87 FC runs great but temp barely registers on dash gauge. Sender?
 

My 87 FC NA car runs excellent but even when I have driven the car for a half hour, the temp gauge barely climbs above cold into the temp bar. It's at like 1/16 scale fully warmed up. It takes about 10 miles before the 1200 rpm cold idle finally drops to 750 rpms.

I am not familiar with the RX7s. Does the dash gauge use the same temp sender that the computer uses to based fuel, spark, etc. on, or does the gauge have its own dedicated temp sender that has nothing to do with the ecu? I am hoping they use the same sender and that might explain the long delay before the ecu brings the idle back down, and why the previous owner said the fuel economy got really bad before he sold (me) the car.

I love cheap all-glory repairs and I am hopeful this may be one!

Thanks

Steve

Your t'stat could be stuck. I had a stuck t'stat and it would take my car nearly 30 minutes to get up to operating temperature.
The ECU isn't bringing the idle down (or the thermowax for that matter) because they are not seeing proper temps either.

The ECU uses a seperate tempsensor located on the waterpump housing.
The gauge's sending unit is on the block blow the oil filter pedestal.

Thanks both of you guys. Stuck t stat is a very good idea, I didn't think of that but now that you mention it, I remember when this happened to a friend's newer GM car and the thing would almost never warm up until we replaced the stat.

Will do!

Steve

the bad fuel economy may be due to a bad o2 sensor. just a guess

in the colder months this isn't surprising. On my 88, the needle will barely get over the cold mark when the factory thermostat opens (opens at 82 C or about 180 F). I have verified this temperature by reading a [recently replaced] factory water temperature sensor right from my Power FC computer.

if you have one of those infrared temperature thingies, check temps with that. But I'd say there's at least a 75% chance there's nothing wrong with your car. The thermostat has barely cracked open but you are at operating temp.

Yeah right now with my setup I see just above the C line when really cold ~40-50 and about 1/8th when it is warmer out.

Even in 70 degree weather the temp gauge barely comes off the "C" line. I'll take a picture of it if I have a chance. The o2 is newer. I do have an infrared gun. Where should I be shooting the gun to get the most accurate representation of the coolant temp?

One thing you could do is start a completely cold engine with the upper radiator cap off, the one that is on the thermostat housing and does not have a spring on it. Then as the thermostat opens you will be able to see coolant moving through there and you can check it with the the gun.