Major MPG Difference - Looking for some thoughts on this...

[kwbills]

I recently retired a '01 SC2 - It had the SOHC and a 5 speed manual. I purchased it years ago with about 85K miles. I would consistently get 40 MPG out of this car. I did run non-ethanol premium in it. I did have two fill ups in a row that I got 44.1 MPG.

Well, I recently gave this car to a friend in need and it just turned over 338K miles. I was still getting 36-37 MPG running plain ol' 10% ethanol 87 octane (I live in Oregon and travel between two cities over a mountain pass. I live at 2600', the mountain passes are 5100' and 4200' and the city on the other end is about 500'.) The 44.1 MPG was going over the 5200' pass.

So I replaced my old '01 SC2 with an '01 SC2 that has 170K on the vehicle and about 30K on a rebuilt DOHC. It also has the 5 speed manual. The thing is, I am only getting 25-27 MPG going over the same two passes.

The car seems to run fine, it does crank a little longer than I feel it should before starting, like for 4-6 seconds. No missing, no sputtering when going up hills. It does lose a little speed going up the hills, but that's understandable. I have not really started looking at the engine, like fuel and air filters, plugs and wires, etc... I have an ICON T7 tablet I plan to plug in and see if shows anything, as well. I'm Just looking for some thoughts on this and what other things besides the obvious maintenance items listed above.

Also, there may be a bit of a loss of power. My old SOHC model I would cruise along at 80 MPH no problem. The new to me car, it seems like it struggles to maintain 80 MPH in the same places.

Thanks in advance.
Ken

 

[Derf]

For comparison, i has a 97 and still have a 95 SC2, 277K and 244K that get 35 on flat highway since i drove both off the lot.

The SC2 is geared differently (different tranny) and feels different in the seat. Hills are not its forte. Go take a drive in WV and you'll see. It is because of where the shift points and rpms are, hence the power bands.

Run a wet and dry compression test to see what you're working with. 30K on a property rebuilt dohc should be 205+ each cylinder.

Assuming no issues there

If you just obtained the vehicle:
OEM plugs NGK 5643 (check part #)
New air filter
New wires if over the resistance limit
Clean throttle body
New fuel filter
Techron in the tank
New fluids all around.
For the low cost, I'd do pads and rotors all around, paying attention to the as found condition of the caliper slide pins. You may have a caliper not retracting all the way.
Check for bearing play
Replace the rear sway bar end links (moogs have grease fittings)

As for seemingly lower than expected mileage

1) Make sure the ects sead something reasonable versus ambient temperature and when running. If not, replace ECTS with brass unit, not AutoZone not advanced auto not O'Reilly's. Get a Delphi and replace the pigtail. The temperature reading that is inferred from this sensor has a huge effect on fuel to air mixture. In fact, it may be part of the reason for the extended crank at startup if it is significantly different reading at ambient temperature.

2) Replace the front oxygen sensor with the denso equivalent of the OEM which is a denso labeled ACDelco. No Bosch. This is the other main contributor to the determination of the air fuel mix. They get poisoned about every 125K and then kill your mileage.

3) Check fuel pressure. If too high, you may be going through fuel too quickly and running too rich a mixture, which again might be affecting startup. Also watch for bleed off of the fuel pressure with key on but engine off. It should stay constant.

4) catalytic converter/resonator/muffler maybe partially clogged.

Where does the temperature gauge read when driving 65 on a flat road? Or just in general what it warms up?

Usually a clogged cat presents with no power going uphill and a higher operating temperature. You can pull off the front O2 sensor and hook up a back pressure gauge to see if there is significant back pressure. Even the resonator and muffler can clog with material from the cat as it breaks up and gets pushed backward in the exhaust flow.

So that is a reasonable summary of what I would look at, what I would do, and how. The front O2 The ECTS and connector, and the stock spark plugs gaps at 040 should have the biggest bang for the buck in terms of performance improvement / recoupment.

 

[kwbills]

Derf, thanks for the info. I saw a couple other posts on the ECT and pigtail -- I was going to explore that tomorrow on the next day. I still need to plug in my Icon T7 and see what it shows me. I've been forced to start wrenching on my vehicles again. My mechanic retired and I wouldn't trust the other mechanics in this small town with servicing a Matchbox car.

The temperature hangs out just below the half mark on the dash gauge. It doesn't seem like it's running hot. I still have plenty of power going up hills, the little 1.9l just doesn't like it. My previous '01 was the same way with hills.

I will order a few of these parts and be ready.

Thanks again, and will update as I go though this process.

 

[Derf]

Ok, I guess I misunderstood about power going up hills.

And solder and shrink wrap the new ECTS pigtail. But splices suck and you need a rock solid connection here.

