I have a 92' Ford Ranger with 2.9 V6. I can't get the engine to warm up. I tried two different thermostats. Here are the facts

1. At 55 mph on the freeway in cold morning I only get up to 155 degrees with half of radiator blocked with cardboard.

2. Double checked to make sure I didn't install themostat upside down.

3. Put thermostat in pan of boiling water and watched it open and then close after taken out of water

4. After installing thermostat "burped" engine for 25 minutes to purge any air bubbles from system.

5. Checked with auto parts store twice to make sure I purchased correct one for make/model.

6. After running engine, upper radiator hose is hot and lower hose is luke warm.

7. radiator flushed twice since purchased new in June 2008

8. heater core replaced this past summer

9. Heater is working fine

10.mechanic tested coolant temp with thermometer and only got 155-160 degrees

11. Don't hear any abnormal sounds from fan clutch which was replaced in late 200

Mechanic says radiator must be working very efficiently and just block radiator partially with cardboard. But the mornings here in Arizona aren't cold enough to warrant blocking the radiator. Something isn't right here and I have to find out what the problem is. I am currently using this customized T-connector from the intake manifold. It sends temp readings to the original electrical coolant temp gauge and an aftermarket mechanical one. I wonder if maybe there is a problem with this setup or some type of blockage or air bubble.

 

hell you dont need heat in arizona!!! try NE OHIO...its -16 wind chill right now with no end in site!!!! might try a hotter 'stat-maybe a 195* seems like you covered everything else

6. After running engine, upper radiator hose is hot and lower hose is luke warm.
after rereading your post i noticed this statement...if i am thinking right the lower hose should be hot and upper COOL till thermostat opens...this tells me its too cool of a stat or stat isnt working as it should even though you "bench tested" it...good luck and let us know what you find!!!

 

Coolant flows into the radiator from the upper radiator hose and is pumped into the engine from the lower hose as the coolant is cooled as it travels down the radiator. Thus, hot upper and cool lower is correct condition.

Even though the fan clutch was replaced in 2000, check to see if it's hung up. If it will not turn, or has an excessive amount of viscous fluid drag, that may be your problem. If the fan is constantly engaged, it will cool the coolant down an excessive amount.

Thermostat should be a 192° unit. Did you replace the "O" ring gasket when you installed the new T-Stat? If no "O" ring gasket, then coolant is constantly bypassing the T-Stat.

 

Actually, the fan clutch was replaced in late 2008. I hear it engaged when I start the engine, then it calms downs shortly after. When I turn the fan with engine off, it has a little drag. I did replace the O-ring gasket when I replaced the T-stat.

This is what I think. I run two coolant temp gauges simultaneously: the original electric one in dashboard, and an aftermarket Auto Meter mechanical gauge. Come to think of it, last winter the gauges indicated low temp all season.

This is how the mechanic set this contraption up. He has this brass T-fitting that is screwed into the coolant temp sender hole in lower intake manifold. Then both lines for the two coolant temp gauges lead away from this T-fitting through the firewall. There must be a blockage or defect with this T-fitting coolant temp sender gizmo that is causing a false low reading. This blockage is possible freed when enough heat builds up since I observe higher gauge readings during warmer weather.

By the way, I notice that both gauges are closely matched or maybe identical in temp readings all the time. This makes me wonder if there is a blockage at the junction of this T-fitting.

This T-fitting is sort of customized piece of equipment my mechanic fashioned for me. I don't know about the reliability of these improvised two-way coolant temp senders. Maybe someone could comment on this

 

Based on what you described, it sounds like the sending unit is getting cooled down some causing the lower than normal reading. Are the sending units for the gauges right at the "T" fitting? Possible air bubble at the "T" fitting? Can you bleed the "T" fitting of air?