6.5 Mpg

[krmiller07]

Since I read your responses to this topic. It got me thinking, I have an 89 Pathy and I replaced the engine with a lower mileage VG-30 with about 40K miles on it. I did a compression and leak down test before installing it, and every thing was in check. However my hi way mileage averages about 6-8 MPG which is horrible. The cat was removed when I bought it, and since the state I live in has no emissions checks, I have left it that way. The AIR pump is still in place with the belt, but the pipe to the exhaust has been capped off. I just did a complete tune up. Plugs, wires, cap and rotor. Checked and replaced all the vacuum lines etc. The truck runs and idles great. In fact, I just had the exhaust manifold gaskets form the 'Y' pipe replaced due to a leak, and the mechanic said it ran really good and strong.( he was impressed )! My timing is set at 15* BTDC.

But the mileage is awful. I even put Warn manual lock out hubs on it, just for the peace of mind in knowing they are disengaged. Someone suggested i replace the O-2 sensor, since it's the original one, and the stock motor literally blew a rod while tooling down the freeway. The guy said it's possible the sensor was contaminated from years of running rich and possible coolant leaks from the old cracked heads I found when doing the engine swap. ( I swapped heads, before doing the entire engine swap ) the exhaust pipe looks so clean, it actually looks like rust is forming at the end.

Does anyone have any ideas or feedback on the O2 theory or any experience with improving the mileage. This is my second pathy, and I know they aren't know for their mileage, but I should be getting better that what I am, especially on the Hi way !

 

Thanks.

Edited March 6, 2010 by krmiller07

[Kingman]

I'd replace the O2 sensor, running rich or having other contaminants in the exhaust system will ruin them fairly quickly.

 

Have you checked the computer for codes?

[BowTied]

Are you running a thermostat? Does it rev high on the highway?

[krmiller07]

Are you running a thermostat? Does it rev high on the highway?

[krmiller07]

Yes, it has a new thermostat and it runs normal and make good power through all RPM's, no backfiring, steady smooth idle, rev's evenly and doesn't 'race' or fast idle. What about the MAF sensor ? I read an entry some where on hear about someone taking theirs off the side of the throttle body and cleaning the wire with isopril alcohol or MAF cleaner.? The only thing I regularly do, is pour a bottle of '107' octane booster in before every other fill up. This seems to make somewhat of a differnce in throttle response and smooths out the idle while waiting in traffic or standing still.

[BowTied]

I'll assume you are running at least a 180 stat.

 

If everything is working so great, perhaps you are getting better mileage that you realize. How many kilometers per tank do you get? Have you verified your odometer is accurate?

 

Is this a carb or FI?

Edited March 7, 2010 by BowTied

[krmiller07]

I'll assume you are running at least a 180 stat.

 

If everything is working so great, perhaps you are getting better mileage that you realize. How many kilometers per tank do you get? Have you verified your odometer is accurate?

 

Is this a carb or FI?

 

I am running a stock Thermostat, and I average around 250-280 miles per tank on the hiway. However the last few weeks, I drive about 30-35 miles and burn through approx 5-6 gals or 1/4 tank of fuel. I verified this when I top off at the pump. In addition this truck has TBI ot Throttle Body Injection, Or 'Electro-Injection' as it referred to on the air cleaner cover. I have never had the odometer verified, but it seems to be working just fine.

[krmiller07]

I'd replace the O2sensor, running rich or having other contaminants in the exhaust system will ruin them fairly quickly.

 

Have you checked the computer for codes?

 

The mechanic I've talked to, have said you can get 'false positives' from this type of ECU, and that those code only point you in the possible or general direction in regards to faulty components ? A new O-2 sensor runs about $ 150-180.00 from Napa ( Bosch ) and once they've been installed, you cant return them. I don't really wnt to throw money at it and hope it improves, I'm trying to pinpoint and replace as necessary.

[frecklecolouredbrain]

I am running a stock Thermostat, and I average around 250-280 miles per tank on the hiway. However the last few weeks, I drive about 30-35 miles and burn through approx 5-6 gals or 1/4 tank of fuel. I verified this when I top off at the pump. In addition this truck has TBI ot Throttle Body Injection, Or 'Electro-Injection' as it referred to on the air cleaner cover. I have never had the odometer verified, but it seems to be working just fine.

 

 

check you math man.

 

280mi=450km.

it has an 80L tank.

out you bu

80L/450km=.177L/km

 

x100= 17.7L/100km.

 

these were rated at 15.5L/100km new, so you're doing pretty good. I'm sitting at 16L now, but i change my fluids more than i change underwear, and try to keep her in tip top shape. o€ne thing I found that might help...don't use octane booster. I don't know about you but mine really likes to burn higher octane fuel.

