Bad Gas Mileage

[Oyaji]

Good intentions, but all you accomplished was to introduce confusion.

 

 

The vacuum canister being discussed here is the vacuum reservoir located behind the front bumper, NOT the charcoal fumes canister back by the firewall. The canister in question DOES store vacuum -- that is its only purpose. Without it -- or if it leaks or the lines to it leak -- any time you go up a steep-ish hill the drop in engine vacuum closes the ventilator doors and shifts all heat or a/c output to the defrost outlets on the top of the dashboard.

 

Roger - guess I should have read more carefully before I replied... But this thread is not about the vacuum reservoir - it is about making improvements to fuel economy. Any comments about how the vacuum reservoir can adversely influence economy? Would that be only through  potential vacuum leaks? (Also, besides providing a reservoir for vacuum to drive the HVAC servos, does it also help the cruise control servo to function during conditions of low vacuum? I can see how that might have an indirect effect on fuel economy as well...)

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Furthermore, neither you nor Hornbrod remarked on my mention about the need for proper overall gearing. Since Jeep owners are pretty well known for their modifications to their vehicles - in particular, the fitment of oversize tires - I wonder how much of the deplorable fuel economy results they posted over on the Fuelly website is due to just that. Looks like their 4-liter 6's average only 15mpg there.  (Compare that to my results on a 1992  Cherokee with HO 4.0: 22.5 highway, 19.4 city+highway, average  economy over 90,000+ miles, every single fuel stop logged.) Since the original poster asked for "every little thing" that could be done to improve his economy, I am interested to hear your takes on the effect of proper gearing. Is it a "little thing", or a big thing? Care to speculate on which has a bigger effect on fuel economy - driving style or tire size?

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In  my experience BOTH those things have  a big effect on economy... on an engine that runs at least decently, more than all other effects combined.       

[Rockfrog]

A question was asked, I answered it ... issues?

 

Then you go on to chastise me, while describing a completely different system. And in the grand scheme of things keeping the engine running close to prime, is important to fuel economy with a Jeep 4.0 ... although IMO economy and Jeep are mutually exclusive terms. But a functioning vacuum system is part of the process, and poor running engine due to a  leaking vacuum system will most likely waste more fuel than an EVAP system will recover in the same time frame. But that's a discussion for another day.

[Knucklehead97]

I am a little confused now, honestly. I didn't figure my slight tire upgrade would be that big of a problem towards gas considering theres people running 31's on stock with little to no problems it seems. I'm going to get the reccomended plugs and everything. Then I should also check all sensors and replace the vacuum canister/all vacuum wires connected that seem old?

[Knucklehead97]

Oh and I'm going to clean throttle body thoroughly and the airbox.

[FrankTheDog]

As far as your tire size and gear ratio are concerned, I'm running 30x9.5x15's also with 3.07 gears.

When I first bought the truck it was 2wd and had 225/75/15's. I averaged between 19.5 and 21mpg over a year.

When I converted it to 4wd and 30" tires it dropped to18-20 over the last year.

The difference in tire size has been corrected at the speedo gear and verified by GPS.

I try not to let the engine drop under 1800 rpm while accelerating or going up hills.

Dropping below that hurts the mileage slightly.

I could probably gain back most of the lost mpg by renewing the plugs, wires, cap and rotor. They have approx 50k on them now.

[Oyaji]

 

HO stands for Highly Overrated anyway. That's lugging it.

 

Uncalled for ignoramus maximus statement that does nothing to answer the OP's question.

Actually, with regard to the topic of this thread, I think he  might be right on target here.       

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Will no one else speak to the OP's problem (poor fuel economy)? I already gave my thoughts: improper overall gearing and/or driving habits.

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As I read his description, the engine seems to be running fine now with the exception of a small exhaust leak - is that  not correct? It would help if the OP could provide some additional background as to just when he first noticed a drop in fuel economy. Was it when the idle went bad?  When the oversize tires were first fitted? Any other clues?

[Knucklehead97]

I noticed bad fuel economy once I started driving it regularly a couple months ago. I've only have the truck since December of '12. I don't have a fuel gauge so I didnt keep up with gas until I really started driving and having to fill up alot.

