than specified spark plug gaps. You are adding resistance as the spark attempts to reach ground. This in turn causes the plug wires to break down and decreases the service life of the rotor, distributor cap, spark plugs and increases the chance of spark scatter within the Distributor Cap.
Think about it, you are setting your spark plugs at a maximum gap even before wear. The higher voltage coil does not reduce stress and wear on your ignition system; it only compensates within a larger margin and then ultimately becomes dependent upon the quality of construction for survival. When opening up your spark plug gap from factory specifications you must be prepared to check your secondary ignition system annually, as opposed to about 40,000 miles with a stock vehicle, or risk performance decreases down the road.
With that being said, IMHO the Motorcraft DG470 TFI Coil is one of the most dependable 48,000v TFI Coils on the market. This is largely due to the quality in design, testing and construction. I have personally had many dependability issues with other imported TFI Coils, such as MSD. Where a Motorcraft Coil has lasted 15 years, I have went through three MSD coils in five years. Your purchase of a TFI Coil should not be totally dependent upon the voltage, but strong consideration should also be made regarding the contruction attributes listed below:

Motorcraft TFI Ignition Coil Attributes
Insulation - Multiple coats on the primary and secondary windings to ensure no internal arcing
Radio-Frequency Interference (RFI) Suppression - Minimize electronic noise
Magnets - Hold strength for the coil to maintain proper energy output
Steel - Used in the lamination stack to ensure a consistent magnetic field needed to develop the required voltage
Coil Housing - Engineered to withstand extremes in temperature without cracking

Low Quality TFI Ignition Coil Potential Issues
Rough Running Engine or Misfires - Causing Check Engine Light
Fuel Economy and Power Decrease – Costs money and performance
Radio Frequency Interference – Affects radio, EEC, sensors, cell phone
Pre-Ignition and Detonation – Can cause engine damage
Weak Voltage Output – Can cause increase tailpipe emissions..."
***

For future mods;
see;
Project M.P.G. in a Centurion 460; miesk5 Note, Results are Comparable to Tests in other Ford Engines
Source: by performanceunlimited.com @
FOUR WHEELER Magazine's Project M.P.G.
Some use this information while others write that the results are not achievable.

COLD air intake - insulate air intake from grille entry area to throttle body.
Air Tube & Box Insulation pics in a 94 5.0; Ken used Reflectix Insulation, avail @ Lowes. etc. ST16025 - 16" x 25 feet. miesk5 Note; Ken installed the K&N® & removed the cold air intake tube that runs to the top of the radiator; but he could have installed the intake tube section later.

Source: by Ken B (Kenny's 94)

Here's the 1988 Bronco Dealer Brochure, part 1 by Ford via member sneal @ 1988 Bronco Dealer Brochure part 1... site upgrade rules...
1988 Bronco Dealer Brochure, part 2 by Ford via member sneal @ 1988 Bronco Dealer Brochure part 2... site upgrade rules...

For any Bronco questions or to chat about it's planned modifications or build, it's better to post each seperately in Noobie Bronco Tech Questions. Flame free zone. This will get more attention and you can build up your post count to get into other sections such as Bronco and Ford Parts/Accessories (75 posts required to participate due to scammers who preyed on our members).

To save you time and for better responses, please fill out your Bronco info with location, year, engine size, transmission type, transfer case type (manual or electric shift), locking hub type (automatic or manual) info & major mods such as a Lift, etc. .
Bronco info is now able to be put under your user name.
Click your profile button in the top right and go to account settings.


On that first page, named Account Details, scroll down to "Vehicle Info" and type in up to 100 characters.

Now you can simply enter your information in the text editor and click save.

Our Forum FAQs includes for example, How To Upload Images To Posts, How to Use Search and more tips!

➡ See Baba Looey's Favorite FSB Links (lots and lots of tech links) including, "how do I get the tailgate glass to...", etc.

Find time to Participate and VOTE in our next Full-Size of the Month (soon to change to Quarter) Contest and later in the year, Full-Size of the Year Contest @ Voting
You will get ideas by those competing.
Al
 

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1988 Ford Bronco 351w, c6, auto locking hubs, manual transfer case, 3inch lift, 33s
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Discussion Starter #9
I calculate the mpg with my GPS, because I watch the gps side by side with the speedometer and I know it's off, so I just take the miles driven with my gps, and divide it by the gallons used at every fill up
 

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95 5.8L MAF XLT, Hedman Shorties/MF SS Y & Muff, E4OD, Man hubs, KYB Quads, 31x10.5x15, 304K miles
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985 Posts
A few simple things to look into would be; what Temp Thermostat you have in there, if it is to cold your fuel system will be running to Rich in Open Loop and never get warm enough to get to Closed Loop. Your Temp Sensor that reads the Temp of the engine might be off. Also your Air intake sensor can have a part in this Fueling problem. These are Basic Parameters that can lead to Low MPG's. Check your vacuum at idle it should be around 20" give or take and the needle should remain steady (depending on what Cam is in there).
 

