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Old 12-17-2017, 06:45 PM   #1
ghostman258
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1995 SW2
Default Poor Gas mileage-Rough idle. Please help.

I've tried everything, but if any here might have some fresh ideas please post back. Here is the issue and what I've already done so far in attempt to correct this poor fuel mileage and rough idle issue...

My Saturn is a 1995 SW2 Wagon. It is in exceptional condition with only 100,000 miles on engine and car. It has a DOHC 4 cylinder motor.

It's very reliable and the RPM range at idle is perfect, but it runs rough. In fact while sitting at idle it nearly vibrates your whole body. here is what I've done so far trying to correct this poor mileage and rough idle issue:

(1) Complete tune up. New plugs, wires, air filter.
(2) Installed new EGR valve thinking that might be the issue.
(3) New fuel filter.
(4) Had the fuel injectors professional cleaned and checked.
(5) Replaced every engine and trans mount on the car. (I thought perhaps that might dampen the vibration of the rough idle.)
(6) New Radiator, coolant flush and fill.
(7) New trans filter and trans fluid change.
(8) Oil change.
(9) I also tried a couple fuel additives and always fill with good mid-grade gas.
STILL. I have this annoying rough idle and after taking a long distance trip I calculated perfectly my current fuel mileage AND I'M ONLY GETTING 22 Per gallon Highway. I feel this has some connection to the rough idle, but nothing I try seems to help.

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Old 12-17-2017, 07:15 PM   #2
Jonasan308
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1999 SC1
Default Re: Poor Gas mileage-Rough idle. Please help.

Have you ever had to use your heat?

Are you familar with something called a "ECTS"?
Its called the Engine Coolant Temp Sensor. Since you never mentioned it, im thinking it would be a good idea to check that because bad gas milage and poor idle is associated with the ECTS. The orignal design is made of ceramic and likes to crack and cause problems. I had to replace the one on my SC1 because i am a northerner that needs working heat.

Its easy and cheap to replace, 13 dollars for the part (flat brass nose in place of the original round ceramic design) and its easy to get to.

...
1999 Saturn SL1-129k at purchase(may 2016)-139k when wrecked(September 2016)

1999 Saturn SC1(CURSED)-156K at purchase(october 2016)-187k currently (August 2018)
Dodging potholes should be a sport

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Old 12-17-2017, 07:26 PM   #3
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Default Re: Poor Gas mileage-Rough idle. Please help.

The heat works great even on very cold nights. Regardless of outside temps of 100F or 20F there is no difference with this rough idle and poor fuel mileage.

I will change the ECTS to see if that corrects the issue. If this sensor is under $40 bucks then it's worth a try even if it does not help.

Thanks for the tip.

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Old 12-17-2017, 07:42 PM   #4
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Default Re: Poor Gas mileage-Rough idle. Please help.

Are the plug wires in the correct firing order, and what condition is the top engine mount? Can you place a ballpoint pen between the top rubber and the underside of the mount top?

Standing at the front of the car (facing the engine), the cylinders are L to R, 1,2,3,4. But the plug wires go 4,1,2,3. The engine firing order is 1,3,4,2.

Also, if you change out the ECTS, do not use anything larger than a 1/4" ratchet. Really tightening hard on that ECTS will cause it to break at the engine.
They already come with thread sealant on it so no need to add ptfe tape.
Examine the sensor plug to see if coolant has invaded it, if so, replace the plug and solder/shrink tape the wires instead of wire nuts or cheapo wire splices.

Last edited by toggenburg; 12-17-2017 at 07:52 PM..

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Old 12-17-2017, 10:14 PM   #5
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Default Re: Poor Gas mileage-Rough idle. Please help.

First step should have been to take compression readings, done while you were inspecting/replacing the spark plugs. If it hasn't been done, do it now...

New ECT sensors are in the $10 range, and easy to change, but be sure to check the harness electrical connector for corrosion, too. The sensor you should be focused on is the 2-wire one. The other (1-wire) is only for the dashboard temp gauge and has no effect on engine operation.

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Old 12-17-2017, 11:15 PM   #6
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1998 SC2
Default Re: Poor Gas mileage-Rough idle. Please help.

Wire Routing
Ignition wire routing as seen when looking at the engine and coils from the front, hood open, your left to right.

Engine
1-2-3-4

4-1-2-3
coil towers

Connect the matching numbers with a plug wire.


The next thing is this sounds a lot like a leaking EGR as you have an electric EGR checking this is easy. Remove EGR and cut up a can or a piece od ductwork etc that will cover both holes in the gasket. Set metal over holes and replace the EGR do not get aggressive with bolt tightening as snug will be fine. Now test drive and see how it runs.

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