Does thermostat temp. relate to mpg.?

[dhp123166]

Hello,

What does conventional wisdom say, (if anything) about the relation between radiator thermostat temperature and an increase/ decrease in

m.p.g.?

 

I searched for this topic and did not find anything.

[Zmanco]

In theory, a hotter engine, all else being equal, will absorb less of the energy from combustion which should mean more of the energy is used to move the pistons up and down. However, I have never seen any testing to verify if this is significant enough in the real world to be measurable and/or if there are other compensating factors at work.

[Brad-ManQ45]

Actually, the venerable L98 Chevy engine is a prime example - they normally run around 210 degrees...

[JSM]

Going from a No Therm, to a 160, to a 180, I noticed a major jump in idle performance. I'm sure each engine has it's optimal operating temp.

[cygnusx1]

In the factory injection and most injection systems, there is added fuel mixture when the engine is cold. If the sensor for the EFI is seeing a cold condition, the car will burn excessive fuel.

[dhp123166]

I thought it was my imagination because I just switched from a 180 to a 160 and have seen my fuel guage move faster than normal...

[Zmanco]

You didn't say what engine and how it's controlled, but as cygnusx1 said, it's quite possible if you have FI that the controller is always keeping some of the warmup enrichment active and so you're running too rich all the time.

 

I'd still be curious to discuss the general question further, for example, how would mileage be affected between running with 180 and 190 thermostats? In theory the higher temp should be more efficient, but I'm wondering if anyone has any experience or data to support that?

[dhp123166]

1976 280Z stock fuel injection, Cygnus X1 was right on the money...

[Tony D]

In the factory injection and most injection systems, there is added fuel mixture when the engine is cold. If the sensor for the EFI is seeing a cold condition, the car will burn excessive fuel.

 

I would disagree on the 'right on the money' portion. A 160 degree thermostat should not make any difference unless you are making VERY short trips VERY frequently in the day, and likely then fuel mileage is shot anyway.

 

I run nothing but 160's in all the Z's, and if you know the settings of the stock EFI system, you will be able to follow the following explanation.

 

Cold start is COMPLETELY gone by the time the engine reaches 177 degrees This is in theory, it can be either way, but the 'official' word is 177. TOTALLY OFF, as in nothing is on the enrichment circuit. EGR is enabled, and above this is the temperature the engine normally operates with a 180 thermostat.

 

What you get with a thermostat is a cracking temperature, and a full open temperature.

 

A 180 Thermostat starts cracking anywhere between 5 to 10 degrees either side of the rated temperature, and will not be fully open until at least 10 degrees above the rated temperature.

 

So what you end up having is an engine that runs up to 175-180 quickly. If you have radiator and flow capacity sufficient the thermostat will be somewhere around half open, or cracked to maintain a stable temperature between 185 and 190F. If it can.

 

With a 160 thermostat, you crack around the same region, and with the thermal outuput of the engine running with a load, will generate a running temperature of 170. My experience has been that this is 'close enough' for the computer to not use ANY of the 'cold start' parameters, and fuel economy is not affected.

 

What WAS affected though, was my underhood temperatures! I rarely seen underhood temps on anything over 180 now. The coil is a full 25 degrees cooler. Same for the distributor and I notice that the 'heat soak' shutdown restart problem inherent in the EFI S30's is FAR less of an issue.

 

I have heard over and over again people saying that running a 160 theromstat shoots your fuel mileage in the arse because it makes the car run on 'the cold start circuit'---while that might be true in theory in practical application (and given the wiring harness condition of many cars) it in effect has no proactical effect on it!

 

Running a 180 thermostat, or even a 190 will exacerbate the 'surging and hard hot-start problem' by a magnitide. Things start cooking under the hood with those thermostats in, and I hear pinging under light loads like going up the I10 grade in Palm Springs. The car runs noticably hotter on the gauge, as well as with an I-R scan.

 

Truthfully, I run 160's in the SoCal summertime, and a 170 when it comes time for late winter as it helps heater efficiency.

