Did tune up and lowered my gas mileage... sweet!

[chopsieze]

Hey, I just put new NGK V-power plugs (set gap to .032), new Bosch premium wires, pcv valve, O2 sensor, fresh fluids (coolant, transmission and engine oils) and a new air filter. Then I took my 96 Prizm 1.8l on an 1800 mile road trip and got under 32 mpg... I typically get 37 and hoped to get at least that considering the tune-up and 51 psi tires that haven't been on the car since my last road trip.

The car was running fine and getting better mpg before I "messed it up" but I wanted to do it cause the car has almost 150,000 miles and I don't think a few of those things had ever been done. I'm going to look at the gap on the old plugs but I thought .032 is recommended. The other thing I want to do, which may be effecting, is get a new thermostat. Not sure what the actual temp got to, but it never got that close to the mid "normal" mark. It's been like that for a while getting a little over half way to the normal mark.

The car seems to run pretty smooth... it does have a little vibration that may not have been there at idle but real smooth otherwise. Going to look into egr and timing if I can find someone with a gun.

Any other ideas or thoughts would be very helpful.

Thanks!
oh, about halfway through the trip I ran some Techron fuel cleaner (never done it before) and plan on putting a new fuel filter on next week. The cleaner didn't seem to effect the mileage and got between 30-32mpg every tank.

[akboss]

51 psi tires, is that safe???

I'm not mechanically inclined enough to assume why your car is burning more fuel after a tune-up, but changing fluids and keeping your car's maintenance schedule is the best way to ensure your ride lasts.

That being said, and in regards to your Techron fuel cleaner, I find these additives are more gimmicks than sure-fire solutions to a better running car. If somebody can show me differently then maybe I'll learn something, but I have tried many different solutions with zero positive effects on mileage. I think most new(er) cars are meant to run pretty smoothly without the need for additives. That being said, specific driving conditions might require premium care - like turbo motors typically require synthetic oil changes and mid-grade or premium fuel, although that is even debatable. Save your money from money-grab additives and put it towards your next tune-up.

[wafrederick]

Plus use the recomended oil grade,it is one way to increase fuel economy.The Geo Metros get 60 MPG to the mile

[akboss]

Originally Posted by wafrederick

Plus use the recomended oil grade,it is one way to increase fuel economy.The Geo Metros get 60 MPG to the mile

It doesn't hurt that the Geo Metro was powered by a weedwhacker engine on 12" lawnmower tires. No offence intended towards Geo Metro's, they did what they intended, it just happens to be about the most nutless car on the road. Now a Honda CRX, that was a fun car that did 50+ MPG's...

[wafrederick]

I know of an used car dealer dealer having a 1998 Volkswagon Jetta with the diesel engine and gets 52 MPG with the manual transmission.Geo Metros have this problem including the Prisms and do not attempt to fix if it has this: the Geo split.The undercarriage rusts out moving the tires out pulling the axles out of the transmission.I have seen one Prism and a few Metros do this so far.

[MagicRat]

A 5 mpg drop is very significant.
I think the plug gap is way too narrow for a electronic ignition. Someting like .045 is more like it. There should be a tune-up sticker under the hood which will indicate the correct gap.

But even that would not explain all of the mileage drop. Take a look at that O2 sensor. Is it plugged in? Did you use sealer on the threads (don't do that). Scan the ECM for codes. You may get a hint about the cause.

Also, you may find the car has two O2 sensors, one above the cat and one below. Did you replace both?

[Airjer_]

Originally Posted by chopsieze

Then I took my 96 Prizm 1.8l on an 1800 mile road trip and got under 32 mpg... I typically get 37 and hoped to get at least that considering the tune-up and 51 psi tires that haven't been on the car since my last road trip.

When was the last time you calculated MPG? Air temperature will play a huge factor in overall fuel economy.

51 psi in your tires is irresponsible and presents potential risks to yourself as well as other on the road that can be avoided with properly inflated tires! There's already enough variables to get into trouble with why would you knowingly add another?

[MagicRat]

Originally Posted by Airjer_

Air temperature will play a huge factor in overall fuel economy.

How will this happen?

Cooler air is more dense, but a modern EFI system will compensate for the increased density and resulting oxygen levels to maintain a proper fuel mixture. So, there will be no appreciable mileage difference, all other factors being equal.

Cooler air will allow an engine to develop slightly more power at wide open throttle with slightly lower fuel economy, but I am assuming he did not drive everywhere with his foot planted firmly on the floor.

Cooler, denser air will increase wind resistance and fuel consumption, but again, this is a marginal difference.

[chopsieze]

My tires are rated at 51psi... so it shouldn't be dangerous. It may increase chance of pothole blowouts airing them past the max but I know some Highway Patrol cars are aired close to 60 psi on 44 max tires (I don't think you gain much past 50 though). They're supposed to wear out faster too over inflating them but I don't even know if that's true. You will feel bumps in the city (could be rough on suspension joints) but you should see how good a car will roll and how easy it is to turn the steering wheel.

I reset my trip meter every time I fill up so I'm always calculating it. The air was actually warmer as I went from Iowa to Dallas but being on the highway most the time the car never warmed up (I know the last couple years it doesn't get to the mid point but I've not paid real close attention... I do think it got higher than it did this trip though) I wish my gauge showed numbers so I new what it actually gets to.