[adamzan]

I'm pretty sure the timing on the TBI pathfinders is supposed to be at 12 degrees BTDC

[Kingman]

Adam, you can advance the timing on the TBIs to 15 degrees from the factory 12 and usually get better gas mileage.

 

Does the exhaust smell rich at all? Also, maybe disconnect the O2 sensor and run it on the highway and see if your mileage increases. My O2 sensor was bad, and I got almost 2 MPG better on the highway than with it connected.

[BowTied]

80L/450km=.177L/km

 

x100= 17.7L/100km.

 

 

 

It is likely that those numbers are conservative. The tank capacity might be 80, but chances are there are several, 10L?, left in the bottom still at refill. So there is a chance you are actually getting 14mpg.

 

OP: does all this ^ help?

 

Reference:

Mpg........ Liters Per 100 K

12.00..... 19.60

12.50..... 18.81

13.00..... 18.09

13.50..... 17.42

14.00..... 16.80

14.50..... 16.22

15.00..... 15.68

15.50..... 15.17

16.00..... 14.70

Edited March 7, 2010 by BowTied

[krmiller07]

Adam, you can advance the timing on the TBIs to 15 degrees from the factory 12 and usually get better gas mileage.

 

Does the exhaust smell rich at all? Also, maybe disconnect the O2 sensor and run it on the highway and see if your mileage increases. My O2 sensor was bad, and I got almost 2 MPG better on the highway than with it connected.

I love your window sticker BTW ! It only smells rich during warm up( Normal ) but after it reaches operating temp. It smells normal ( I think ). I don't know if you can adjust anything on the TBI except cold ( Fast ) and normal idle speeds. All I know is that I would think I should be able to drive more than 30 miles with out burning through 1/4 tank of gas !?. BTW, wouldn't diconnecting the O2 sensor throw off the ECU and send it into a tail spin ?

Edited March 7, 2010 by krmiller07

[BowTied]

A tail spin? Not likely, there would be some safe mode pre-set fuel map - which would likely be conservative (rich) to avoid a dangerous lean condition.

 

If you are getting 400-450km in a long trip but crappy mileage in a short trip, the truck may taking too long to warm up. I snagged the following from a different thread where I was helping someone who thought they were revving high on the highway...

 

When you first start your truck (cold) the engine operates in "open loop mode" This bypasses readings from the O2 sensor and the engine runs rich to help it warm up. During this time the automatic transmission's lock-up torque convertor will not lock up. This causes higher RPMs. Once the engine reaches the correct operating temperature the O2 sensors come on line and lean out the mixture and the torque convertor locks up which drops the RPM a bit (feeling like a shift into OD). This allows the engine to warm up and the truck to go down the road in the best manner possible until things are warmed up.

 

If the warm up period is taking longer than it used to, it could be:

Very cold outside

Thermostat stuck partially open (most common in my experience) or incorrect thermostat

engine temperature sensor malfunction

 

 

Some O2 sensors also have a heater circuit in them.... My pre-96 vehicles did not, I am not sure about the early pathys. This is another failure mode that can use fuel on short trips.

Edited March 7, 2010 by BowTied

[krmiller07]

A tail spin? Not likely, there would be some safe mode pre-set fuel map - which would likely be conservative (rich) to avoid a dangerous lean condition.

 

If you are getting 400-450km in a long trip but crappy mileage in a short trip, the truck may taking too long to warm up. I snagged the following from a different thread where I was helping someone who thought they were revving high on the highway...

 

When you first start your truck (cold) the engine operates in "open loop mode" This bypasses readings from the O2 sensor and the engine runs rich to help it warm up. During this time the automatic transmission's lock-up torque convertor will not lock up. This causes higher RPMs. Once the engine reaches the correct operating temperature the O2 sensors come on line and lean out the mixture and the torque convertor locks up which drops the RPM a bit (feeling like a shift into OD). This allows the engine to warm up and the truck to go down the road in the best manner possible until things are warmed up.

 

If the warm up period is taking longer than it used to, it could be:

Very cold outside

Thermostat stuck partially open (most common in my experience) or incorrect thermostat

engine temperature sensor malfunction

 

 

Some O2 sensors also have a heater circuit in them.... My pre-96 vehicles did not, I am not sure about the early pathys. This is another failure mode that can use fuel on short trips.

Thanks, but the truck is a 5speed and I religiously allow it to warm up for at least 5- mins or until I see the temp gauge starting to rise. I have checked to see that all the 'cold start' equipment is funtioning properly, and it is. Do you have any input on targeting the O2 sensor, besides going off the code from the ECU ? I'm at a loss here !

