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Old 09-03-2013, 07:07 AM   #1
sdowney717
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Default MPG dropped about 10 to 15% suddenly

When we first got this SL2 1995, I replaced the engine temperature sensor with the better quality solid brass one. That was maybe 8 years ago.
Do you think that sensor has failed?

What ohm measurements should they read at various temperatures from cold to hot?

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Old 09-03-2013, 07:18 AM   #2
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Default Re: MPG dropped about 10 to 15% suddenly

I'm sure others with more diagnostic abilities will chime in but you may consider various sensors/devices and their age. These three are inexpensive and easy to change.

ECTS (as you mentioned)
Oxygen Sensor
Thermostat

as well as things that can get clogged like...

exhaust system
fuel filter
air filter

...
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Old 09-03-2013, 07:26 AM   #3
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Default Re: MPG dropped about 10 to 15% suddenly

I replaced the air filter and thermostat about 6 months ago. Engine reaches normal temp when viewing the gauge. Exhaust is definitely not clogged, engine power and sounds normal.

There are no codes. I have never replaced the fuel filter.

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Old 09-03-2013, 11:17 AM   #4
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Default Re: MPG dropped about 10 to 15% suddenly

32F = 9420 ohms
77F = 2796 ohms
212F = 177 ohms

these are approximate, yours may vary a bit

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Old 09-03-2013, 12:42 PM   #5
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Default Re: MPG dropped about 10 to 15% suddenly

Quote:
Originally Posted by sdowney717 View Post
I replaced the air filter and thermostat about 6 months ago. Engine reaches normal temp when viewing the gauge. Exhaust is definitely not clogged, engine power and sounds normal.

There are no codes. I have never replaced the fuel filter.
First it would help us if you define things for us better. When you state that you "suddenly" had a drop in MPG is this all tanks since day XX or an isolated event?

What variables if any have you isolated other than what is listed above? Have gas brands changed, location you get it, driving conditions, work hours, new anything other than the above, etc?

What is your MPG now and in the past, and what method do you use to calculate it?

The first thing I'd do is take a look at longer term trends. MPG often cycles up and down somewhat with weather and especially with AC use. Looking at the trend of the last 10 or so tanks often shows a better picture of the overall. Eliminate any variables you can think of.

If you can't think of any major changes in any variable, I'd start with the basics of checking plugs and basic tune up type things. Check the ECTS connector, spark plugs, fluid levels, etc. Some of the earlier cars (we own a '95 SOHC) didn't throw a code as quick as the later cars, and you can have an issue that the computer hasn't flagged with a code yet.

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Old 09-04-2013, 07:13 AM   #6
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Default Re: MPG dropped about 10 to 15% suddenly

Thanks for the numbers
Its dropped and stayed down for 3 months. So I kept wondering if it would fix itself, but has not.


What variables if any have you isolated other than what is listed above? Have gas brands changed, location you get it, driving conditions, work hours, new anything other than the above, etc?

No, the car is driven as it always is. Same WAWA gas, everything.


What is your MPG now and in the past, and what method do you use to calculate it?

MPG is about 23 to 24 now, I have seen as low as 21.
Before MPG was 27 to 28.

What we always have done for years, fill tank, read odometer, calculate.
Wife noticed gas gauge dropping faster and she could no longer get the range from the tank she use to get.

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Old 09-04-2013, 08:12 AM   #7
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Default Re: MPG dropped about 10 to 15% suddenly

I doubt the coolant sensor failed or changing for the worse. As long as the connector is intact and free of any corrosion, it should be fine. The act of disconnecting and reconnecting 'exercises' the electrical connections anyway for better or worse. You can try using the chart below but remember there may be a small tolerance error to consider when measuring sensors.