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86 BKO orig-5.0efi swap to 91 5.8efi w/AOD longtube w/2.5in to 3in hiflo cat 3in hp shorty flomaster
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Yeah 7 sounds low...I was managing around 14 with my 86 351efi swapped with long tubes and flowmaster hp shorty on 30s. Made me happy.
 

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Knowing how to accelerate efficiently helps. As does paying attention to braking habits.
But that won't get you down in the 7 range.
You might also want to be sure you don't have any brakes dragging, that kind of thing.
Assuming you do not have gear oil in the crankcase.
My 90 averages about 12-13 overall. Mostly open road, not a lot of city type driving.

Sent from my Pixel 3 XL using Tapatalk
 

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TBH IDK what you can really do with the setup you have. That C6 is the biggest problem. No overdrive and no lockup means it's gonna suck gas. Plus the old batch fire injection and speed density isn't helping either. On mine I did a free flowing Y pipe, sixlitre tune up, and I can get 13 towing, but I have a mass air 94 with overdrive.
 

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'88 XLT. 2" lift, 3G, Saginaw Pump, Headers, High flow 3" cat, 3" exhaust, 6 litre tune, K&N
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188 Posts
I would pull the sparkplug on each cylinder to read it, and compression test any cylinders that are suspect. One dead cylinder would explain 7 mpg. The other 7 cylinders are working harder to make up for the one cylinder that is just spitting out unburned fuel mixture. The dead cylinder could be from loss of compression or lack of spark so thoroughly check the spark plug wires, distributor cap, and connections on any cylinders that don't look like they are firing.

While you are swapping out injectors, I'd check your throttle position sensor, and look for any large vacuum leaks.
 

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I have a 1988, 351 running on 33s and consistently getting 7mpg. I have searched and searched, but I'm a noob and cant seem to find things I can directly apply to myself. I bought this because a bronco was always my dream vehicle, knowing they didn't have great mpg, I was not expecting 7... it was totaled, so I got it cheap, replaced side panels and hood and whatnot, to make it driveable, then noticed the drinking problem. I did a tune up, got a k&n air filter, removed the cat. And recently replaced the FPR because they are cheap. I don't know the history of the motor and what has been done to it, it's the c6 tranny, which definitely isn't helping. I was doing reading and saw that leaky injectors can be a common issue, and could smell fuel if I got on it. Soooooo doing not quite enough research, I went to summit racing and spent a pretty penny on 24lbs/hr injectors, not realizing that they weren't the stock 19lbs/hr. I have the truck torn apart, should I send them back and get 19? Or should i get a tune with the 24? Which would help mpg most? Nothing seems to be improving the gas mileage.

Anything is appreciated! Thanks
So assuming the vehicle is in good tune, the biggest factor that affects gas mileage is "the nut behind the steering wheel". That being said, 7MPG is pretty bad. I used to get 8MPG in my '76 Crew Cab F250 4x4 with a worn out 390 in it. My comparison, my '86 Bronco has a 351 - C6 - 3.55 and 33s and I got 12MPG going to eastern WA and back last summer. So start with the basics: make sure ignition components are in good shape, clean air filter, no dragging brake, and tires aired up properly. Then mind how you drive, I long ago realized that I'd rather have money in my wallet than look cool by hauling ass from every stop light.
 
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Stankyjay,

All of my Bronco stuff is carbureted, so I'm not too much help. After reading through the other posts (thanks everyone!), I'd say start with a new O2 sensor. 7MPG isn't right. K&N isn't going to fix it, new plugs isn't going to fix it - you've got a bigger issue here.
 

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Without a doubt get rid of 24 lb injectors, tune up, and poss consider changing rear end gear? If you break 10 mpg big win.
 

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Make sure there are no exhaust leaks ahead of the O2 sensor. From personal (read poor welding job), it does not take a very big exhaust leak to get lean readings at the O2 that make it go rich. It has the most effect at lower engine speeds and load, right where gas mileage is made.
 

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1988 Ford Bronco 351w, c6, auto locking hubs, manual transfer case, 3inch lift, 33s
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Discussion Starter #19
Thank you all so much! Lots of good info, I replaced the o2 sensor, no leaks in the exhaust, returned the 24lb injectors and ordered the 19s, so I'll do an update once I can drive it again!
 

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1988 Ford Bronco 351w, c6, auto locking hubs, manual transfer case, 3inch lift, 33s
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Discussion Starter #20
Any recommendations on a thermostat to get? From what I've read it seems helpful to change it
 
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