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Old 03-15-2018, 10:18 PM   #1
rocket98
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Default Low MPG... Really low MPG.

'98 SL2 auto. To preface, mileage over the last month or so has been almost entirely city, with the occasional trip a few miles up the highway - once a week. Driving like this, a full tank will get me 220 miles, on the dot. And by full tank, I mean the entire tank (I ran out once. Gotta love the fuel indicator that never comes on!). 220 mi./12.1G ≈ 18.2 mpg. Of course it won't be as efficient in stop-and-go traffic, and it's a 20-year-old car, but wow... This is a good 6 or 7 mpg below rating, and it's been running this way the whole time I've had it. I've been checking the fluids regularly, including for leaks on the ground underneath, and there doesn't seem to be one; I do catch the occasional whiff of fuel, but nothing alarming to indicate a leak there, I don't think. It sounds like 2nd and 3rd generations usually operate at just over the 1/4 mark on the temp. gauge, exactly where mine has been, although I have read a handful of comments saying it should be closer to 1/2. The needle occasionally jumps and drops erratically, pointing right to a thermostat, but my mechanic insists he put a brand new one in when I had the radiator replaced. Maybe it wasn't the 195˚? Seems to me like it's a bad/wrong thermostat, but could it also potentially be the ECTS? And how much of the drop in efficiency could be chalked up to it just being an old engine?

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Old 03-16-2018, 05:05 AM   #2
Waiex191
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1999 SL2
1998 SC2
Default Re: Low MPG... Really low MPG.

If you see erratic temperatures, that is not likely the thermostat. It does not change that fast. I'd change the ECTS. Especially if you have the old plastic one. I don't think the old engine is an issue.

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Old 03-16-2018, 07:57 AM   #3
trottida
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Default Re: Low MPG... Really low MPG.

Wow, lots of activity lately around poor fuel mileage and the relationship to engine temperatures. I'm going to recycle this post that I wrote this morning in another thread. It will likely be relevant to your situation as you seem to be running rich. Inaccurate temperature readings and/or low coolant temperatures greatly affect fuel mileage and result in excess carbon deposits which eventually fouls air injection (applies to gen3 only)/exhaust manifolds, oxygen sensors and catalytic converters.

Quote:
Originally Posted by trottida View Post
While I don't have a source for the information you've asked for I did find this post where someone took the time to map out the varied temperature points.

The temp gauges aren't terribly accurate and they differ between model years. My 99 SL2 reads lower than my 2001 SL1 at just above 1/4 vs 3/8ths and I've tested the 99 to ensure that everything is functioning properly.

There are 4 points to check - the ECTS, the ECTS connector, the thermostat operation and the actual measured temperature vs the scanner temperature.

You've already stated that the ECTS is new but did you check the connector? Has it been replaced before? If replaced where the wires soldered together or did they use butt connectors which are prone to failure. Does the connector look fouled in any way? Green stuff at the connection points is evidence that coolant leaked through the original resin tipped sensor (prone to cracking). If fouled then replace it and solder the connections into the wire harness.

You can test the thermostat to determine if it is leaking while on the car. Starting with a cold engine start it up and feel the lower hose just beyond the thermostat. If the hose begins to get warm before the engine gets to 188.6 F then your thermostat is leaking. It should be closed up until that temperature then it begins to slowly open. It will be fully open at 195 F. If it is leaking then your engine will not get to operating temperature and you'll be running rich.

You can also test the coolant temperature by measuring the temperature of the coolant flow from the return line in the reservoir. This should be relatively close to what you're reading on your scan tool. If it's not comparable then suspect the ECTS or the connector has failed as that is the source of the measurement you're getting through your scan tool.

If all this checks out it is safe to assume that your temp gauge reading is normal for your car.

...
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Old 03-16-2018, 11:55 AM   #4
Saturn Night
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Default Re: Low MPG... Really low MPG.

Erratic Temperature gauge readings, such as a spike from low to high or vice versa, can be indicative of the failing ECTS.

You smell fuel, which can be indicative of a leak in the fuel system OR running rich air-fuel ratio.

I would check live data with a scanner, to see if the car is getting out of "open loop".

It would also be advisable to pull the plugs and check them. If you are running rich, they may smell like gasoline & the oil will smell like gas too.(Unburned fuel can be pushed past the rings, and will be absorbed by the oil when you run rich. This will cause premature wear on the rings.)

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Old 03-16-2018, 01:55 PM   #5
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Default Re: Low MPG... Really low MPG.

Low fuel light that never comes on + twitchy temp gauge might indicate a wiring or ground issue for your instrument panel.

Quote:
I do catch the occasional whiff of fuel
I keep three gas cans, a snowblower, and a lawn tractor in the garage, and I never catch whiff of gas. Your gas might be evaporating slowly out of a leak - possibly a pinhole in a fuel line, rust in the filler tube, etc?

ECTS is generally the more common issue; just throwing out ideas in case that does not pan out - I would check your engine temp using a thermometer. Check out the whole thread that trottida linked to.

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Old 03-18-2018, 09:41 PM   #6
Rez
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1996 SW2
Default Re: Low MPG... Really low MPG.

Might want to check the exhaust as well. Mine was getting around 245 miles on a tank after I replaced the ECTS and the thermostat and come to find out I forgot to put a bolt back on the exhaust by the cat when I was trouble shooting

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Old 03-18-2018, 10:49 PM   #7
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Default Re: Low MPG... Really low MPG.

Also... my '97SW2 was running inefficiently at about 170kMi.
I tested the fuel pressure with a gage, seemed OK.

But when i installed a New press regulator, the fuel consumption returned to Good, which is where it stays IFF the SES/Check.Engine lights stay Off.


In 1998 the regulator is inside the Fuel Filter, which may be due for renewal.

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