Cold weather mileage - discussion

It has been well below zero around here for several days and I commute 130 miles a day. Even with a new thermostat, my engine temp never gets above about 150. I am pretty sure that keeps it running open loop (Renix).

My mileage dropped from around 20mpg to around 10mpg. I'm getting ready to put a piece of cardboard in front of the radiator.

Anybody else ever see this.

Maybe there is something in the gas? I definitely lost a few mpg to the recent cold weather too. I don't have issues getting my '99 to temp, but my intown numbers dropped from about 14 to 9.5 on my last tank. I was assuming it was a mix of winter gas, and the fact that I'm usually where I want to go just after she gets hot. Glad to know i'm not the only one seeing this though.

old_man and Lightfoot; I believe some will say there is a thermostat correction in order, or anti-freeze is dying, and not to block radiator like what was customary in times past. I'll leave that to the experts.

In Alaska, when the fuel mix is changed for winter.., come summer, (we only have a few weeks of spring, and fall), the fuel tanks often have ice cubes, or slabs of ice banging around until they melt away. What I do, as many others, is to get a 'turkey baster', and a gallon can of acetone from the hardware store. Before fueling, I introduce a couple of ounces of that, and then mix it all up with high octane. In layman terms; Dries out any water, and I believe it helps to atomize the fuel, have no icebergs, and engine runs 'mo-'betta. Usually get an extra mile, or two per gallon of fuel, and a bit more in the summer. I've used it in a '74 Chev., '79 J.Cherokee, 2006 Kia, and currently in my '89XJ. Has not been deteriating any fuel 'O'-ring seals, rubber hoses, etc., to any vehicle I've been running.

So far this year, my routine at minus 10 degrees f., is to separate the electrical plug from an oil pan 250watt Moroso heating pad. After idling for about three minutes, it takes my XJ about 3 miles to get heat out of the dash vents. XJ won't go below 16mpg at this minus temp., and rarely gets 20 in the summer, (sans the Moroso).

I think it's worth the extra pocket change to get the highest octane possible from the pumps in order to insure a complete burn in extreme cold. LOL.., I've other major worries. Moose, drunks, cops, slippery roads, and half way think something is going to break! Also, I try not go faster than 45mph, (which also helps the mileage figures), but sometimes have to go to 50, 55, on a 50 mile two lane road.

I'm running a fairly new 195 degree thermostat.

IDK about Renix, but my 00 will run at 195* even at 25 below Zero. Partially blocking the radiator with cardboard helps speed the warm up to temp and gives a bit more heat in the cabin.

If you have consistent 150* temps verified with an IR temp gun, I would suspect a thermostat problem.

It may be a "newer" thermostat, but if you are never getting above 150, the thermostat is not working right. Speaking from many years of experience in running my '88 XJ with a three core in temps well below zero! I've even run into this problem with a new out of the box thermostat. Superstant or OEM only is all I use now.

You can block portions of the radiator with cardboard as a stop gap before you get a chance to replace the thermostat. Been there, done that, got the Tshirt... I zip tied mine in behind the grill and it helped, but did not cure the issue.

Even with a new thermostat, don't expect miracles in the fuel mileage department in below zero weather to what you see the rest of the year. All the fluids (motor oil, gear oil, transmission fluid, etc...) are cold and thick, and guarenteed if you didn't check your tire air pressure in the real cold, its down a few psi. Just part of dealing with real cold. My fuel mileage is much lower in January than November, and its all due to the temps (all winter fuel at the point...)

Any other mods to the cooling system that may factor in?

Renix will go closed loop well below 150 F, probably as low 70 F from my experience (evidenced by the live O2 sensor data), but it does use a little more fuel at 150 F, maybe a 3 mpg loss. If the O2 sensor is not working properly, the MPG drops by 50% in my experience with Renix, like from 18 mpg to 9 mpg.

Get you old analog meter out and check the live O2 signal, and check for 12-14 volts to the electric heater in the sensor (large orange wire and a black ground, there are two black ground, 4th wire is the data, 0-5 volt, should be crossing the 2.45 V, back and forth at idle, peak swing at idle about 1-4 Volts). Also, in my experience, renix will run very cool (yes even 150, mine at 160 F, on 100 F degree days with AC on, when the O2 sensor was not working.

That said, It also sounds like the new T-Stat may be a dud, as well, but checking the O2 sensor first is fast and easy.

old_man said:

I'm running a fairly new 195 degree thermostat.

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I was, too. It was a Stant that I bought from Napa. What gave me a clue was the lack of heat in my wife's XJ vs my MJ. Got out the laser temp pointer and verified about only 160* at the heater hoses. I bought a new stat from the dealer, 83501426, and installed it. It's got a vent hole which needs to be positioned at 12 o'clock and that hole has a "jiggle valve" in the form of a BB.

Heater is hot now.