[Knucklehead97]

And yes the engine seems to run great other than a small exhaust leak, and my oil leak I'm tryimg to find. Is there a certain smell the truck has when it's running rich? Also when I crank the truck the rpm's shoot up to 2000 and then go back down to about 550-700, is that what its supposed to do?

[Oyaji]

I was brought up on a farm,  so there was always a lot of equipment to keep running. Since he also worked repairing military airplanes, Dad had a very particular approach to maintaining our rag-tag "stable". When I started turning wrenches at age 8, I also learned to keep logbooks on everything we worked on, too - in some ways, similar to  the logbooks kept for planes. The early training stuck, and for nearly 50 years now I have kept logbooks for everything. I log not only all repairs/maintenance/parts for my vehicles, but every drop of gasoline and oil added, too. (Really, notes don't take but a minute, and I have found  that the minimal time spent in keeping them to be  quite worthwhile.)

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Any drop in fuel economy is noted at the next fill-up; if  3 fill-ups in a row show a in drop fuel economy that I cannot account for (like a comment "pulling heavy trailer" or "strong headwinds crossing Texas" next to the fuel price - yes, I log that too!), then I go looking for the reason why.

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One simple and useful tool in tracking down engine running condition is to "read" the spark plugs. Pull them and lay them out in order, and note the color of the ceramic insulator. What you want to see is a soft dry deposit light in both color and amount of residue, which indicates an engine running well. Here is a chart of examples rated from cold/rich to ideal to lean/hot:

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Barring an exhaust gas analyzer (as a substitute, I have a simple guage connected to an O2 sensor that reads air/fuel ratio in near real-time - it was damned useful when jetting 4 independent barrels of carbs on my 4-cylinder 914! :D ), this is probably the best place to start looking for evidence of engine running condition. Once you are assured your engine is running fine, you can move on to looking for other causes for your poor fuel economy.

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And finally, to answer one of your  latest questions - yes there is a telltale odor: an engine running too rich will smell of unburnt gasoline in its exhaust - the richer running, the stronger the smell. (Unfortunately, I don't have direct experience with the Renix system, so I cannot comment on a lot of specifics  like your question about RPM behavior on startup - but where general experience can answer, I do what I can.)

[HOrnbrod]

Some questions to the OP:

 

You swapped from stock ~27" tires to 30" tires. Have you installed the correct speedometer gear based on your new tire diameter and rear axle gear ratio?

 

How are you calculating your MPG? By dividing the gallons used between fill-ups into the total odometer mileage? If you haven't installed the correct speedo gear, the odometer results will be skewed.

 

First thing you should do (if you haven't already) is get everything calibrated, then check actual speed and mileage against GPS if you can. Establish a valid MPG baseline and work from there. Your mileage might not be as bad as you think.

[Oyaji]

Some questions to the OP:

 

You swapped from stock ~27" tires to 30" tires. Have you installed the correct speedometer gear based on your new tire diameter and rear axle gear ratio?

 

How are you calculating your MPG? By dividing the gallons used between fill-ups into the total odometer mileage? If you haven't installed the correct speedo gear, the odometer results will be skewed.

 

First thing you should do (if you haven't already) is get everything calibrated, then check actual speed and mileage against GPS if you can. Establish a valid MPG baseline and work from there. Your mileage might not be as bad as you think.

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Absolutely.

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Meantime while waiting for the replacement speedo gear, you could also work out the percentage difference between stock tire diameter  and multiply your current mileage data  by it as a correction factor. (Since  diameter has a linear relationship   with circumference, you could use either for your correction factor.) Note that tire size varies somewhat between manufacturers for tires of the same size designation, so for best accuracy, see if you cannot "Google" up information for the exact tires you have. 

[FrankTheDog]

With my truck when I compared it to the gps the speedo was right on but the odometer was reading 9.5 miles for every 10 miles traveled.

So I take my total miles traveled and divide it by 20 then I add that to the total miles and divide by the amount of gas.

[Knucklehead97]

Is there a formula for calculating difference in miles without using a gps? I sadly don't have a gps. And I think I smell the smell of it running rich... so o2 sensor needs to be checked? I'll be checking everything if I ever get a break from prior work. I really need to fix that gas gauge...

