Does a cooler running engine reduce fuel economy?

[Ludichris]

Just curious. I need to replace my t-stat as of about a week ago or so, just haven't gotten around to it yet. On the highway, she's hovering around 175 degrees or so (she used to run at high 190's, low 200's, but I had an overheating due to a cracked coolant elbow, and I think the spring in the t-stat got weakened or something). I've noticed a significant decrease in fuel economy, just by eyeballing the gas gauge so far (i.e., I've almost used 1/4 tank...and I'm barely at 100 miles on the trip odometer. Usually, at that point I'm at LEAST at 150 miles, sometimes closer to 200). Also, I've read different things about winter mix fuel and summer mix fuel, and not sure how much truth there is to that...but mainly just wondering about a cooler running engine knocking down fuel economy, because if it does - I was thinking about replacing the T-stat with a 180 degree Intense T-stat, but not if it's going to knock down fuel economy.

[01bonneSC]

Yes, it can. The ambient temps around us are dramatically dropping so I am willing to bet thats where your decrease is, along with winter blends.

I ran a 180 drilled and never had a problem except for long warm up times in the winter.

Personally I would just run a stock temp one in the winter and put a 180 in when daytime temps get back up to low 70s in the spring.

[00Beast]

Or just run a stock 195* T-stat, as it's obvious you're not really after the performance affects a 180* 'stat gives...

[Ludichris]

I see. So, another couple questions then: What 'are' the performance benefits of a 180* t-stat? Also, is there a 'sweet spot' for ambient air temp? I'm figuring there must be, because we don't like our engines to suck in hot air, but if I'm understanding correctly, if the air is too cold, that has a negative effect too, is that about right? So is there like a 'sweet zone' where the outside air temp is between 65-90 degrees or something where our engines will operate optimally?

Also, even though I DO have a 195* t-stat in right now, but the spring is weak, and it doesn't even get UP to 195* (like I said, somewhere around 175 is where it is normally) but I still have plenty of good heat...is it just not good for fuel economy and the engine in general to run a 180* in the winter? Because right now...the t-stat that's in there, is more or less behaving like a 180* t-stat.

As far as fuel economy, we'll see how she does this weekend, I'm taking her on another 400+ mile round trip to Illinois this weekend.

[01bonneSC]

Also, a 180 stat doesnt really work unless your PCM is tuned for the fans to come on at a lower temp.

I would just throw in a new 195 stat for now since you seem to think the spring is weak. Theres no reason to try to "mod" when the car is not operating right in stock form.

[nos4blood70]

I'm running a 180* and I have the fans tuned to run at that temp too. To be honest, I didn't notice a drop in fuel economy at all. Although, as mentioned by Jake, I did notice it does take a bit longer to warm up fully during the winter.

[2000Silverbullet]

If the engine does not get up to the programmed PCM temp, it will remain in open loop and ignore the O2 sensor, running very rich.
You can buy an electronic device to fool the PCM or have the PCM reprogrammed so the engine goes into closed loop at a lower temp.

[Ludichris]

If the engine does not get up to the programmed PCM temp, it will remain in open loop and ignore the O2 sensor, running very rich.
You can buy an electronic device to fool the PCM or have the PCM reprogrammed so the engine goes into closed loop at a lower temp.

This makes sense. I'm not a gearhead, but I DO understand that. Okay, then my fuel economy is probably down because it never gets even close to 195* right now. I'll change that t-stat as soon as I can, too bad I have to go on a 400 mile road trip before that will happen (aka: I'm about to spend more money on fuel than usual.).

[yonkerse]

The stat only takes like 10 mins to change...should have time to replace it before leaving. Could even change in parking lot of autozone with a couple tools!!

[Ludichris]

The stat only takes like 10 mins to change...should have time to replace it before leaving. Could even change in parking lot of autozone with a couple tools!!

I thought about that, but doesn't a bunch of coolant leak out all over the place when you pull the hose off the housing? Also, is it just an O-ring type seal, or is there another gasket for the housing? I just didn't want to be a bad citizen and leave a big puddle of coolant in a parking lot somewhere.