Although its being disputed as to which t-stat to use, a replacement may be the key to raising coolant operating temperatures back to where it should be if its running cooler than normal. The main reason to maintain optimum coolant operating temps is to allow the EFI system to run at its best, all other things being the same (wawa gas, driving style, same roads, etc.). In general, best coolant operating temps has always been close to 200F to have the EFI system run as lean as possible in its closed loop mode of operation when the engine's at operating temps. An older t-stat wears out and tends to run a cooler operating temperature, lowering overall operating conditions and possibly contributing to lower fuel mileage. The temperature gauge may tell you - if the t-stat is running cooler then the temperature needle will be closer to the 1/4 mark instead of higher up, 3/8ths.

And there's always the tune-up, spark plugs and general cleaning of the ignition system to ensure clean spark all the time to avoid misfires.
Attached Images
File Type: jpg ects chart 1.jpg (85.6 KB, 12 views)

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Old 09-05-2013, 07:54 AM   #8
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Default Re: MPG dropped about 10 to 15% suddenly

The T-Stat has a seal on one end that fails. I usually replace T-Stat at 100,000. It is not if it will fail, but when it will fail. I recently had a 80,000 mile engine installed. The thermostat would not go above 1/4 on gauge and was lower at idle. I could see the gas gauge moving when I was driving.

I went with a Napa (Stant) thermostat. Before that always installed OEM.

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Old 09-05-2013, 11:43 AM   #9
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2000 SL
Default Re: MPG dropped about 10 to 15% suddenly

Have you recently changed where you got your gas? Fuel quality will impact MPG that much. I notice 10-15% better mileage by not using cheap gas. Ethanol blends drop MPG in about that range.

Of concern is not only cheap (Sam's Club, Meijer, etc.) vs non-cheap (BP,Mobil, Marathon), but also the specific station. I gained 2-3 MPG (about 10%) by filling up at a BP near my home vs near my work. Same supplier, but underground tank quality and other things can affect what comes out.

Also, I notice that my car, anecdotally speaking, likes certain brands vs others. Marathon and Speedway and BP, with BP sometimes being even better in summer.

My experience is that the weight of your foot and what you feed it are the more significant factors in MPG. (Not even remotely trying to imply a good tune-up isn't valuable, but they cost more money)

...
Thanks! David

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2000 SL 5-Speed 214k miles; Silver

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Old 09-05-2013, 04:59 PM   #10
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Default Re: MPG dropped about 10 to 15% suddenly

Still filling the same place we always do which is Wawa or Sam's club.

My replacement ECTS has about 90,000 miles on it.
So maybe it is done?

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Old 09-05-2013, 08:15 PM   #11
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Default Re: MPG dropped about 10 to 15% suddenly

If the engine starts up instantly, warms up quickly, and shows the temperature needle above the 1/4 mark then its working as designed. Look elsewhere as suggested.

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Old 09-06-2013, 10:37 AM   #12
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Default Re: MPG dropped about 10 to 15% suddenly

Gas stations do switch blends of fuel from winter to summer which is about the time we noticed the drop off in MPG.

So what do you get for MPG on a saturn SL2? Mine is a 1995.

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Old 09-06-2013, 11:39 AM   #13
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Default Re: MPG dropped about 10 to 15% suddenly

Quote:
Originally Posted by sdowney717 View Post
My replacement ECTS has about 90,000 miles on it.
So maybe it is done?
Nah. Brass tipped ECTS should last life of the car. Mine is the original with 183K miles.

...
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Old 09-06-2013, 11:42 AM   #14
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Default Re: MPG dropped about 10 to 15% suddenly

Quote:
Originally Posted by sdowney717 View Post
So what do you get for MPG on a saturn SL2? Mine is a 1995.
Shouldn't really compare to others as it depends on how you drive and where you drive. Comparing to yourself is the only way to do it correctly.

My MPG dropped from the same time last year from averaging around 38 MPG to now averaging around 36 MPG. But, I know this is more likely because I've changed jobs and my new place of employment has more traffic/traffic lights, etc. It is closer to home, so I use less gas, but the MPG did take a little hit.

