I recently bought a 98 Monte Carlo Z34 to replace my other L36. However I noticed fuel economy is terrible compared to my other L36. (98 Oldsmobile intrigue). I already replaced a few cheap things such as upstream O2 sensor, thermostat, coolant temperature sensor, and FPR. but fuel economy still seems pretty bad. It's about 16mpg 70%hwy/30%city. no CEL.
I thought the coolant temp. Had something to do with it. Because the gauge on the dash doesn't move much further than 100 Even after operating for well over 30 mins. But after replacing the thermostat and CTS, Nothing is different, the needle is in the same general area.
The gauge on the dash goes from 100 to 260 I installed a 180 t-stat so I would think it should be smack dab in the middle. Can anyone confirm if my temp. gauge is working properly? Picture was taken after about 45 mins of highway driving at 0* Fahrenheit. Could this be the cause of poor mpg? If so what am I missing that's making it run so cold? I don't believe I've ever seen my fans running so they can't be constantly cooling the engine.

Anyone have any input it would be much appreciated.

 

Welcome to the forum!!
Do you have heat when you turn on your defrost/heater?

 

Thanks!
Yeah heat works pretty decent. though I'll admit it could be a little better.

 

It could be possible that you got a bad themostat.
https://montecarloforum.com/forum/mo...ght=thermostat <--click

 

When I replaced the thermostat. I noticed the gauge dropped and stays steady a little bit colder than the previous thermostat. I believe OEM is 195f which is what I'm assuming was in it when I bought it. I used a 180f and the drop the gauge made makes me belive both thermostats were functioning properly. Which is what made me think CTS... well after replacing that and driving around today it's still showing cold.
This has got me stumped. Not sure if previous owner tuned the PCM to make it run cold. If that's even possible. Or if temp is fine and the instrument cluster gauge is inaccurate. Which would mean something else is behind my poor fuel economy.

Just for clarification. My temp gauge is showing pretty cold? Or do Monte Carlos just have funny temp gauges?

Also anyone with a Z34 or L36 for that matter. What mpg are you getting? My Olds was about 23-25mpg. That's why I'm thinking 16 mpg is **** poor. However I know my Monte should get a little less than the Olds. At least according to the dealer papers. But still.

I'll probably set up an appointment with my mechanic and let them figure it out, but I'm still open to ideas.

 

Do yourself a great deed... get a OEM stat and be happy! Reason is you are running to cold and staying in open loop fueling, every computer is set up to reach/achieve a certain temp in a certain amount of time. I won`t go into the total details but the higher temp is required for everything to function properly. If you have to much heat then you increase the capacity of the radiator and leave the stat at the factory required temp.

 

If the temp gauge on the cluster is accurate. I was thinking it might be stuck in open loop. However it was still running cold with previous t-stat. With the 180f it evens out well below where 180 should be.
I'm not convinced both thermostats were bad.
I also ran a 180f in my other L36 with no ill effects.
The only thing I can think of that I have not tinkered with is the fans. But I have never seen them operate. Correct me if I'm wrong but I believe there set to turn on at 210f-220f. So if coolant doesn't reaches that temp it would make sense why I've never seen them run.
And that's where I hit a dead end. nothing else I can think of that's keeping it cold. If only I had one of those fancy tech 2's I could probably instantly pin point the culprit.
I have another 180f t-stat in my intrigue that I know functions properly so I'll swap that in just to be sure.

 

I told you straight up. EXAMPLE: your torque converter will not go into lock-up until 188* your fuel trims will not go into closed loop until 192*..... I can go on but I think it is time for me to stop helping people.

 

Torque Converter temp is not based on engine temp, it's based on trans fluid temp.

Closed loop kicks in at 170* AND when the oxygen sensor reports a specific voltage (meaning it is up to temp as well), also after a specific amount of time it will kick on.

He is completely safe running a 180* thermostat on a stock tune, though it is beneficial to make PCM adjustments for a 180, it doesn't hurt anything.

I agree though, throw a 195 in there. 195 is stock, and should be the middle point of the gauge. These gauges are also notorious for being inaccurate, if you have a way to scan through the OBD2 port I would do that and see what the actual reading is.

If the car never goes into closed loop, you will get a CEL.

 

Here the problem is it takes to long to get to closed loop. To long to reach temp so the converter is freewheeling even the readiness test are postponed until the required temp is met. The thermostat controls the engines internal temp, not the cooling system temp. So until the cooling system temp reaches the required by the computers temp it will stay in open loop. There may very well be a pending code as for the time to reach operating temp but it ignores /clears it as it does get up to 180* as it should normally.


Your car should reach a full operating temp of 210* at that temp the fans should be on low power at this time. At this temp your engine is efficient, your oil is hot enough to evaporate the water in it and the emissions are basically at their peak performance. Beyond this temp the fans control the cooling system temp until an overheat is detected at which time the computers will attempt to help, limp mode, CEL and even cutting fuel.