I don't normally drive over 70mph because I like to get the most mpgs. I occasionally punch the pedal to get past people and just because it's sometimes fun to show off the power of these tanks... ok, so on to my point...

I was working in Florida this week and I got out of there rather late on Friday to head home. It was 90 degree ambient temps. I had to drive from Tampa to Walterboro, SC to pick up my four wheeler by 6pm, so I had to speed. I had my truck running 80-82 mph and 2350-2400 rpms for hours at a time. I noticed my temps were 13 degrees separation. ECT 190 and EOT 203. At 87 mph and about 2400 rpms, it crept up to 204. I drove for about 3 hours straight with the EOT at 190 and ECT at 203 running 81mph on the Scangauge II.

I thought, well, it looks like my flushes didn't help and I'll have to report to all the guys on the forums that all my work was for nothing. I was going to be eating crow. Once I reached my destination in Walterboro, I loaded up my 800 lb beast in the bed and hit the highway headed home. I set my cruise at 75 mph and headed out. I was running about 1900-2000 rpms. My temps were ECT 190 and EOT 199 and would fluctuate to 186 and 196 and 188 and 198. When I got closer to home, I dropped the cruise to 70 mph and the temp spread was between 8 and 10 steady.

So my question is:

Is this normal expectation to have 3 degrees in only 10 mph? This is the first time I have kept my engine speed and rpms this high for hours at a time. I'm still sitting at 10 degrees when I perform the normal ECT v EOT highway test, so I guess I'll just monitor it. The other thing I noticed is that my ECT has started to drop. When I originally installed a new thermostat, my ECT was 192, then it dropped to 190...188... and now I'm seeing 186. It's only been about 5000 miles. Anyone else seeing this all the time?

 

Yep... The engine has to work a lot harder to ram that bar door thru the wind at 75+.

MPG drop like crazy above 72 in my experience. And that applies with my Maxima too, above 72 and mpg is under 25, under 72 and mpg is 30+

Josh

 

Your experience is completely normal, as are your temps.

Brian

 

I actually averaged 18.2 mpg's on the lie-o-meter in a 1300 mile round trip. On my trip down I averaged 20.1 mpg's. Then by the time I left Tampa, it was down to 18.4 from all the in town driving I was doing. Tampa traffic lights suck!

When I hand calculate, I normally run about .5 under that. I haven't done it yet because I need to fill up one more time to get the numbers correctly. If it's like the last 8 or so fill ups, It will be about 17.6-17.8 mpg. That's not bad considering the trip back I was just killing it. ha ha.

 

My temps are similar to yours. I do have 2.2 degrees different at cold engine. My temps go a lot higher towing my trailer in hot weather in mountains.

 

My temps behave the same way when driving 75+ compared to 70.

You also probably had a headwind from the weather front.

Sent from my Sprint PC36100 using IB AutoGroup

 

Bill I wouldn't comsider the cooling flush a Waist at all everything is Cleaner now

Its rare anyone Really Closes the ECT vs EOT Spread with Cooling system flush Alone

That Oil Cooler is Just Like a P-Trap Best Coolant filter on the Truck LOL

 

Haha very true. Just thought after all that 'talk' about how clean my system was, then I'm looking at the temps and I'm like maaaaan! I have to tell those guys my temps have gotten worse. Lol

So what about my coolant temp showing 186 now when driving 65-75? Colder weather maybe? I wonder if it's weak again?

 

Well the ECT @ 186 could be playing into it

At those speeds with a Good thermostat my truck would run 6-8*f this time year maybe even touch 10* spread depending on Ambeint temp


Just an Example here
Also lets say you run 85mph and Have spread of Say 15*F if you slow down to say 70MPH and see EOT drop to say 10*f spread then that is also a Sign of the Oil Cooler Cooling the Oil and working Properly

When I had that cut up Oil Cooler in those Pics yesterday or whenever it was My Spread was 25*F at all Times If I pushed it it would hit 30*f Spread And NEVER Cool the Oil until truck was shut Off

 

That's part of the key to watching the ECT/EOT is how well does the oil cooler respond to load changes.

Josh

 

To add more reassurance to your findings your numbers are essentially identical to mine at the same speeds. Just drove 600 miles yesterday at around the same speeds.

 

Mine does that. During the day, if it's hot outside, I'll see 190* at 65 MPH on flat ground. Then at night when the ambient temps drop, so does my ECT to around 184-186. I'm flushing this weekend, and replacing the thermostat and a doing bunch of other crap.

 

Don't forget to allow for the sensor differences.... After a cold soak, my sensors are about 3* different....oil higher than coolant.