...
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Old 09-06-2013, 11:50 AM   #15
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Default Re: MPG dropped about 10 to 15% suddenly

Quote:
Originally Posted by Tiger View Post
I went with a Napa (Stant) thermostat. Before that always installed OEM.
I got an aftermarket (made in germany) from Autozone and it has outlasted the OEM that came with the car.

...
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Old 09-06-2013, 02:46 PM   #16
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Default Re: MPG dropped about 10 to 15% suddenly

Quote:
Originally Posted by TXSaturn02 View Post
Shouldn't really compare to others as it depends on how you drive and where you drive. Comparing to yourself is the only way to do it correctly.

My MPG dropped from the same time last year from averaging around 38 MPG to now averaging around 36 MPG. But, I know this is more likely because I've changed jobs and my new place of employment has more traffic/traffic lights, etc. It is closer to home, so I use less gas, but the MPG did take a little hit.
What is going on with my car? 36 mpg??
It used to get 28.
How many here get 36 mpg in their 1995 SL2 all around driving?

Drove the car to Myrtle beach about 800 miles total and best it got was 31, return trip 28.

This trip was after the local mileage in our town it fell to 23 to 24 and I have seen low as 22.

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Old 09-07-2013, 09:19 AM   #17
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Default Re: MPG dropped about 10 to 15% suddenly

Quote:
Originally Posted by fdryer View Post
If the engine starts up instantly, warms up quickly, and shows the temperature needle above the 1/4 mark then its working as designed. Look elsewhere as suggested.
The ECTS is different from the head temp sensor. Does gauge run off the upper head sensor?

I took out the ECTS and the air box sensor which is the same thing and compared the two. The old air box sensor seemed closer to the numbers, so put that in and put the ECTS into the air box.

A few years ago I had added an ECTS sensor in parallel to the airbox to fool the engine into thinking the incoming air was hotter. There was a long discussion about this on this forum number years back. I found it disconnected. Maybe I did unhook it a few months prior. So hooked it up again and will see how this week goes on MPG.

The theory on that was hotter incoming air means engine computer compensates mixture by leaning it out since hotter air is less dense that colder.

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Old 09-07-2013, 02:33 PM   #18
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Default Re: MPG dropped about 10 to 15% suddenly

No matter which sensor is screwed into any hole, the difference between the gauge and pcm sensor is the gauge sensor uses the single wire one while the pcm must use the two-wire one. Confusion with which sensor plugs into which connector can result in the wrong connection with the temperature gauge needle sometimes going to midscale immediately with the cooling fan turning on when the ignition switch is turned ON.

There's another thread discussing the same hot air intake mod using the intake air temperature sensor to help with fuel economy (towards the end); http://www.saturnfans.com/forums/sho...d.php?t=194170

Last edited by fdryer; 09-07-2013 at 02:41 PM..

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Old 09-22-2013, 07:45 AM   #19
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Default Re: MPG dropped about 10 to 15% suddenly

updating this thread.

MPG is back to 28MPG!

Fix was hooking back up the added on ECTS in the air box and
swapping one of the airbox ECTS into the engine.

I had added a second ECTS into the airbox a few years ago
about 4 months ago on a trip I had disconnected it (unrelated issues) . That dropped the MPG for months but I did not link what I did to the MPG as it slipped my mind.

A few years ago there was a thread about boosting MPG by adding a second ECTS into the air box circuit in parallel which fools the computer into thinking the incoming air is hotter and the computer then leans the mixture.

It is either that or my ECTS was faulty and doing the swap made it run like it should.

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Old 09-22-2013, 09:43 AM   #20
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Default Re: MPG dropped about 10 to 15% suddenly

Disconnecting the air temperature sensor was probably the reason for lower fuel mileage. The worst problems are caused by shooting ourselves in the foot while attempting to maintain our cars. Finding these mistakes can be an eye opener to human errors. At least you're back to getting good fuel economy and may decide to review what you were trying to accomplish at a later time. Or not.

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