[Oyaji]

Is there a formula for calculating difference in miles without using a gps? I sadly don't have a gps. And I think I smell the smell of it running rich... so o2 sensor needs to be checked? I'll be checking everything if I ever get a break from prior work. I really need to fix that gas gauge...

Well, not a formula, but a method: distance divided by time, for instance "miles per hour". You will need an exactly  known distance,  a stopwatch, and a very steady foot on the accelerator pedal. Best is flat country where roads are in a N/S-E/W grid pattern - that way you can have confidence in the distance measurement (mile markers are NOT trustworthy in my experience, but country roads in the flatlands are  very regularly spaced, often with crossroads exactly 1 mile apart). As long as the distance is known exactly, you will be fine - curves tend to cancel out (left handers are slightly longer distance, rights slightly less, but there are usually about as many lefts as rights - and if you average in your return trip too, any error will cancel out), but hills can make it hard to hold a steady speed.

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I suggest a distance of at least 10 miles to provide a long enough sample for accuracy (the farther the known distance, the better), and a double-check on the return trip. You can check your odometer for accuracy in measuring distance., and then take that distance, divide it by  elapsed time, and convert it to MPH. Just make sure you are at your intended test speed when you enter your test run and that your start/stop times are accurate, or you cannot have confidence in your measurements when you are done.

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With regard to the  cause of the smell of unburned gasoline in the exhaust,  a bad oxygen sensor could be  the cause... But before fiddling with it, why not pull your spark plugs and read what they tell you first? It is easier to read your plugs than test the O2 sensor, ain't it?

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I am a believer in reading  spark plugs with the seasons (and any time trouble is suspected) and adjusting their gaps and squaring off the center electrodes  with a file  while they are out (note that I won't have platinum electrode plugs in my engines - I just use regular Bosch or NGK plugs - I have had much better results with them). You can keep tabs on the condition of any changes in how your engine is running that way (cylinder by cylinder - and if you install an underhood remote starter switch you can easily check compression while the plugs are out if you make the time - don't forget to log  the results, too!), and keeping the electrodes square-topped   and the gaps in spec reduces misfire. (Would you believe that even an engine that is properly tuned and is running well still misses fire about 3% of the time?)

[87MJTIM]

Is there a formula for calculating difference in miles without using a gps? I sadly don't have a gps. And I think I smell the smell of it running rich... so o2 sensor needs to be checked? I'll be checking everything if I ever get a break from prior work. I really need to fix that gas gauge...

 

If you have a smart phone, you can down load a running or bicycling app that has built in gps.  The app's are free to inexpensive. You just set it on you dash and drive.

[FrankTheDog]

 

Is there a formula for calculating difference in miles without using a gps? I sadly don't have a gps. And I think I smell the smell of it running rich... so o2 sensor needs to be checked? I'll be checking everything if I ever get a break from prior work. I really need to fix that gas gauge...

 

If you have a smart phone, you can down load a running or bicycling app that has built in gps.  The app's are free to inexpensive. You just set it on you dash and drive.

 

That is what I did. It's on my iphone.

[Knucklehead97]

I'll be pulling the spark plugs and checking them asap. And soon will be changing them just to do it. And hopefully my phone can run a good gps app. Ah the joy of having a MJ :D

[Oyaji]

Earlier, I made mention about the desirability of sharp square edges on spark plug center electrodes. Here are a couple pertinent lines from an article by a spark plug manufacturer on the subject:

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In addition, electrons are emitted where the electrical field strength is greatest. This is from wherever the radius of curvature of the surface is smallest, for example, from a sharp point or edge. It would be easiest to pull electrons from a pointed electrode but a pointed electrode would erode after only a few seconds. Instead, the electrons emit from the sharp edges of the end of the electrode and as these edges erode, the spark becomes weaker and less reliable.

 

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 The article makes a good case for iridium-tipped plugs (maybe I should give them a try - but I had very poor performance with old ignition systems and platinum electrode plugs and have been leery of hype ever since). You can read the whole article here: http://www.thumpertalk.com/topic/886615-takai-rtype-spark-plugs-series-i-ii/.

[mvusse]

I have tried a number of different types of spark plugs, and at least for my Renix 4.0, it seems to run best with the good old copper Champion plugs.

 

I had the same problem with my 94 Suburban. After spending oodles of money of 7 different brands and types of spark plug, it ran best with the stock AC Delco ones, and they were the only ones that lasted more than 6 months.

