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Old 05-14-2006, 08:30 PM   #1
David R
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2009 Astra XE
1997 SW2
Default Poor fuel economy

Hello all, I've had a good run of mostly problem free operation on my 97 SW2 for the last six or longer months of use. Unfortunately I'm back at it again. Fuel economy hasn't recovered since the winter weather has gone. I usually tune out the fuel consumption once the Winter fuel goes into the tank. I've had several highway runs and my economy hasn't come back. Normally get 42 mpg (cdn) on the highway and about 27 to 30 mpg in the city. We'll my city is right on the money, but my highway sucks. I'm getting about 30 to 34 mpg.

I've plugged in my limited OBDII scanner, no codes. Long term fuel trim is -3.5%, short term is about -8.5% at idle, going to zero or slightly positive at 2500-3000 rpm. Intake temp 75-80F at idle, Coolant is about 185F after about 15 mins of idle. Plugs were changed back in March when I started to suspect, but no effect. New air filter at sametime. Put a new muffler on in March (?) didn't seem to effect the car, Walker brand. Engine has plenty of power and accelerates without problem. Tire pressure is 32 psi.

Any one have any ideas?

David

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Old 05-15-2006, 09:22 PM   #2
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Default Re: Poor fuel economy

^^^^^

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Old 05-16-2006, 12:33 AM   #3
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Default Re: Poor fuel economy

If I'm interpreting the trim data correctly, the puter thinks your engine is running a little rich. If it really isn't running rich and it trims it back, you will have poor economy same as if it were running rich. I'd check the O2 sensor. A lot of cold running in the winter may have been the last straw for it. How many miles on the O2?

What is 42 mpg "Canadian" equal to in US measurement? (Is it the gallon that's different?)

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Old 05-16-2006, 12:36 AM   #4
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Default Re: Poor fuel economy

oops.

42 cdn is 33.6 US, I think.

What kind of plugs and what gap?

Maybe check EGR. Run some Chevron Techron fuel system cleaner for a couple tanks if injectors are suspected. Check ignition to see if its retarding spark, too.

Not familiar with fiel trims to comment on that. My obd II scangage doesn't show that.

Last edited by cheapybob; 05-16-2006 at 12:43 AM..

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Old 05-16-2006, 03:12 AM   #5
Titanium48
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1998 SL2
Default Re: Poor fuel economy

Quote:
Originally Posted by cheapybob
42 cdn is 33.6 US, I think.
Yup. 1 US gallon = 3.78 L, 1 imperial gallon = 4.54 L. Imperial gallons were used in Canada until conversion to metric in the late 1970's.

The official metric fuel efficiency unit is L/100km but it never really caught on, probably because it is a reciprocal measurement (lower is better) and it is more difficult to calculate in your head. So you get people saying things like "I got 40 MPG on a 500 km trip at 110 km/h".

I prefer km/L - higher is better and it is a simple division of a number in the hundreds by a number between 20 and 40. My saturn gets 14-15, my metro used to get 20 if I kept it under 100 km/h.

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Old 05-16-2006, 03:23 AM   #6
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Default Re: Poor fuel economy

185 after 15 mins sounds like a possible tstat problem. Unless fan has come on and cooled it WAY down. Tstat should open around 195.... if stuck open, could take forever to warm up to norm oper temp, if ever....

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Old 05-16-2006, 06:55 AM   #7
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Default Re: Poor fuel economy

Your fuel trims are within specs. It's just showing that the O2 sensor is seeing a slightly rich condition and the fuel trims are trying to shorten the injector pulse widths to get you close to zero on long term. If the long term trims to zero, then your 'puter and sensors are working, more or less. What was the O2 sensor sweep range and frequency?

Your coolant temp is not right. It's taking too long and is not reaching operating temperature. It might be the sensor/sender or a t-stat issue.

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Old 05-16-2006, 07:22 AM   #8
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Default Re: Poor fuel economy

Hmmmm, thermostat or coolant temp sensor it sounds.