[95naSTA]

An actual test and some reading on the subject:

https://www.easyperformance.com/support ... nt-testing

[Ludichris]

An actual test and some reading on the subject:

https://www.easyperformance.com/support ... nt-testing

I just read that exact page about 30 minutes ago! It's a good page though, a lot of good info. For those that aren't into clicking on links though, I especially found this helpful - it's an excerpt from the above link -I highlighted a couple things that especially caught my attention:

Points of consideration

There are some other issues that do arise with running a cooler than stock thermostat. To begin with, all OBD-II computer controlled engines (all vehicles manufactured from ~1996 and on) monitor the Engine Coolant Temperature (ECT) with a temperature sensor. If the engine fails to reach the programmed lower temperature set point, then the PCM will set a Diagnostic Trouble Code (DTC) in the computer's memory and illuminate the Service Engine Soon (SES) light in the instrument dash panel. For most GM vehicles, this is typically around 170ºF. Furthermore, all the while the PCM sees the engine running below this set point, it considers the engine not warmed up and the PCM runs in what is called Open Loop Operation. This open loop condition has the engine running with preset fuel injection and spark advance values that are not optimal for performance and economy. Consequently, running continuously in open loop condition is a bad thing. Therefore, if one would conclude that running a 180ºF thermostat is good and running a 160ºF should be better, may find themselves with setting off the SES light and not achieving the performance and/or fuel economy they other wise would have.

Another issue that arises with the use of cooler thermostats is how the PCM uses the cooling fans behind the radiator to keep the engine cool. Nearly all contemporary cars now use electric fans to pull air through the radiator to cool the engine coolant. Only when the vehicle is at higher speeds (typically above 35 MPH) does the vehicle's motion naturally force air through the radiators and removes the need for the fans. Problems can even occur at higher speeds when drafting behind another vehicle and your car experiences less frontal air pressure. At lower speeds or in those special situations, the fans are necessary to keep the engine cool. However, these fans are controlled by the PCM and do not run all the time. Furthermore, the PCM typically controls the fans at two different speeds, Low Speed and High Speed. For many GM vehicles, the PCM will turn the fans to Low speed when the PCM sees the ECT reach ~215ºF and switches to High speed when the engine temperature reaches ~225ºF. Consequently, having a 180ºF thermostat will NOT have the engine running at ~180ºF when you are traveling below 35 MPH or in stop-n-go traffic. In this condition, the thermostat will be wide open, coolant racing through the radiator but without any airflow through the radiator, the coolant is not cooled down. Thus, the engine begins to superheat over the thermostat set temperature. To counteract this difficulty, a cooler thermostat must be complemented with a manual override cooling fan switch. With a cooling fan switch, you can command the fans to run when the PCM would otherwise have them off. With the manual fan switch activating the fans, you will be able to maintain the engine at the thermostat set temperature regardless if the car is standing still or traveling in stop-n-go traffic. Enthusiasts who enjoy trips to the local drag strip further take advantage of a cooling fan switch by allowing them to rapidly cool down their engines between passes down the track.

[Ludichris]

Okay, if any of you late-night folks are still up, or early risers - I just need a little help. I'm gonna try replacing the t-stat tomorrow morning before heading out to Illinois (200 miles one way). I've never done it on this engine and it looks pretty straight forward and simple, BUT - I've read where the couple of tubes that are right above the t-stat housing can get in the way of scraping the gasket material off of the upper intake side - but to me they look like they'll just bend out of the way (looking at photos online, it's cold outside!)....is that going to be an issue?

Second - there's an O-ring AND a gasket, correct?

And third - maybe most importantly - how important is it to drill that hole into the flange of the new t-stat? Should I do it, what purpose does it serve, and what size is that hole supposed to be again? Is it beneficial to have the hole drilled, or just leave it alone without the hole?

Oh, also - will the system burp itself of any air bubbles/air pockets and I shouldn't worry about that, or is there a "proper" way to get the air bubbles out?

I'm not a mechanic, but it looks easy and I've done my own brakes, plugs, wires, etc., so it shouldn't be a problem. Any advice or pitfalls to avoid, even for a simple procedure like that, is appreciated though. Thank you!

[95naSTA]

Series 3 engines stopped using the gasket and only use an o-ring (same one) for the thermostat. So, the gasket isn't really necessary.

Drilling a hole helps the system bleed but isn't 100% necessary. It'll keep the engine from heat soaking for a little longer but will also increase warm up time.