 

Thanks again. : ).

[SC88Pathy]

The only thing I regularly do, is pour a bottle of '107' octane booster in before every other fill up. This seems to make somewhat of a differnce in throttle response and smooths out the idle while waiting in traffic or standing still.

 

This might be part of your problem. These engines are designed to run on 87 octane. Higher octane fuel burns slower, so not all of your fuel will be getting used.

 

If you want the "cleaning additives" that the octane booster supposedly gives just fill up with name brand gas like Chevron or the like.

[krmiller07]

This might be part of your problem. These engines are designed to run on 87 octane. Higher octane fuel burns slower, so not all of your fuel will be getting used.

 

If you want the "cleaning additives" that the octane booster supposedly gives just fill up with name brand gas like Chevron or the like.

Thanks, but I don't think it's the octane booster ! The idea behind higher octane and a 'slower' burn is to increase the amount of energy per cumbustion cycle. It dosen't effect any 'left over fuel' in the cumbustion cycle, that would fall under a poor general state of tune within the engine it self.

Thanks though.

[Kingman]

After experimenting with my O2 sensor, I figured that around town a disconnected sensor will cause the engine to run rich, but on the highway it will lean out as it's only running off of the MAF sensor and there's a ton of air flowing past it into the throttle body. At highway speeds the engine runs lean without the O2 sensor to sense it's running lean and even things out. That's the only explanation I have for my truck running extremely lean and getting 22-23 MPG on the highway with the sensor unplugged, and worse mileage with a new one plugged in. On the flip side, in town mileage went back up with the new sensor.

 

With such a low compression ratio, running fuel with over an 87 octane rating is basically pissing your money away a little at a time.

 

The O2 sensors are a 3-wire heated unit.

 

You cannot adjust the air/fuel mixture on a TBI during open-loop mode, unless you open up the ECU and re-map it.

[krmiller07]

Does your sensors connection mount on the top of the passengers side fender ? BTW- what year and version of the V6 do you have and is it a 5 speed or AT ? The other question that comes to mind is. Since it's my undersatnding the the memory in these ECU's are ROM ( Read Only Memory ), would you, or do you diconnect the battery every time you disconnect your O2 sensor, as so the ECU 're-learns' it's new operating parameters. I've read alot of posts on here with regards to emissions as well as the TBI's and the one thing that remains fairly consistant, is every one disconnects the battery for at least 12 hours, before running the engine. Apparently, this helps the computer to learn the 'New' readings ?

[BowTied]

Thanks, but the truck is a 5speed and I religiously allow it to warm up for at least 5- mins or until I see the temp gauge starting to rise. I have checked to see that all the 'cold start' equipment is funtioning properly, and it is. Do you have any input on targeting the O2 sensor, besides going off the code from the ECU ? I'm at a loss here !

 

Thanks again. : ).

 

 

Oh, didn't know it is a manual, good. However, if your thermostat is lazy or stuck partly open, or if the temperature sensor is not acting properly, your warm up preparations will not aid you. Unless you know that your engine is operating above approx 75 deg C (~170F) it could be running too cool or taking a long time to get to this temp affecting your mileage.

 

If you have a code indicating an O2 malfunction replace the sensor - that has always worked for me.

[BowTied]

Thanks, but I don't think it's the octane booster ! The idea behind higher octane and a 'slower' burn is to increase the amount of energy per cumbustion cycle. It dosen't effect any 'left over fuel' in the cumbustion cycle, that would fall under a poor general state of tune within the engine it self.

Thanks though.

 

 

I agree that octane booster is not likely changing your fuel economy from 15mpg to 6mpg. However, your comments on octane are a little misguided, not your fault likely as this is commonly misquoted on the net. Fuel of differnet octanes does not burn faster or slower, though you will hear that on the internet often as it does seem like that is the case. The speed of the flame front propigation is essentially the same. What is different is that the energy input required to initiate the chemical reaction that is combustion is higher with higher octane rated fuels. This allows for a more controlled combustion and resistance to autoignition due to high compression in performance cars. From Wikipedia:

 

Many high-performance engines are designed to operate with a high maximum compression, and thus demand high-octane premium gasoline. A common misconception is that power output or fuel mileage can be improved by burning higher octane fuel than a particular engine was designed for. The power output of an engine depends in part on the energy density of its fuel, but similar fuels with different octane ratings have similar density. Since switching to a higher octane fuel does not add any more hydrocarbon content or oxygen, the engine cannot produce more power.

 

 

So, I agree that this is not likely your problem as the reported mileage of 6.5mpg if accurate is way too significant in my opinion to be caused soley by using octane booster. It is however not likely doing anything beyond lightening your wallet.