If you get an aftermarket coolant temp sensor, make sure its got the brass tip, not plastic. The plastic tip on mine was cracked causing bad readings, and the bad readings will cause the computer to adjust as though the engine were cold.

If you need a new thermostat, I have a brand new, never installed one from Saturn in a box to sell. Make offer, paypal accepted.

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Old 05-16-2006, 03:30 PM   #9
David R
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Default Re: Poor fuel economy

I've always prefered the, how far can I go for a fixed amount of fuel, method of measuring fuel economy. I think in the aviation industry its the other way around, how much fuel do I need to get from here to there.

This car has always run a bit rich, even after swapping in a new/used set of injectors, didn't take long to settle in a slightly rich.

Spark plugs are 040", NGK don't remember the model number off my head. Been down the platnium road waste of money on this car.

Wish I could be more precise about the O2 data, but the OBDII is one the ELM chips attached to an old laptop. The stream isn't regular enough to use that way. There is about 70000Km on both O2 sensors. I would have just changed it, but the front O2 sensor is a bit more than $160 cdn now. A little to expensive to change on speculation.

I don't think engine temp is that big a deal in this case. Once on the highway, the temp gauge is between 3/8 and 1/2, which according to the OBDII is between 190 and 200+. I ran with a pot that I could switch in parallel with ECTS during the winter, really helped the city fuel economy a lot, as going to work was only 10 to 15 mins, 20 on a bad day. Didn't make a hell of a lot sense to put the brakes to a car rev'n at 1000 to 1200rpm at a
stop sign. It ran quiet smooth at 800ish after a minute. Its on the highway where my worst fuel economy occurs. Then again I'm on my third thermostat.

Any one know it the temp sensor on the autotrans gets used for fuel modes?

Been kind of wondering if I've got a small leak in the fuel system. Driving around in the city for 1/2 an hour and there, won't show a leak. Driving on the highway for 3 or 4 hours might add up. Anyone hear of leaks showing up on saturns?


David R

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Old 05-16-2006, 04:06 PM   #10
marcusa
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Default Re: Poor fuel economy

I would think a leak would be more apparent in city/short distance driving. Because then everytime you turn it off it would leak until the fuel system pressure diminished.

Other than a new OEM ECTS (less than $10 if I remember correctly) I'd go for the OEM Tstat if yours isn't. Also, how is your coolant? Low levels would retard the timing.

Also, try running some Techron, 2 bottles in one tank if you've never run it before. Also, try to put it in when you'll be driving short distances so it can soak in and clean good.

And based on the fuel trim data, try checking your air filter, etc. And check your individual plugs to see if any one cylinder is running richer or leaner.

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Old 05-16-2006, 09:59 PM   #11
David R
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1997 SW2
Default Re: Poor fuel economy

ECTS is OEM brass tip, tests good, connector looks good as well. Might do the Thermostat again, it is also OEM. Coolant level is good as well.

When the weather dries out I'll check the plugs for deposits.

I'll check the price on the techron, I usually add preston or canadian tire injector every third or fourth tank.

Question about the possibilty of a leak. In the city, would the fuel pump run all the time or just once in a while. I'm just thinking that if there was a leak on the return side, it would only be an issue when pumping.

David R

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Old 05-17-2006, 12:22 AM   #12
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Default Re: Poor fuel economy

If you had a leak you'd smell it.

I can't imagine your injectors are clogged, but might wonder if they were "fixed" to flow too well. Buy a used set from a junkyard and see if MPG improves?

Have you checked the EGR? Its not difficult to check and clean.

Replace O2 sensor?

How old are the wheels/tires? What brand, model and size, if not stock? Weight of wheels/tires and rolling resistance can affect MPG's a lot. I put Kumho 712's on the wifes PT cruiser and MPG dropped by 12%.

Have you checked each wheel to see if they spin freely or anything is dragging?

What oils are you using in the motor and trans? Synthetic oil is worth 1 or 2 MPG.

