Terrible mileage 99 GLE

Replace the coolant temp sensor. Mileage will come back after about 3 tankfuls (computer re-adjusting to new sensor readings).

CD

Thanks!!!! I will ask my nabe mechanic; he's fairly honest (I think).

Or is this so technical that I should go to a Nissan dealership (shudder)?

Do you have a ballpark on cost, so I don't go in "naked"???

Aspasia

DIY. Part costs like $20 at autozone. If your mechanic does the job will take him like 10 minutes of labor to install.

CD

Oh, and you really should be using 91 octane or better. The Maxima can run on 87, but mileage and power suffer.

CD

. . . . .and you'll find the sensor here (scroll down and you'll see the PIC);

CD

You guys have been way kewl in identifying the sensor problem(s)! I thanked you earlier, and want to do so again.

I am relieved to learn this is not a major $$$, so I can go to my nabe mechanic with advance information..

I discussed this long distance with my son (who knows about cars), and he wondered why the mechanics at CarMax hadn't done a simple test with the diagnostic gadget that reads out problems. I told him I didn't know WHAT CarMax mechanic had done; all I know is that they replaced the spark plugs and changed the oil, air, and fuel filters. So when I take the car back to CarMax (so they can fix the antenna they bent), I will try to find out whether they did a diagnostic test, and if so, what data it yielded..

QUESTION: One poster suggested I use high octane (91) gas; said lower octane is harmful. This has been argued back and forth FOREVER. I don't mind spending the extra $$$ if there's any serious research validating his point. Is it purely a matter of opinion, or can anyone refer to such "serious research"? Again, thanks, amigos - your advice much appreciated.

Aspasia

I don;t know if you've noticed, but it's been all me with the responses:)

Anyway, to answer your questions:

First, a computer diagnostic sometimes will not reveal a problem such as this one. These are issues where sensors go semi-bad, but not fully enough to trigger a code. I have expreienced this in several nissans (Maxima, Altima, Sentra, Quest) with coolant temp sensors and O2 sensors. The sensors don't go bad, they just go "funny."

Secondly, as for what octane your car takes, refer to the manual. It wil state that for maximamum performance, that includes for gas mileage, 91 octane or higher is better for the car. The Maxima's computer *will* adjust to take advantage of the better fuel.

Try a tankful of super and drive normally. Then when the tank empties, fill up with 87 octane. If everything else is equal, you will see that the tankful of super unleaded gave you better gas mileage . . .enough to offset the higher price of 91+ octane.

CD

I've done two years of serious research tracking my fuel economy. I used only 93 octane for several years before switching to 89. My fuel economy did not change at all.

I imagine under very heavy load there is a difference, but for routine driving you will not see anything.

Dave

Dave:

David Geesaman wrote: [...]

The higher octane rated, or equivalent to chemically pure octane, has no benefit to power at all. Nor is there a difference in the quality of the product. The octane rating numbers have nothing to do with quality or power produced.

Higher octane rated gasoline has one primary purpose in the internal combustion piston engine. The higher rated fuel burns slower.

If you have a higher compression engine, or that achieves higher mass charge and cylinder pressures due to super/turbo charging, the lower rated gasolines burn far too fast. In fact the sudden burning can seriously damage the pistons by melting and predetonation. The piston tops can be hammered into dents by pre-ignition. The higher octane rated fuels burn more slowly and evenly, and the ignition timing may be set to ignite in advance of TDC producing a longer burn in faster revving engines.

If you burned septane or heptane in a modern engine with more than a 9:1 compression ratio there would be a lot of pre-ignition detonation, and that would be quite audible as pinging or knocking. The method of slowing combustion with those fuels is to retard the ignition. Later ignition sparks reduces pre-ignition, however, the fuel is not burned so efficiently.

The computer makes the ignition delays, and other changes to induction timing and fuel metering, etc., and you may not be aware of the changes made to the engine functions to avoid the knocking.

Using low octane fuel in a high compression engine without a computer will result in knocking. The engine will not be using the fuel as efficiently and producing as much power with the low octane rated fuel.

The manual states the minimum octane for that engine. Its not a matter of quality or fuel economy; its a matter of the durability of the soft aluminum engine pistons over time. The advantage is with the high compression ratio. That creates higher cylinder pressures, greater charge and more power. If the engine runs with the ignition retarded pressure are lower and the power produced is less.

Its a matter of safety. A melted, holed or dented piston top can wreck the engine. You may not be able to hear the sounds of pre-ignition that the computer knock sensor detects. Stay with the recommended anti-knock fuels. Formerly, the chemical called tetraethyl lead was used to raise the octane rating, however, that shouldn't be used in engines that have catalytic converters. Fortunately, the engine designers built in the computers, and that makes it possible to use lower rated fuels. If so, stay off the throttle and minimize the charge in the cylinders to avoid knocking. Mixing rated fuels of the same type is Ok, until you run the proper rated fuel.

A higher rated fuel than what the manual recommends has no effect on power or fuel economy. Nor does it have any additional quality. Higher rated fuels than necessary are simply money wasted.

