Bewildering Cooling Issue - Too Cool

The ghost town that is 300zxclub.com has failed me so I'm pulling out the big guns and asking the most expert car forum on the web. 

Here's the issue - the ol' 1990 300ZX seems to be running too cool - below operating temperature. I've verified the low temperature with a infrared thermometer (read 120F on the upper radiator hose) and with the fact that the heater doesn't blow hot, so I don't think it's the gauge. 

The gauge reads about 1/4 up, should be 1/2 I believe. Originally I suspected the thermostat was sticking open, so I replaced it. However, there is no change. If anything, it runs even cooler now. The fan clutch seems to be working normally. 

What else could it be? Seems like the only possibility is fan clutch or thermostat. Looking for confirmation that it's the thermostat before I drain the coolant and pull the damn thing out again. 

Not stoked on driving my car around like this as I know how bad it is for the engine/gas mileage. 

I don't know about in a Z but in some cars it's possible to install the tstat backwards or upside down. If it's not that, maybe your replacement was defective. You can throw it in a pot of boiling water and verify.

What are the temps on various points on the engine itself? Possible obstructed flow somewhere?

Low coolant will cause these symptoms.

I will admit that when I installed the new thermostat, I wasn't able to install the jiggle valve 100% "up" as specified. The way it was clocked it only wanted to go in with the jiggle valve at roughly 2 o'clock position. Didn't think it would make a difference. Also didn't use a gasket, only RTV. 

Edit: Thanks, I'm going to check coolant level when I get home. : ) 

Coolant level is the biggy. You will see the same issue if there is a pressure leak on these cars. 

But thermostat upside down is more then likely the culprit. Not sure how buried it is but they make inline ones that  go on the radiator hoses that are quick and easy installs if it is really really buried in the car like some 300zx's./ 

Your upper rad hose is a bad spot to check the temp of the engine.  It will only get hot when the thermostat is open.

A blocked radiator will have a pretty cold upper rad hose too.

Update: On the advice of gearheadmb, I rechecked the coolant level - with the heater on, the car fully warmed up, and on a slight incline. It took a few tablespoons but not much. So I'm pretty confident in ruling out low coolant level. 

I suppose it's worth noting that the car has an aftermarket radiator with no bleed screw. 

The only part you know is working properly is in a properly functioning car.

Take the thermostat out and test it in a pot of boiling water. Maybe the new one is stuck open.

What temperature thermostat did you install? Most modern vehicles seem to like the OEM temperature rating best.

170F just like OEM. I actually tested the new thermostat before using it and it functioned fine, just like the old one. 

I drove around a little bit and the temperature is still low. The upper rad hose doesn't feel pressurized, which is odd. I'm thinking I may have a massive air bubble somewhere, maybe because I installed the thermostat with jiggle valve at 2:00 instead of 12:00. 

Sounds like a big air bubble then.

Get yourself one of these: https://www.amazon.com/Stanley-Proto-JFF160-Spill-Free-Funnel/dp/B00BNSJ90A

I use one at work, no more air bubbles in anything. It lets the engine come up to temp with the radiator cap off and purge all the air out into the funnel which stays full of coolant.

They're great and worked a treat on my old Fiero which was usually absolute hell to bleed the cooling system.

How cold is too cold? I put a 40k transmission cooler on my TBI 350 chevy truck, running through the radiator prior to the cooler. Even with the in-line thermostatic bypass on the transmission cooler, it pulled down the engine temperature quite a bit. 

I only have the stock temperature gauge, so I don't have specific numbers, but my other question is where on the engine to check the temp with my laser thermometer. 

In reply to Floating Doc :

I usually check the thermostat housing itself if it's metal.  Hardlines for the heater core are pretty accurate usually.

In reply to wae :

Makes sense, I'll check it out. Any idea what the minimum operating temperature should be for my engine?

Thanks for all replies, I'm going to try to burp the system once more, elevating the nose this time. Worth a shot before changing the thermostat again. 

Nose elevated and system burped THOROUGHLY this time, made no difference. 

A few more readings with the infrared thermometer - this time from the upper coolant pipe, not the radiator hose - were around the 200F mark. The heater blows warm but not hot. 

Not sure what to think. The engine seems hot so maybe it's the gauge, IDK. 

There could still be air in the heater core. I burped and purged my e36 five, maybe ten times before the heater truly blew hot. What also helped was a bit of water wetter.

200 def F seems about right to me.

Possibly both a slightly off gauge and a crappy heater core or stuck blend door?

1990 you won't be able to pull scan tool data from, but maybe go old school and ohm out the ECT sensor on a hot engine and compare to factory specs.

UPDATE: I think it was the gauge temp sensor! 300ZXs have a separate one next to the ECU temp sensor... I removed it, tested it on the stove with a multimeter and thermometer and the resistance reads above FSM spec for a given temperature. 

Won't know for sure until my replacement sensor comes in ($4 shipped on eBay; not gonna spend 5x on an OEM one, it's a simple part), but I think the gauge was just reading low this whole time. 

So the prize goes to Run_Away, I think his diagnosis of a slightly off gauge + slightly clogged heater core is correct. 

Just an observation:  My '89 Mazda 626 always ran right around a 1/4 of the way up on the gauge.  Nowadays it seems manufacturers have moved the "normal" reading much closer to midway on the gauge.  So, unless you noticed a distinct change before/after, I'd suspect that cars from that era showed normal temps lower on the gauge than they do today.

Duly noted, however operating temp on a 300ZX is halfway. 

Typical if you add 50% or so to the power of the existing engine your temp

gauge will climb, I recommend this.

I'm finding 195-200 at the water neck, so likely okay. 

Didn't occur to me at the moment to check the oil pan. I'll check that the next time I drive the truck. I would guess that oil temperature is most important when discussing the possibility of engine temperatures being too low. 

Temporarily install a good quality mechanical gauge to verify your temperature.   Stock gauges are not to be trusted.  (Just ask GRM on their Miata.)  A stock gauge should only be used as a reference point after finding what the engine is actually running.  As an example my Jetta has a gauge that has a range from 160 to 230.  The hash mark on the gauge between those two numbers would be 195 if the hash mark had numbers.  And that is where my Jetta runs normally.  But I don't really know if that is actually 195.  I am only concerned if she starts running above that or below that.  That would not be normal.  But a good gauge on your Z.  Find out what temp it is really running at.  And call that spot on your gauge "normal".  This thought just in, put a piece of cardboard in front of your radiator and see if you can force the gauge to read higher than 1/4 of the way.