high mileage-extremely poor fuel efficiency

[hikerpat]

I have a 1997 Saturn SC2 w/approx 117,000 miles on her. She has been immaculately maintained. I still get approx 27 mpg in-town. But over-the-road? Last week, I drove 170 miles highway and mountains. She has always loved mountain driving. With 150 miles in-town, plus 170 otr/mountains, I averaged 29 mpg. It should have been up near 35.
The dealer stated she's suffering from high mileage; that the computer adjusts for it so she runs flawlessly.
I use Shell gasoline.
Is there anything I can add or have done to get the mileage back to or near normal?

[RC1488]

Dont go to the dealer...or at least that one. Always come here first

I would buy that car from you in a heart beat. 117K....that car is a teenager. if treated right...expect near 300K out of it.

Replace your ECTS. Yours is cracked. New ones are best bought from the dealer. 13mm deep dish socket on a cold engine and youll be good to go. Full tune up may be needed. plugs, wires, throttle body cleaning, EGR cleaning, etc. Tires should be inflated to proper psi. NOTE: Tire compound effects mpg up to 2mpg. I use Michelin X Radials which are fantastic!

Operating temperature AFTER you change the ECTS? SHould be 1-2mm above 1/4 looking at the gauge. If not..replace your thermostat. 195F

[hikerpat]

One of the problems with on-line forums is that, there are some who believe they can "diagnose" without testing or visualizing.
The car has been gone over with a fine-tooth comb. ECST has been and still is reading fine. Tires are kept properly inflated - it's poor advice to install tires that were not used when the car's safety handling was rated. I continue to use the proper tires-updated ones, of course. And, there are less than 6K miles on them.
Spark plugs are fine.
No leaks in fuel system.
And I misstated mileage - it averaged 28 mpg, with more than half highway.
If anyone else has an idea, I will consider it.
Thanks.

[RC1488]

Originally Posted by hikerpat

One of the problems with on-line forums is that, there are some who believe they can "diagnose" without testing or visualizing.
The car has been gone over with a fine-tooth comb. ECST has been and still is reading fine. Tires are kept properly inflated - it's poor advice to install tires that were not used when the car's safety handling was rated. I continue to use the proper tires-updated ones, of course. And, there are less than 6K miles on them.
Spark plugs are fine.
No leaks in fuel system.
And I misstated mileage - it averaged 28 mpg, with more than half highway.
If anyone else has an idea, I will consider it.
Thanks.

I can only base my diagnosis off of what you have diagnosed and the data you have collected. Any and all data is important.

And as i said before each tire is made of different compound. I guarantee you that my cars safety handling would be MUCH higher with a pair of slicks on it than the factory Firestones.

Many dealers never check the ECTS. It will not set off any code either. What is your temperature gauge reading?
If the ECTS has never been replaced, I am 99 percent sure THIS is your problem. For 8 bucks at autozone its worth it to change out.

What kiind of plugs are you using? NGK Coppers are the best. Platinum or iridium do not work well with the waste spark ignition system on the s-series. How are the spark plugs "fine"? What color are they at the electrode?

[magnetman]

Originally Posted by hikerpat

One of the problems with on-line forums is that, there are some who believe they can "diagnose" without testing or visualizing.

Then why did you come to the forum for a diagnosis?

Every Saturn forum I've been to the first response you would get, to the poor fuel mileage question, is exactly what you got from RC1488. Start with the ECTS then the Thermostat. The computer adjusts the richness of the mixture to the temp of the engine. If your engine is running cold the mixture will be rich. If the ECTS is bad it may be sending faulty information to the computer. Check the ECTS with a mirror or something. It should have a brass shoulder, at the top of the nut:



Center part is the good ECTS, right one is the old style that fails. (ignore the PCV valve on the left )

So, start there and go on.



Hey RC, just found out that you're on the Saturnfans forum too!!

[s1120]

Would this be the same switch in my 94 SL2??? Millage has been dropping [about 24mpg!!] and with 180K Im sure something is bad.

[RC1488]

Yupp this can be your problem You car has two sensors, replace the one with the two wire connector. Also check your operating temps. Should be at 1/2. If its low replace your thermostat with an OEM one. This too can hurt fuel mileage.

[s1120]

Originally Posted by RC1488

Yupp this can be your problem You car has two sensors, replace the one with the two wire connector. Also check your operating temps. Should be at 1/2. If its low replace your thermostat with an OEM one. This too can hurt fuel mileage.

Guage runs right about 1/4. SHould be higher???


I also find the fan running a lot. After driving a bit, AC off, but guage at 1/4 and cold out. Also pinggs a bit....


Sorry for the post Hijack.

[RC1488]

Originally Posted by s1120

Guage runs right about 1/4. SHould be higher???


I also find the fan running a lot. After driving a bit, AC off, but guage at 1/4 and cold out. Also pinggs a bit....


Sorry for the post Hijack.

Change your thermostat. Yours is stuck open. OEM Only.
Replace the ECTS as mentioned above. It controls many subsystems of the car including fan control and air fuel mixtures (could be the cause of the ping).