Ralph Hertle

Yes Ralph, all that is true and well-known. However, the Maxima's knock sensor will trip if you run lower octane fuel and adjust timing. The net result can be loss in both power and fuel economy. My experience, supported by the data I've collected, is that stepping down from 93 to

89 octane has not caused the computer to adjust and lose fuel economy.

Also, fuel economy wasn't the only thing I was interested in learning from tracking my mileage - I also know exactly what I'm spending on fuel per month and how many miles/day I drive. Makes oil changes very easy to keep track of.

Dave

Going down from 91 to 89 octane is a small step down, so any loss should be minimal, and, if you haven't noticed a power loss perhaps your driving habits are not so "enthusiastic" :)

I know that once I had 87 octane in my 98 Maxima and the moment I floored it, all computer hell broke loose. The car started pinging, then the computer adjusted and I felt a huge power loss. It didn't like 87 octane at all.

Another factor we should keep in mind is where the Maxima is located. A maxima at 5000 feet above sea level does not need 91 octane, whereas the Maxima at sea level can take full advantage of the richer air and needs to richer fuel to maximize power and economy.

CD

First of all, Maximas never got good fuel economy. My lifetime average with my 2000 Maxima with 100,000 miles is 21 MPG with a 5 speed. This is

*right on* the revised estimates from:

The old combined mileage rating was 24 MPG for the 5 speed. It is good to see the EPA got a clue and is posting more realistic ratings. The automatic transmission 2000 Maxima had a combined rating of 23 MPG but was recently revised down to 20 MPG under the new testing criteria. If you beat that in city or mixed driving just count yourself lucky.

That government website also lists the Maxima as using regular gas, but I know that the actual EPA tests are conducted using the manufacturer specified fuel (91, in this case) so the listing is just a typo and shouldn't be ammo in the 87 vs 91 debate. I noticed this mistake in several cars' listings anyway.

Which leads us to my word on 87 vs 91, for what it's worth:

From the 2000 Nissan Maxima shop manual:

page GI-8: "PRECAUTIONS FOR FUEL Use premium unleaded gasoline with an octane rating of at least 91 AKI (Anti-Knock Index) number (research octane number 96). If premium unleaded gasoline is not available, regular unleaded gasoline with an octane rating of at least 87 AKI (research octane number 91) can be used. However, for maximum vehicle performance, the use of premium unleaded gasoline is recommended."

page EC-141 lists Low Octane fuel as a possible "Poor Fuel" trouble diagnosis.

page EC-34: "The knock sensor retard system is designed only for emergencies. The basic ignition timing is programmed within the anti-knocking zone, if recommended fuel is used under dry conditions. The retard system does not operate under normal driving conditions. If engine knocking occurs, the knock sensor monitors the condition. The signal is transmitted to the ECM. The ECM retards the ignition timing to eliminate the knocking condition."

It is clear from the foregoing that use of 87 octane is not meant to be done in the long term.

The manual... says *use premium unleaded* says use regular when premium is *not availiable*

*does not say* to use regular when I feel premium is too expensive *does not say* to use regular when I can't feel a difference with the premium says low octane is in the same category as *poor fuel* says the knock sensor retard system is designed for *emergencies* says the knock sensor retard system is not meant to operate *under normal driving conditions*

If you pump 87 octane gas expecting the knock system to continuously retard the timing for you just know it is not meant to be used that way.

Another thing, the phrase "Maximum vehicle performance" does not just mean acceleration either. It means durability and economy as well. You affect all of these negatively by putting 87 octane in the tank. Sure you can *feel* a loss of power and you can *measure* fuel economy, but how do you know what impact pumping 87 has on durability? Well if the car is slower and gets poorer mileage you can expect a negative effect on durability as well.

I also don't care what your local Nissan tech has to say. I have access to the same training materials and information as he does. His opinion is no better informed than mine. The engineers are the final word, not your local Nissan or garage tech. Argue with the Nissan engineers all you want, I'm not going to. If you pump regular gas *in the long term*, you are potentially damaging your car for the sake of a few bucks. Not catastrophically but why do it at all if you don't have to? Besides, the guys who built the engine don't think it's such a hot idea either.

There are a lot of folks here and on many internet forums who argue about the value of pumping 91 octane in the Maxima. I am not saying I'm an expert but neither are they! Unfortunately every piece of advice is just a guess because your situation contains many factors, some you don't even know about like gas *quality* and *formulation* (not octane). Driving habits, altitude, ambient temperature, condensation in the gas, and seasonal changes to the formulation (in NY they add 10% alcohol part of the year) will greatly vary the fuel's knock sensitivity and change tank to tank.

There was a news special on NBC in NY about gas station pump money errors where the reporter discovered that drivers were getting premium gas out of a regular pump when the station ran out of regular. This was even tested by an expert they brought in because they were looking for the reverse situation. If that was your car you might be spreading the gospel of 87 octane not causing a knock without realizing you really had

91 in there the whole time. This happens more than you think because stations do run out of one type of gas and substitute another because the wrong truck showed up to fill the station's underground tanks. Of course, the reverse is possible.