 

Carburetted cars it's a no brainer! With the quality and terrible vaporisation problems in most of today's fuels, running a 180 thermostat in 110+ heat just makes the fuel rail and float bowls boiling hot with radiant underhood temperatures.

 

When averaging 80mph, towing an 800# trailer, with three people in the car, using a 3.9 differential, and early 5 speed, I averaged over 22-23mpg on the interstate. Slowing down to 65through Wisconsin netted me 25-26mpg. In town anywhere between 17 to 20 was what I got using that same combination.

 

With a 160 degree thermostat.

 

How much more do you think I would have returned with a 180 or 190 in there? I know driving across the Desert Southwest would have been far more adventuresome due to fuel vapor issues! As it was my 'floor temp light' came on once on that trip---driving for 16 hours a day will do that I suppose.

 

My 260, on a similar trip years before peaked at 27mpg through Wisconsin, and got 24 on the freeway as a minimum, save for the section from Oglalla NE to Iowa, which averaged 19mpg at an average speed of 1XX Mph...I udnerstand that decrease in economy. That was with a 2.6, late ZX five speed, 3.7 gears, and a car loaded so heavy you couldn't see out the back, and two people in it.

[dhp123166]

I would disagree as I replaced my 160 thermo back to 180 and mpg is back where it should be.

 

Based on my oil temp gauge, the engine is 5-10 degrees F. hotter.

 

I have cowl induction hood vents so there is adequate ventilation and have never had any heat soak issues.

 

I have a federal vehicle (no EGR), I wonder if this affects anything?

[Tony D]

I would check your actual operating temperature. And also this has to take in to account the type of driving you do...and if your system is working properly in the first place. Oil temperature is not necessarily related in a direct correlation with the water temperature setting. Generally is can be noted they rise equally, but different under some conditions.

 

As I have stated, the cold start is all OUT of the situation by 177 F water temperature.

 

My car is a Fairlady Z, there is no EGR, as a matter of fact, the EFI system on it is from a 1976 Federal (non Catalyst) 280Z.

 

You don't give any numbers, where is 'where it should be' and how does that compare to what I've been getting since...about 1990?

 

I can't count how many 160's I've put into cars without any negative impact on fuel economy. Not to mention that Nissan offered a 170F thermostat for standard fittment, meaning to me that the range of temperatures seen during running with a 170F thermostat installed would have to be FAR away from any influential skewing of the EFI system.

 

So from that, one could logically deduce that if everything is working correctly a 10 degree change in thermostat operating range (from 170 to 160) should not have any appreciable effect--if any.

 

Actually, from what I've seen when dissecting OEM EFI setups, most have fueling back to normal in the 155 to 160F water temperature range...that being considered 'normal operation' in most cases.

 

Nissan uses 177F for the thermotime switch...meaning that is when the cold-start will never actuate on cranking, and the EGR will be enabled given proper conditions. My thought on this is that Cold Start Enrichment follows along with most other OEM's in that fueling is run to 'normal' due to emissions concerns well before EGR is enabled. And that would be around the 150 to 160 range. Easily sufficed by a 160 Thermostat's operating range.

 

I know starting cars sitting with the hoods open in the desert sun will give some strange operational troubleshooting issues as the thermostat housing heat soaks to around 140-150, and the rest of the car is only 100F. And this causes coughing and spitting, like the engine is running lean.

 

Knowing what multipliers I use on my standalone, REAL enrichment (the fuel mileage affecting kind) happens below 120F. Once you are above that point, you can start the car and run it on normal fueling with no problems.

 

I know my 260Z is a BEAR to start below 80F with no starter system engaged. But at 110F it fires up and drives away cleanly.

[dhp123166]

I'll keep my 180, thanks for the knowledge though...

[JSM]

Tony, where does radiator efficency play into this? My current setup is a 3 row w/a 180. With another 73 I had it had a 2 row and I ran a 160. It seemed that w/ the more row radiator the 180 ran better w/ the hotter thermostat.

 

On a side not, I would test your thremostats. This is very easy w/ a microwave and a cup of water. I noticed big variations on a 160 I had. Both in temp. and speed of opening and closing. It read closer to a 180 and opened and closed very slowly to another brand 160 I bought.