Warm the engine up halfway before trying to remove the front O2 using an O2 sensor socket, And do not use anti-seize on the threads. Should probably pre-soak several rounds with PB blaster the night before and then hour before working on it

 

[kwbills]

An update: I replaced the front O2 sensor, plugs with NGKs, wires, ECT and pigtail and the air filter.

The O2 sensor was in bad shape appearance wise. The voltage from the data stream was hovering at 0.100 and would dip down to about 0.050. I may do the rear O2 as well, the voltage readings were mainly in the range, but would occasionally drop below the minimum range value for short durations.

I am assuming the ECT was functioning properly, the coolant temp would run from about 180 to 205. When I removed the pigtail connector to inspect it for any oxidation, the white plastic piece on the ECT broke off. I already had the part so I replaced it.

Plugs had a little fouling to them. When I was removing the plug wires, one of them broke, so clearly they had some age to them and were in need of replacing.

I plan to do the fuel filter and clean the throttle body when the rain subsides.

The results thus far: First, it seems to be running smoother. Before I did the above, I was getting about 25-27 MPG. Also, when cruising along at 70 MPH, the RPMs were 3500 give or take. It is now at 2950 RPMs at 70 MPH. I also have more power when climbing a small hill when leaving town.

I have a little device called an Ultra Gauge. I picked it up about 15 years ago when I bought my old SC1. It plugs into the OBDII and gives you data stream readings. It will give you the instant MPG and the average MPG. I didn't have it hooked up before I tuned up the car, so I just did the math when I filled up the tank. So far, for the 40 miles I've driven since tuning it up, the UG reports I'm averaging 36,8 MPG. I looked the Ultra Gauge up yesterday and see it is still available for purchase, just do a web search if you're interested in it. I'm not affiliated with the product in any way, so there is no shameless self promoting going on. It will also read and clear trouble codes.

Derf, thanks for your recommendations.

Ok, I guess I misunderstood about power going up hills.

And solder and shrink wrap the new ECTS pigtail. But splices suck and you need a rock solid connection here.

Warm the engine up halfway before trying to remove the front O2 using an O2 sensor socket, And do not use anti-seize on the threads. Should probably pre-soak several rounds with PB blaster the night before and then hour before working on it

 

[Derf]

No problem.
Was likely running way rich, since the 02 was under registering the O2 present, calling for more gas all the time.

So bye bye gas mileage.

There are bank 1 sensor 1 codes for certain misbehaviors of the sensor, as well as rich codes, but apparently none applied in your case.

 

[Charles G.]

Hey, I have a similar issue with my 95 SC2. I was wondering where I might obtain the following:
"1. ECTS with brass unit, not AutoZone not advanced auto not O'Reilly's. Get a Delphi and replace the pigtail. "

Looking for part number, good place to order from, and so forth. My initial searches have not been terribly fruitful. So far as I can tell my ECTS was replaced twice, once in '04 and once in '05.


2) Front oxygen sensor with the denso equivalent of the OEM which is a denso labeled ACDelco. No Bosch.

Went here https://www.partsgeek.com/catalog/1995/saturn/sc2/fuel_injection/oxygen_sensor.html
and found this:
Denso Oxygen Sensor
Part Number: W0133-1613813

This what I need?

THanks for your help!

 

[kwbills]

I ordered my ECTS and the front and rear O2 sensors from Rock Auto. I got a Denso from them. I just looked under the 95 DOHC and they have Delphi, ACDelco, Walker, Bosch and NTK for your upstream O2. The ACDelco part number at Rock Auto is 2131186.

I also got a Delphi ECTS, I just looked, it's available for the 95 DOHC, as well. The part number to search for on their site is TS10193.

Hope that helps.

Ken

Hey, I have a similar issue with my 95 SC2. I was wondering where I might obtain the following:
"1. ECTS with brass unit, not AutoZone not advanced auto not O'Reilly's. Get a Delphi and replace the pigtail. "

Looking for part number, good place to order from, and so forth. My initial searches have not been terribly fruitful. So far as I can tell my ECTS was replaced twice, once in '04 and once in '05.


2) Front oxygen sensor with the denso equivalent of the OEM which is a denso labeled ACDelco. No Bosch.

Went here https://www.partsgeek.com/catalog/1995/saturn/sc2/fuel_injection/oxygen_sensor.html
and found this:
Denso Oxygen Sensor
Part Number: W0133-1613813

This what I need?

THanks for your help!

 

[Derf]

Exactly my feelings after looking exactly where you did.

