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Discussion Starter #1 (Edited)
I have already read about all of the other posts on mileage and what causes it to go down and I've tried most of the suggestions short of throwing parts at it.

Whats been done since I bought it:
Sixlitre tune-up
Replaced fpr
New 31's
Checked for vac leaks/all silicone lines
Checked for codes (just 11 and 111)

Whats gonna get done soon:
All bushings, bearings, and seals on front axle (except axle pivots)
All ball joints
Tie rod ends and adj sleeves
Coil springs and shocks
New rotors and brake pads
Fuel filter
Oil/filter change
Alignment

Whats wrong:
Filled up my tank about a week ago and got 200 miles from a 33 gallon tank (6 mpg :cry) When I first bought it in Oct of 2013, after a little cleaning and tuning up, I was getting 10mpg combined. What happened? I'm sure the fuel filter is pretty bad, but the tank and fuel pump were replaced my PO a few months before purchase (and I do have receipts so its legit). I know it wasn't taken care of as well as he said but it doesn't seem like it was abused too bad.
Questions:
1) I know I have a valve tick but could that cause such bad mileage? Its been there since I bought it but slowly getting worse.
2) Coolant temp sensor. Could this cause mileage issues if its the wrong temp? I know these engines need a 195 deg. to run best but I'm not sure if PO knew that (also replaced by him w/receipt).
3) Cooling system issues? Truck seems to take a while to warm up but I don't know whats normal for broncos as this is my first. Also some coolant ended up on my pas. side floor mat after pressure washing to get rid of salt and brine after snow. Heater core/core housing was not wet or dripping though. Has not happened since. I have been thinking about a cooling system flush though.
4) My brake booster hisses when pedal is held down at stop light or after I park and shut truck off. All vac leaks out within 20 seconds after shutting off if pedal is not held down, and takes about 8 seconds with pedal held. Is this normal for these? Bad check valve maybe? I still have a feeling there is a vac leak somewhere but I cant find with propane.
5) Can a bad alignment and front end parts (ones that will be replaced) cause bad mileage? Any thing else I should do while I'm down there?

Any other suggestions without just throwing parts at it? I'm sure it needs lots of parts but I need to find the mpg ruining ones first. When I have something else for a daily driver and can leave the bronco apart for longer than one day I will dig in and clean/inspect everything I can esp. in and around upper end of engine.

Truck runs strong and doesn't seem like anything is wrong, just the fuel gauge moves way too fast. And this is adjusted for 31's and 4.10 gears with the closest speedo gear I could get (its 10% off)

Any help is appreciated greatly. I love the bronco it just drinks too much for my liking.
Logan
 

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Coolant temp sensors are not rated temperature specific. You're thinking of the thermostat.

Bad alignment can lower your gas mileage. Think about it, your truck is having to expend more energy because it's fighting to go in a straight line when it's also trying to go to the left or right
 

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4) My brake booster hisses when pedal is held down at stop light or after I park and shut truck off. All vac leaks out within 20 seconds after shutting off if pedal is not held down, and takes about 8 seconds with pedal held. Is this normal for these? Bad check valve maybe? I still have a feeling there is a vac leak somewhere but I cant find with propane.
Not normal. You have a significant vacuum leak causing the engine to run rich. Replace the Brake Booster.
 

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Should be blowing warm air within about 5 miles regardless of temperature.
 

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aka: kemicalburns
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confirm you have a 195* tstat and if the coolant temp sensor is giving a false reading it will tell the ecu to richen the fuel mixture thus using more fuel. fix the booster as that vacuum leak will also have an affect. o2 sensor is still connected in the stock location correct?
also why did you change the speedo gear? doing a gear change doesn't make the speedo inaccurate, only running larger than stock tires will do that.
 

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Does anyone know if the valve tic is affecting his gas mileage?

What needs to be done to stop the ticking?

Thanks
 

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Discussion Starter #8
Coolant temp sensors are not rated temperature specific. You're thinking of the thermostat.
Oh, right, thanks.

Bad alignment can lower your gas mileage. Think about it, your truck is having to expend more energy because it's fighting to go in a straight line when it's also trying to go to the left or right
Right, but would it really make that much of a difference in mileage?

Not normal.
Yeah I didn't think so just couldn't find anything definite on it. Would that be the booster itself or the check valve?

Should be blowing warm air within about 5 miles regardless of temperature.
Really? It takes forever for my air to even get warm. Probably 10 miles of driving or 5 miles after idling for 5-8 mins.

confirm you have a 195* tstat and if the coolant temp sensor is giving a false reading it will tell the ecu to richen the fuel mixture thus using more fuel. fix the booster as that vacuum leak will also have an affect. o2 sensor is still connected in the stock location correct?
also why did you change the speedo gear? doing a gear change doesn't make the speedo inaccurate, only running larger than stock tires will do that.
How can you tell if its 195* or not? Also if I have the back window down the exhaust is pretty strong whether I get on it or accelerate normally in the first 10-15 mins of driving. And yes I believe O2 sensor is in stock location (little cross tube just before the Y). Not by much but I do have larger tires (31's); so did PO but he never changed the speedo gear. I thought regearing did require a different speedo gear? I did the calc for speedo gear teeth (can't remember what it was now or how many teeth I needed) and got that gear.

Does anyone know if the valve tic is affecting his gas mileage?

What needs to be done to stop the ticking?
This. I'm assuming its just a backed off rocker nut but haven't been able to look at it yet. Cause of bad mileage?
Logan
 

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Well if it's not stamped on the thermostat, stick it in a pot of water with the heat on it and have an instant read thermometer in the water. When something happens, that's the temperature it's operating at. Or just buy another one at that point if you're removing it, they're only $6.
 

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Discussion Starter #10
Yeah I'll probably just replace it if I take it out to test it. Any other ideas?
Logan
 

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Discussion Starter #11
Yeah I'll probably just replace it if I take it out to test it. Any other ideas?
Logan
 

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have you hooked it up to a scanner at all?
 

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aka: kemicalburns
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Sorry but that is wrong. Here is a calculator to show you the difference in indicated speed versus actual speed after a gear change.

http://www.csgnetwork.com/rgpspeedcalc.html
I guess I am confused. my 94 bronco has a tone ring on the carrier that provides reading to the speedo and trans. only when we install larger diam tires we then have to reprogram the PSOM. if I was to leave the stock tires on and change out the gears the PSOM wouldn't need reprogramed.
 

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:stupid
I guess I am confused. my 94 bronco has a tone ring on the carrier that provides reading to the speedo and trans. only when we install larger diam tires we then have to reprogram the PSOM. if I was to leave the stock tires on and change out the gears the PSOM wouldn't need reprogramed.
 

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OP has a '89. Speedo is mechanical with gear in t-case tailhousing IIRC. So, for him, R&P and tires will both effect the speedometer calibration. '89 also has a tone ring in the rear end, but it's for the RABS, not speedo.
 

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Discussion Starter #16
OP has a '89. Speedo is mechanical with gear in t-case tailhousing IIRC. So, for him, R&P and tires will both effect the speedometer calibration. '89 also has a tone ring in the rear end, but it's for the RABS, not speedo.
Yup.

Any other ideas? Can the valve tick be a serious thing? Does it need to be fixed immediately?
Logan
 

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It can be a serious thing, sure. Any noise can be. It can also be a 'personality' of the motor to the effect that it doesn't really hamper the performance of the engine and is just not serious enough to warrant a teardown. I've seen older engines with ticks continue for tens of thousands of miles just fine.
 
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