Furthermore, the knock sensor in the Max (and all modern cars) intervenes to retard the timing *before* the knocks become audible, making a detonation diagnosis by ear dicey at best. The malfunction light does not even illuminate when the knock sensor operates (page EC-392).

Know these facts:

• The Nissan Maxima owners manual and shop manual say to use 91 octane fuel. Nissan engineers and *not* marketers wrote this. Marketers would very much like to conceal this fact since it does not help sell a car.

• You may not hear any knocking when running 87/89 octane fuel but that doesn't mean the computer is not invisibly retarding the timing and reducing performance (power/economy/durability)

• Retarded timing is *not the most efficient timing* or it would run that way all the time. Retarded timing will reduce fuel economy as well as increase combustion chamber temperatures.

Do what you will with this. I decided that the price increase for premium gas isn't enough to avoid it. As the price of all gas rises the difference in price shrinks. At least it does near me. My advice is to run 91 octane and ease up on the lead foot if you need to save gas. It also saves on wear and tear.

codifus,

I hope you're right. I bought my car ('96 SE) when it had 37K miles and I would get about 425 miles on the trip odo. After a couple of years, it suddenly dropped to about 375 on the trip odo. No codes, everything runs great. I live with it. Then I had some problems with what I found out to be the MAF, but in trying to find the problem, I ripped out everything from the CAI all the way down to the intake manifold and cleaned everything real good along the way (IACV, EGR port, etc.) and replaced all the gaskets too. Replaced the knock sensor since I was in there too. Put everything back together and with a brand new MAF and till this day I get about 350 on the trip odo. So with all that cleaning, it got worst. Again, no codes, running 92 octane, and everything runs fine. I got an O2 sensor code a while ago for the rear and I replaced front and rear since I was at it but no affect on the MPG. My MPG is still pretty good but to think that it just suddenly drops like that is totally weird. I take pretty good care of my car and keep up with all the maintenance, but my MPG issue has been on my mind and now, 7 years later with 127K on the odo, I can't figure it out. I'm going to buy the ECT since it's cheap and easy to replace and we'll see. But if this works, I might have to send you a case of beer...:) Everything I buy is only genuine Nissan stuff I might add.

Thanks for reading.

Update:

I had CarMax (where I bought the car) replace all the filters, change oil, and install platinum spark plus. No change in mileage.

CarMax showed me the records where they had gone over the car to the equivalent of a 90K checkup when they took it in from previous owner.

I called the corporate office in the East, and they suggested clogged fuel injection.

Went to my nabe mechanic with a note about the sensors that was suggested here, as well as clogged fuel injection suggestion.

He went over everything, including putting in the scanner for codes.

He said everything was fine except the fuel injection, so he "unclogged" all six, and also replaced (?) connection wire(s) on one injector.

He said I should fill it up, take it out at freeway speeds for at least 1/2 hour to an hour, then fill it up again and measure fuel consumption. (This is unwittingly humorous, since freeways around LA are all "clogged" at any hour of the day or evening -- go find one that permits "freeway speeds"!)

So I will do that tomorrow and HOPE that the $505 (!!!) I spent will up the mileage.

He also pointed out that my 99 GLE has a LARGE V6, which I didn't realize. I know I won't get the mileage I had with my late, lamented, totaled Toyota Corolla, but I KNOW there has been a decrease since I bought the Nissan, so wish me luck!

Aspasia

FASCINATING site! This NG is so educational!

So which way is the mistake? And why isn't it ammo in the 87 vs 91 debate?

[...discussion of 87 vs 91...]

Aspasia

He also pointed out that my 99 GLE has a LARGE V6,

Odd that your mechanic would say that. The fact is, all Maximas from

1992 to 2001 have the same exact size engine, a 3.0 liter V6. All 1995 to 199 Maximas have the same exact 3.0 liter V6. Not only that, the 2000 and 2001 Maxima's V6 is essentially a 95-99 V6 with a variable intake manifold. There's more to it than that, of course, but that the basics of it.

CD

I have a feeling you'll see improved mileage real soon. Remember, the CTS replacement takes like 2 or 3 tankfulls to take full effect.

CD

Good news -- I think! I followed nabe mechanic's instructions to fill up car, set trip odometer to zero, drive freeway -- I did an hour at

60-80 -- and fill up again.

Bottom line: I got 28 MPG! Incredible.

Mechanic said, however, that in town I might get 15. Which is what I was getting in the first place -- 13-14. So for what did I spend $505??

I will burn a tank around town and report whether there has been any improvement.

Aspasia

You're right; I have checked the stats. He may have meant that it has a "large" engine compared with my beloved little 4-cyl Corolla (RIP)

Whatever...I'm stuck with it now; still have a lot to pay off, and who's going to buy a 99 luxury V6 in these times!

I wish I could find one of those little street-legal gizmos that the parking cops ride around in! I would use it for around town and use the Maxima for freeway.

Anybody got a clue where they can be bought?

TIA

Aspasia