The one thing about the AC Delco O2 sensor back in the day when it was guaranteed to be ACDelco in the box is that it worked. All of this AC Delco professional, gold, silver crap, it's all sold to the highest bidder to fill the boxes. There's a chance you don't actually get an ACDelco but you get some other brand. For this reason I might go for the Delphi O2 as I've never had issues with any of their parts.

Don't forget the pigtail connector for the ects.
Butt splices suck. Solder and shrink wrap.

Watch carefully which warehouses the parts are coming from. Every Warehouse that is different incurs its own shipping charge. So if you buy three ports from three different warehouses, you pay three times the shipping as if they all came from one place. You may be able to find some of the parts now that you have part numbers for cheaper including shipping at other online stores.

 

[Charles G.]

Re-engaging with this (my SC2 isn't driven much, so fixing this issue hasn't been a high priority).

I have another question on the O2 sensors - I see the ones I'm looking at listed as being "upstream" or "front" - for the 95 SC2 is there a separate "downstream" or "rear" O2 sensor, or is the same type used in both positions?

Also, where is the "rear" sensor located - the front is pretty obvious but I'm guessing the "rear" sensor is probably only accessible from underneath the car. Thanks!

 

[Derf]

95 was the last year of OBD1. While OBD2 has a front sensor before the cat and a rear sensor to make sure it's doing its job, OBD1 only has a front sensor. There is no policeman rear sensor.

There should not be anything for a 1995 listed as downstream because they don't exist.

The one that you want is a Denso brand O2 sensor. The one that looks identical in the picture to the AC Delco which is much more expensive. The OEM part is in fact the Denso sensor so there's no point in paying an extra 30 bucks for the AC Delco because all you get in the box it's same sensor. If you are on RockAuto, it should have a start next to it for being the most popular for exactly this reason.

Don't know about the sohc, but my dohc is absolutely choke on Bosch O2 sensors, so if cost is an issue, stay away from Bosch.

 

[Charles G.]

95 was the last year of OBD1. While OBD2 has a front sensor before the cat and a rear sensor to make sure it's doing its job, OBD1 only has a front sensor. There is no policeman rear sensor.

There should not be anything for a 1995 listed as downstream because they don't exist.

The one that you want is a Denso brand O2 sensor. The one that looks identical in the picture to the AC Delco which is much more expensive. The OEM part is in fact the Denso sensor so there's no point in paying an extra 30 bucks for the AC Delco because all you get in the box it's same sensor. If you are on RockAuto, it should have a start next to it for being the most popular for exactly this reason.

Don't know about the sohc, but my dohc is absolutely choke on Bosch O2 sensors, so if cost is an issue, stay away from Bosch.

You know what, something seemed odd about my O2 sensor - I found it difficult to believe that it hadn't been replaced in the entire time I owned the car, and sure enough I came across a misfiled repair receipt from 2023 showing that it had been replaced - visually when I looked at it earlier today I thought it might be a Delphi.

So, I very much doubt that my O2 sensor is causing my poor MPG. What else can it be? My ECTS was replaced last in 2005 at 141,796 miles (currently have 191, 608, so pretty much 50K miles ago).

One other thing I've noticed for awhile now is that my coolant temperature gauge on the dash seems to show the engine running a little hot. So the first image shows approximately what I see after the engine warms up, while the second image shows what I recall as a more typical temperature for my car, and I think is likely a standard temp under normal operating conditions. Is there something mechanically wrong or is there a sensor issue of some kind? Seems likely that if the engine either thinks its hotter than normal, or actually is hotter than normal, it's going to run rich and tank my MPG. Any insight would be most welcome!

 

[Derf]

If the temperature sensor was not a Delphi was not an AC Delco, Delphi etc but was purchased at AutoZone or advanced, people have basically tested them and proved that they don't read properly. Basically their calibration curve doesn't match the OEM sensors. This may seem minor, but to your point with the gauge pictures, they are indeed different. Please remember that the gauge needle travel is not linear. I've never tried to figure out what the temperatures are at different spots but I vaguely remember someone who took the time to do it, it may have been on Saturnfans or elsewhere. A search of the web might bring it up. I would look under ects Saturn S-Series or ects Saturn calibration. I don't know and I will look as well. Actually come to think of it, I actually posted a resistance versus temperature curve. It's somewhere on this forum probably in the tech section. That doesn't address the needle travel, but it allows you to check the approximate resistance being read at the PCM.
The ects circuit may not seem all that critical, but as the temperature increases, the resistance of the ects decreases. I don't know what the PCM is programmed for in terms of minimum resistance it measures on that circuit to turn on the auxiliary fan under the hood behind the radiator. Looking at the temp gauge, sometimes my 95 dohc will actually go past the third tick heading towards the red. But the fan always kicks in and brings it back to where your second picture shows it. That's where my temperature gauge usually sits as long as I'm moving. The second picture is also where it sits with the AC running because the underhood fan is running 100% of the time when the AC is on. Couple all that with the fact that the gauge display is not linear and then the question becomes how much of a temperature change is that.