And yes, at the temps you're describing, the Renix is staying in open loop.

I will try and swap the tstat this weekend. I am running a old school (before the new cheap crap) 3 row radiator. It regulates at 195 when it warms up, but 85 mph (65 mile each way commute) in zero temps only gives me 150 degrees. Screw the volt meter, I will pull out the SnapOn brick scanner and take a look at the open vs closed loop condition.

Screw the volt meter, I will pull out the SnapOn brick scanner and take a look at the open vs closed loop condition.

That's the best way to do it. Let us know the results.

cruiser54 said:

And yes, at the temps you're describing, the Renix is staying in open loop.

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Just to clarify cause and effect here, it is NOT the 150 F keeping Renix in open loop, it is a SIGN of an O2 sensor problem, KEEPING Renix in open loop!!!!... and probably a bad (new) T-stat.The 50% drop in fuel mileage tells me the O2 sensor or circuit has a problem! IIRC he replaced his Renix ECU very recently!!!!

LOL, "Screw the volt meter", be sure to use dielectric grease instead of KY, LOL!!!:laugh3:

If he isn't getting heat and the temp is staying around 150 when running, the issue is in the cooling system. The Oxygen sensor is not and cannot keep the engine temp at 150. Even with a failed Oxygen sensor, my rig had no issues staying hot. Bad fuel mileage and idled poorly yes, but the heat was fine.

Fix the cooling system issue first.

MNgopher said:

If he isn't getting heat and the temp is staying around 150 when running, the issue is in the cooling system. The Oxygen sensor is not and cannot keep the engine temp at 150. Even with a failed Oxygen sensor, my rig had no issues staying hot. Bad fuel mileage and idled poorly yes, but the heat was fine.

Fix the cooling system issue first.

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I agree to fix the cooling system first.

But, I think you're misunderstanding Ecomike. He's saying it is going into closed loop even at 150* but that the oxygen sensor is the cause of the poor mileage.

I'm not sure I agree. The only way I'll find out is to hook up my DRB-II and monitor the system from start up and see what temp it goes into closed loop at.

Those of us who have eliminated the heater control valve. I'm assuming since it always runs through the heatercore, that we are going to experience decreased mileage?

This is not an issue I have, but just wondering if changing over to dual electric rather than the one belt driven fan might help.

Well it depends, if you can cut both fans off to prevent it from over cooling, than yes. I've noticed that as the weather here is getting colder, that I am no longer reaching 210. I might consider checking my thermostat, but my heat works fine. I have straight lines to the heatercore instead of the Valve though. I'm going let it run a good while and see what sort of results I get, but when I drove to work this morning it didnt reach temps.

I'm not so sure that the thermostat is that bad. I think that running the heater at full blast when it is below zero provides enough cooling to keep the temps down, even if the thermostat is pretty much closed. You would be surprised how much heat a heater core can dissapate.

The next time you are on the trail and you are overheating, turn on the heater full blast and watch the temp drop like a rock.

My issue, that I've had in the past is the thermostat sticking open, thus causing way low temps.

Anyone that is thinking of trying Alaskan89XJ's trick of adding acetone to your fuel. Should reconsider it after what I am about to tell you. I am a aerospace and sae engineer by trade and have run into many issues from people adding chemicals to gas and not knowing how it will react. You might want to think twice about doing this

Do you know how 95% of all Acetone is made? From oxidized Isopropel Alcohol, or IPA. Meaning IPA is left out in the open to oxidize until it turns into Acetone as a chemical reaction to the atmosphere.

This is not Ethynol Alcohol with is in most fuel mixes of concentration of 10 - 15%. Acetone and IPA are known moisture collectors and chemically react when mixed with Gasoline. Acetone oxidizes metalic surfaces when it comes in contact with Hydrocarbon based chemicals as it reacts to the hydrogen in the gas.

In otherwords, this will eventually take the metal parts of your fuel system. Your tank will get messed up and contaminate your fuel. It oxidizes your fuel ines, the fuel rail, causes deposits to form on fuel injectors, and in some cases can take out the fuel pump.

DOT has numerous articles regarding this and even warns that the materials used in fuel lines, rails, and steel tanks prior to 2005 were uncoated and have no protection from todays modern fuel mixtures and advises changing fuel lines in all vehicles prior to this date to prevent fuel system blockage.

I was skeptical too after reading this DOT material until I had to change my fuel pump in my 93 XJ a year ago and saw the inside of my fuel tank and how corroded it was. And that was just from fuel and occasional injection cleaners.

I changed out my fuel lines too. I cut one of my lines open and saw it was as ugly as my tank. My tank has since been coated and all my lines replaced.

Now I live in the Seattle area and had people add anything from IPA, MEK, and Acetone to their fuel only to have their vehicles eventually fail to start and in some cases die on them while driving.

My advice. Know what you are putting in your vehile and what the hazards are when doing so.