There really shouldn't be much in the way. and you will want to drain some of the coolant out of the radiator petcock before you start.

For bleeding, I typically start the car with the rad cap off, wait for the fluid to expand and rise to the point of it tricking into the overflow tube, put the cap on, wait for the fans to come on, and bleed the remaining air out of the thermostat housing bleeder screw.

[markwb]

I've noticed a significant decrease in fuel economy, just by eyeballing the gas gauge so far (i.e., I've almost used 1/4 tank...and I'm barely at 100 miles on the trip odometer. Usually, at that point I'm at LEAST at 150 miles, sometimes closer to 200).

Just remember the fuel gauge is not precision, or linear. The best way to check fuel usage is to fill the fuel tank to full, drive the 200 miles then fill up again as soon as you arrive at destination. Then divide the miles you traveled by the number of gallons it took to re-fill the fuel tank. (MPG) You should get at least 27 MPG HWY give or take in your Bonneville.

[markwb]

I've noticed a significant decrease in fuel economy, just by eyeballing the gas gauge so far (i.e., I've almost used 1/4 tank...and I'm barely at 100 miles on the trip odometer. Usually, at that point I'm at LEAST at 150 miles, sometimes closer to 200).

Just remember the fuel gauge is not precision, or linear. The best way to check fuel usage is to fill the fuel tank to full, drive the 200 miles then fill up again as soon as you arrive at destination. Then divide the miles you traveled by the number of gallons it took to re-fill the fuel tank. (MPG) You should get at least 27 MPG HWY give or take in your Bonneville.

[MattStrike]

Ideally, because it's a linear variable resistor, the fuel gauge would be linear. But the fuel tank is not square and over time it will change as the sender bushing wears out.



Because it has two 'feelers' that swipe on the resistor pad, it could have a sudden change to it's normal readout if one of the feelers stopped making contact. This would indicate that the sending unit is almost ready to be changed out if you gas mileage remains unchanged. Eventually it will stop making contact altogether and the fuel gauge won't work at all. They can be fixed without replacing the entire sending unit, but most of the time they are too rusty to justify repairing (at least where I live).

But I wouldn't suspect it was an issue in your case, considering everything else, but you can test it by checking your gas mileage.

[01bonneSC]

Do you have good heat coming from the vents?

[Jrs3800]

Currently doing 26.3 MPG in the 95 SE... It has a 97 PCM tho... 180F T-Stat..

The 26.3 is a mix of highway 55-60Mph.. 40Mph, 35 Mph and 25 Mph zones with several lights...

And yes the 180F can drop your MPG's slightly.. You really should not notice much of a difference if any at all.. But for the winter months I would run a 195 in your neck of the woods...

[Ludichris]

Thanks everyone. I did change the T-stat out right before my road trip. Went with a 195*. IF the fuel gauge is any indication, not only did my fuel economy come back, it did slightly better (because by the time I used 1/4 tank.....I was at about 170 miles...WITH two adult passengers and a trunk full of luggage). The t-stat itself was so simple I don't know what I was concerned about. The biggest hassle was the people at NAPA not being able to find the gasket, and wasting 20 minutes of my time. But the t-stat swap itself could not have been simpler. It remained at about 197* almost the whole time, and is running like a champ again. I might post a photo of the old t-stat - it was off center, we noticed it right away, like pushed slightly to one side.

Here's another thing though, and I would LOVE to know the answer to this: What relationship does oil pressure have with the cooling system??!! I'm not talking about at start up, because I know oil is thicker at start up and therefore oil pressure will most likely be higher - BUT - I posted a separate thread about high oil pressure and cooler running temps. I SWEAR to you, I'm not crazy, and as soon as I swapped the T-stat out, the oil pressure dropped down to where I was used to seeing it (slightly above 40 PSI at start up, down to 40 PSI at warm up, and at about 60 PSI at cruising speeds, as opposed to about 60 PSI at startup, STILL at about 60 PSI at warm up, and at about 70 PSI at cruising speeds). It could have just been a coincidence I suppose, but all I know is, as soon as the engine was running cooler (about 177* or so), the oil pressure was acting funny. Change the T-stat out, and it went back to normal. Coincidence, or is there some relation there?