Back to the ects circuit. Because the resistance gets down to somewhere around 300 ohms when extremely hot, you don't want any stray or extra resistance so to speak being added to that circuit. Another known issue is the pigtail that connects to the pins of the ects. Those develop corrosion overtime and increase the resistance of the ects circuit which causes the temperature to be perceived higher than it is. The engine then is adjusting the mixture not only wrong at that temperature but also affecting all the temperatures, although as the engine cools down the contribution from that corrosion is a smaller percentage of the total resistance.

Sometimes the wiring where it connects to the pins inside or as it passes through the outside of the ects connector will also get partially broken, becoming an effectively higher resistance because there are fewer conductors present to carry the same current. I know all this sounds very esoteric, but people that put in the cheap advanced and AutoZone brass sensors are actually screwing up their air fuel mix the entire time but don't know about it.

The above, since it affects the air fuel mix, that mix generates x amount of oxygen, which is then sensed at the O2 sensor, which then sends feedback to the PCM telling it to incorrectly balance that offset from the temperature reading and I'm not sure if the top of my head if a higher reading on the ects leads to a rich or lean condition more than it should. You can look at the fuel trims with a diag scanner that can read real time data.

So it's critical to know which brand ects and which brand of oxygen sensor are in there. Should be marked with part numbers.

Other possible culprits:
Wheel bearings
Brakes not releasing fully/caliper pins not properly lubricated.

Lower compression in each cylinder equals less bang for each compression stroke and therefore less overall power, which usually ends up in you putting your foot down further on the gas pedal.

Oil control rings seizing in place allowing oil to pass into the cylinders which burns, changing the O2 present that is read by the sensor.

Traces of oil that don't burn in the cylinders burn in the catcon, destroying the honeycomb and having it break off, fly to the back, and block the exit somewhat, causing back pressure issues and requiring more fuel to keep the vehicle moving forward.

I think I mentioned not using OEM plugs could be an issue. I apologize for possibly repeating things, but I just noticed after 3 days that I never hit post when I originally wrote this, so the second half I am tacking on and trying to include most things in case I missed them.

Ignition coils combined with the ignition control module may be producing weak spark which leads to not as much bang for your buck per cylinder per cycle.

EGR valve may not be recirculating enough of the blow by back to the intake manifold to cool the combustion temperature therefore making the burn less efficient.

I could pick things apart all day if I really wanted to, but that's part of my point. You can chase stuff like this in 100 directions at the same time, but the causes are almost always the obvious ones.

So what brand of ects do you have? Was the pigtail changed with the ects? What is the temperature read through the diagnostic port versus actual temperature? What brand of O2 sensor is in there? Just because these components were swapped out with new ones doesn't mean they are the correct or optimal parts for the vehicle, nor does it prohibit accelerated wear and for the O2 sensor, if there is a significant amount of oil being burned, that sensor gets poisoned very quickly and underreads how much oxygen is actually in the exhaust gases and then of course mis adjusts the fuel mix.

What is the fuel pressure at the Schrader valve on the fuel line right before the fuel rail to which the injectors connect? What is key on fuel pressure and what is key on engine on fuel pressure? Are they in spec? The fuel pump could be weak, the filter could be clogged, the fuel pressure regulator under the hood may have failed. It is vacuum controlled and easily replaceable just don't drop the o-ring.

With that huge RPM difference, I really don't know what to say. I can't believe it will be that much of a difference at the same speed based solely on the things you mentioned swapping.

Are you sure you do not have any leaks at the exhaust manifold gasket, the exhaust manifold itself, or the intake manifold gasket? Sucking the unmetered air in at the intake manifold will cause a lean condition in whatever cylinder it's occurring but the net result is less oxygen being sensed at the oxygen sensor which then says oh hell I need to rich in the mixture to make up for that. Also listen to the exhaust note it should be pretty smooth. If you can hear the individual cylinders firing, they may be misfiring. There must be a misfire counter for each cylinder somewhere in there stored by the PCM but it's probably only accessible with a GM tech 2 or similar device that can truly probe an OBD1 based PCM. I would like to know what you're using as a scanner for the 95. I need to get something before they disappear off eBay

 

[Charles G.]

Thank you for the thorough response! Lot to digest, here.

Regarding the ECTS, I don't know the brand but the work was done by my local Saturn dealership (Saturn of Torrance), so I can only assume they used the correct parts for their work. I can pop the hood in the morning and take a closer look.