I dont know if it started with the change to the 3923's or not, but my truck's been running stupid cold lately. A few weeks ago, I drove from Arlington To Azle (probably a good 50 miles, going 70 MPH the whole way), and it never got above the first hash mark. From home to arlington on the surface roads, it'll usually make it about 1/3 way up the gauge, but only if I leave OD off. If I go on the highway, or turn OD on on surface streets, the needle basically doesn't even move.

I got home today, the needle was about 1/8" past the first hash mark, and it was 35 outside. I got the IR gun from the race box, the top hose and driver side tank was 150, the fins went from 130 on driver side to 90 on passenger side, and lower hose and passenger side tank was 90. I didn't aim it at the heads or block, and I forgot to feel the hoses. All temps Fahrenheit with engine running.

Dad thought the sending unit or gauge might have been bad, until i gave him the readouts from the IR gun, now we're both confused...especially since it's supposed to have a 195 stat, right?

Trying to figure out what gives... (3.9 V6)

 

Same here dont know why used to run about 190 on the gauge now it shows about 160-170

 

what confuses dad and I, with the stock 195? thermostat, the stat wouldn't even be opening..... and with the temps I'm getting, even with a 180 stat, its hard to say if our IR gun "lost" 30 degrees through the rubber hose, to read 150 on the hose....

 

Yeah it hard to get a true reading threw a hose. I havent been too worried about it im still getting about the same heat. I havent let mine idle to see if it will meet the 190 temp im going to try that tom. Ill have to check mine with the ir gun too just to see how it compares it to yours.

 

To the stock Stat's fail open or closed? Something my dad thought was maybe the stat is failed open? It falls on it's face a little in the morning, but not too bad...My last tank was half highway half streets, and I got something like 14.75 on it, been averaging in the mid-high 13's, low 14's, but slowly progressing up as we go into winter, but i think the plug change was part of that, too.

 

At 70mph plus the fact that it's 35* outside, you have quite a bit of a 'chill factor' going on with the air coming thru the radiator which is going to drop the temps of the coolant. If the thermostat was not opening, then you'd be running hot. Sounds to me like everything is operating normally and there's nothing to really worry about.

As for the mpg decrease, that's normal during the winter months also. When you combine the winter mix of the fuel plus the colder outside air temps thats causing the PCM to increase the fuel mixture, you won't see the mpg come back up until Spring.

 

mine runs around 190 even at highway speeds in dead of winter, I've never noticed a large drop. I do have a tranny cooler in front of my radiator though. Have you checked for air bubbles in the system? Mine seemed to be running cold earlier this season, asked around at my local parts/repair shop and he mentioned air bubbles. I put in a new T-stat, flushed my heater core (heat wasn't working well either) and purged the system; that fixed whatever problem I was having.

 

I replaced the stat, water pump, antifreeze, and hoses on mine this past summer and it seems to be running at a lower than normal temp. I can't remember what temp stat I installed. It would have been what ever is stock for it. Running stock Champion plugs now, had bosch+2s installed. Same temp. No idea what the difference is unless the stat is leaking.

 

Mine runs just a hair above the first mark.

 

Even if hes driving a 35 degrees the engin will still warm up to operating range it will just take a much longer time. Your thermostat is going or has gone bad. A new one id only like $10 it's a pretty simple fix.