[chopsieze]

Originally Posted by MagicRat

A 5 mpg drop is very significant.
I think the plug gap is way too narrow for a electronic ignition. Someting like .045 is more like it. There should be a tune-up sticker under the hood which will indicate the correct gap.

But even that would not explain all of the mileage drop. Take a look at that O2 sensor. Is it plugged in? Did you use sealer on the threads (don't do that). Scan the ECM for codes. You may get a hint about the cause.

Also, you may find the car has two O2 sensors, one above the cat and one below. Did you replace both?

I didn't replace the 2nd O2 sensor yet, thought it was just to notify you of a bad cat. and I did put a little thread compound on the one I did swap. What will putting it on do?

I will check the gaps on my old plugs and try to find the recommended tonight.

Will an auto parts store computer check give a detailed reading enough to give me a clue... the check engine has been on for the past few months but it was still running great and getting great mpg. I just assumed it was O2 or something and meant to reset it before the trip to see if it came back on.

[Airjer_]

Originally Posted by MagicRat

How will this happen?

Cooler air is more dense, but a modern EFI system will compensate for the increased density and resulting oxygen levels to maintain a proper fuel mixture. So, there will be no appreciable mileage difference, all other factors being equal.

Cooler air will allow an engine to develop slightly more power at wide open throttle with slightly lower fuel economy, but I am assuming he did not drive everywhere with his foot planted firmly on the floor.

Cooler, denser air will increase wind resistance and fuel consumption, but again, this is a marginal difference.

When temps drop from 80 degrees to -20 degrees Fahrenheit it makes a huge difference in overall fuel economy. just the amount of energy to get everything moving on a cold morning will suck your tank dry! I can go from 34-35mpg on the freeway in the Saturn in the summer on the freeway at 70 mph to 28-29mpg under the same circumstances only 80 to 100 degrees colder. So if you want to come to Minnesota on a nice crisp winter morning we'll give your theory a shot but I can tell you it won't hold water!

[MagicRat]

Originally Posted by Airjer_

When temps drop from 80 degrees to -20 degrees Fahrenheit it makes a huge difference in overall fuel economy. just the amount of energy to get everything moving on a cold morning will suck your tank dry! I can go from 34-35mpg on the freeway in the Saturn in the summer on the freeway at 70 mph to 28-29mpg under the same circumstances only 80 to 100 degrees colder. So if you want to come to Minnesota on a nice crisp winter morning we'll give your theory a shot but I can tell you it won't hold water!

That's not relevent in this situation, and you know it, so stop giving bad advice.

If you have a decent working thermostat in the car, warm-up can occur in just a few minutes (Yeah, I live in Ontario with the same winters as yours).
So, you will get decreased mileage only for that first few minutes and NOT for 1800 miles!

Originally Posted by chopsieze

I didn't replace the 2nd O2 sensor yet, thought it was just to notify you of a bad cat. and I did put a little thread compound on the one I did swap. What will putting it on do?

I will check the gaps on my old plugs and try to find the recommended tonight.

Will an auto parts store computer check give a detailed reading enough to give me a clue... the check engine has been on for the past few months but it was still running great and getting great mpg. I just assumed it was O2 or something and meant to reset it before the trip to see if it came back on.

Okay, that's useful info.

The thread compound will prevent the sensor from having a good electrical contact, because it is grounded through the exhaust system.
If this occurs, the sensor will not work at all, which usually makes the ECM go into a failure mode and causes a richer fuel mixture.
Often when this occurs, you get a 'check engine' light and increased fuel consumption, even though the car will often seem to run just fine.

Get a small wire brush. Remove the O2 sensor and thoroughly ckean off all the sealer from the sensor (be careful not to brush or damage the end of the sensor that projects into the exhaust) and the hole in the exhaust. Reinstall the sensor.

Disconnect the car battery overnight, then hook it back up and go for a drive. Then go to an auto parts and service place that dies a free scan and have them scan for codes. Often the codes require interpretation to be meaningful. You may be able to find further info on any codes on-line.

Also, if you had a pre-existing 'check engine' light then there is likely other issues as well.

[Airjer_]

Originally Posted by chopsieze

My tires are rated at 51psi... so it shouldn't be dangerous.

Your car isn't rated for 51 psi! Every tire manufacturer and every vehicle manufacture tells you to inflate the tire to the vehicle NOT what it says on the tire! The amount of savings you get in fuel economy are not worth the sacrifices you make in safety. An over inflated tire reduces the footprint of the tire which results in a decrease in traction. During an emergency maneuver you may find the tires skipping or skidding instead of responding to you command to turn! over inflation also will result in greater breaking distance. For that small of a car 51 psi is grossly overinflated!

You said you haven't had those tires on since the last road test?

[Airjer_]

If you don't think the vehicle is coming up to temp than get that checked out. A cooler engine requires more gas!

[Airjer_]

Originally Posted by MagicRat

That's not relevant in this situation, and you know it, so stop giving bad advice.

That's not bad advice, we didn't know where he road tripped and he is from Iowa and the temps have been cooler in the upper mid-west lately have they not?

I think we can all agree that there is not one problem that is contributing to a ten MPG drop in fuel economy. As we poke and prod we start finding out little bits and pieces that start to reveal that there are several things that are not right. Had I not suggested that temp may be a factor we may have not know he went to Dallas which was revealed a couple posts later!