 

I'm sure there's a lot of good science behind new spark plug technologies, I'm also sure a lot of it is hyped beyond factuality. I will stick with stock replacement from three years ago on.

[FrankTheDog]

What ever the engine came with originally usually works best in my experiance. I run the stock champions in all my 4.0 HO's.

Never had ignition trouble.

[Oyaji]

One caveat about Champion spark plugs: do not use them in aluminum heads!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

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The simple reason is that, since they are plated with zinc, and since zinc alloys with aluminum, they will bind to threads in an aluminum head as if soldered. This is particularly troublesome in air-cooled engines because their head temperatures can run as high as 600 degrees, which makes the binding/"soldering" worse. When the plugs are next removed, the threads sometimes come right out with them! For that reason I recommend NGK or Bosch plugs, which are not zinc plated.

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I agree about using the same type spark plug as originally came in the engine. Perhaps the reason  I suffered with platinum plugs used with old ignition systems is that platinum plugs came out with modern high-energy ignition systems, and that the older systems deliver insufficient voltage for them. Can anyone check me on that?

[HOrnbrod]

What ever the engine came with originally usually works best in my experiance. I run the stock champions in all my 4.0 HO's.

Never had ignition trouble.

:agree:  I pull the plugs randomly and regularly to read. Throughout the years I've tried the Iridiums, NGKs, and others. Nothing gives a more consistent plug pattern the the standard Champion copper cores. The only thing I've changed is to drop down one heat range with the stroker due to the slightly hotter coil and the Unichip I have. The Champions give the best plug pattern and A/F ratio throughout the RPM range. Nothing else I've tried work as well.

 

EDIT after reading the above. The only alloy head manufactured you can get for our I6 engines is from Hesco. For $2K. They recommend using the copper core Champions with Permatex silver grade anti-seize. Air-cooled engines are not a problem with MJs, so who cares? (Obligatory multiple exclamation points)

[Oyaji]

Do I even need to explain here "who cares"? Some people drive other vehicles than Comanches,  doubtless many  (most? all?)  readers here do too - and many of those  vehicles DO have aluminum heads. Some people offer brand allegiance blindly - but in the case of zinc plating, that blind allegiance might have costly consequences for users of Champion plugs when they find themselves installing helicoils to replace stripped spark plug threads in their aluminum heads. Mention of a potential problem just might save some folks here some grief, so I threw that in in hopes of being helpful.

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Incidentally, I have always had good experience with Champions in old engines  with cast iron heads (especially 50+year-old farm equipment), but  there is no material difference between how they are made compared to any other copper-cored spark plug (besides the potentially troublesome zinc plating, that is). I grew to like NGKs because my Japanese motorcycles  ran well with them, and Bosch because my VWs, Porsches, and Jeeps  ran well with them... but all I use in all my old engines are copper-cored plugs with standard electrodes, regardless of the  plug maker.

[mvusse]

Yes, I do drive other vehicles. Besides a Jeep Comanche I also have another Jeep Comanche and a Jeep Cherokee.

 

I also have a 91 GMC Vandura camper, it has AC Delco plugs in it after I learned from my 94 Suburban.

 

Bosch plugs did great in my Suburban, for about 2 months. They worked great in my Geo, for about a year, and they didn't work worth a crap in my Renix 4.0, inconsistent idle and random misfires, even after rechecking the gaps and then trying another set to see if I got a defective batch. They were the last plugs before I changed to the stock copper Champion ones after replacing the distributor cap, rotor, wires and coil did not fix my problems.

[Oyaji]

For anyone whose  interest was piqued by the mention of the importance of sharp edges on the center electrode, the owner of a shop I worked at in my youth had a  racing tip to share, which I pass on here. He used to carefully bend the ground electrode out of the way, drill a shallow hole into the center electrode of his plugs, then carefully bend the ground electrode back and reset the spark gap. This adds significantly to the amount of sharp edge offered to enhance spark (though the removal  of material does speed up erosion of the electrode). When  I first saw this, I was skeptical  of the benefit, just as I was of that of sharp square edges... but in the intervening 35 years everything I have read  along with  my own practical experience and that of others has borne out the truth of this concept.