I'm about out of suggestions. You could ditch it and get a 5 spd SW1 of a similar vintage. That would surely get better MPG's.

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Old 05-17-2006, 11:59 AM   #13
David R
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1997 SW2
Default Re: Poor fuel economy

The tires are Michelin, two years, just had them rotated and one resealed for a rim leak. The service department commented about my front pads, I like to think they would comment about a bad bearing or dragging brake.

Running Mobil 1 5w30, just had that changed as well.

I've been giving serious thought to retiring the car as the AC compressor isn't compressing any more.

I'm going to check the plugs when the weather is good, think I'll pull the EGR and give it a clean. In theory, if it doesn't work the PCM should be able to identify it, but if its marginal it may not set a code. Just guessing.

Could bad power steering, labour the engine enough to impact the fuel economy. Again just guessing. It has always made an annoying hissing sound and groans on warm days.

Does anyone know what the timing advance should be at idle with the engine warm?

We'll see what turns up.

David R

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Old 05-17-2006, 10:05 PM   #14
David R
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2009 Astra XE
1997 SW2
Default Re: Poor fuel economy

I managed to pull the plugs (between storms). I've attached images, they look pretty good to me. Don't think there is anything wrong on the emissions side. Spark plug one is actually cleaner than it looks, its more camera angle hiding the clean side.

I might be going for an extended highway run this weekend. I think I'll reset the pcm before I go and see what happens.

David R
Attached Images
File Type: jpg Plug 01.jpg (86.8 KB, 23 views)
File Type: jpg Plug 02.jpg (79.8 KB, 16 views)
File Type: jpg Plug 03.jpg (82.3 KB, 12 views)
File Type: jpg Plug 04.jpg (79.0 KB, 16 views)

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Old 05-18-2006, 11:05 AM   #15
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Default Re: Poor fuel economy

Everything sounds good, I know everyone likes to point at the T-stat, but as in your case, as long as your running 190-200 on the highway, thats normal. Plugs look normal. Your fuel trim points to something I had, which may be your case, and harder to detect. I had a leaking intake manifold gasket, which caused the lower MPG and negative fuel trim readings. I'd take a look at that and any other vacuum leaks.

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Old 05-18-2006, 11:14 AM   #16
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Default Re: Poor fuel economy

If there was a vacuum leak, wouldn't MAP be low?

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Old 05-18-2006, 11:40 AM   #17
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Default Re: Poor fuel economy

Maybe it's not low enough to trigger something. Various auto parts store lend you tools, I know of an easy way to test a vacuum leak; stick the gauge dealio it in the oil dipstick hole and let her run. I'd also test your PCV at this point, just pop it out and shake it, if there's a noise get a new one. Check the pipe for it as well, as well as the air in to the crankcase from the intake. Finally, do your brakes seem like they've deteriorated, are mushy/non-resistant, or sink gradually when pressed? The pedal shuld be taut at pretty much all heights.

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Old 05-18-2006, 08:02 PM   #18
David R
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Default Re: Poor fuel economy

Vacuum leaks isn't something I've persued, as idle has always been smooth, 750 in gear and 850 in neutral. However its worth a try. Anyone tell me the easy spray product that helps on leak detection?

Marcusa, I think you've got something on the brakes. The pedal is a little mushy and the rpms bounce around if I pump the brakes. If I push the brake pedal hard then quickly release the rpms jump 25 to 50 rpms. So what should I look for on this one?

David R

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Old 05-18-2006, 09:21 PM   #19
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Default Re: Poor fuel economy

Spray some brake cleaner around the manifold to head gasket with the engine running. The engine will sputter or stall if there is a leak.

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Old 05-19-2006, 03:43 PM   #20
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Default Re: Poor fuel economy

Quote:
Originally Posted by FrozenPilot
Spray some brake cleaner around the manifold to head gasket with the engine running. The engine will sputter or stall if there is a leak.
Excellent thought, I'd forgotten about that. Doesn't starting fluid (really a spray) work too?

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