Hey all,

83 720 with a Z24 engine. Motor has been running very cool since I got it, temp gauge barely gets to the bottom mark and takes 20 minutes of driving to do so. Heat blows lukewarm.

Replaced the thermostat which was a 180 degree with a good Stant 195 degree and noticed no difference. Old one I took out looked in good shape, didn't appear to be stuck open. Coolant level is OK.

Any ideas?

Welcome to the forum. You might as well check the engine temp with a real thermometer or other temp measuring device and find out what the temp really is. The gauge on your dash might be bad. They make infrared guns that will read the temp, or just get the engine warmed up and carefully open the radiator and put a thermometer in the coolant. The laws of thermodynamics cannot be broken, so I doubt your engine runs cooler than whatever your thermostat temp is unless you live on the north pole. Unless, maybe . . . Wait! Could it be? . . . Are you the real Santa Claus? Cooool! I want a new BB gun and a pony!

Mine does the exact same thing, and I replaced the thermostat with no change. I have a new temp sender that hasn't gone in yet because I'm what you call Lazy. I'll put it in on Saturday morning and report back.

BTW-- Fastboatman, thanks for all the great tutorials you're putting on this site. In addition to that temp sender I'm also putting in new ball joints this weekend, emboldened as I am by your thorough instructions. I appreciate what you and all the other members are doing around here to make this forum such a valuable resource.

wallaccx wrote:BTW-- Fastboatman, thanks for all the great tutorials you're putting on this site. In addition to that temp sender I'm also putting in new ball joints this weekend, emboldened as I am by your thorough instructions. I appreciate what you and all the other members are doing around here to make this forum such a valuable resource.

Thanks for the kudos! It's always nice to know that people are actually getting helpful info from the DIY posts. It makes it worth the trouble.

These trucks are easy to work on and mine still has lotsa life left in it at 213K miles. We gotta keep'em running!

Thanks for the suggestions. I idled it for about 30 minutes to let it warm up. After 30 minutes, taking off the radiator cap and using an IR thermometer the coolant was 108 degrees. Definitely too cool.

I checked the coolant temp sending unit and got a bit over 100 OHMs which is good according to haynes, and shorting the lead to ground caused the temp gauge to climb all the way to H which according to haynes means the sensor and the gauge are both OK.

Could the cooling fan be cooling too much? Or could a bad water pump cause the heat to not be distributed enough? Though not quite the north pole, still getting frigid around here, dying with no heat

An update: today I kinked the upper radiator hose with a pair of vice grips and started and ran the engine. Let it run for 1 hour at 1800 RPM. After one hour, the temperature gauge still didn't creep above the bottom. The top of the engine block was warm to the touch, definitely not hot though. I took off the vice grips and squeezed the hose a number of times with my hands and it didn't feel like there was any pressure or flow inside, indicating the thermostat still closed.

Completely 100% baffled. Never experienced anything like this. Ideas?

ben1,
OK, I'm just gonna think out loud. If coolant is not flowing AT ALL, (blocked passages or bad water pump) then the engine would get extremely HOT, but the radiator coolant would not. You can point those infra-red guns right at the engine and test the block temp. That's probably not the problem.
If your plugs on the exhaust side are not firing because the coil is bad, then that might cause the temp to stay down. Maybe. I really think it would not make much different though.
This is a puzzle.
A thermostat installed upside down would cause the engine to run hot because it doesn't open.
We'll eventually get it figured out.

Anyone else?

Thanks for the suggestions. Top of the engine block around where the thermostat house is was warm but not hot enough to burn my hand. Nothing in the engine seemed hot (except the exhaust manifold). I tried unhooking each coil one at a time, when either coil is unhooked the engine runs, but very poorly, so I think they're both firing. Runs good only when both are hooked up.

Something's just not adding up. Expoding gasses create lots of heat. Is the coolant fluid standard antifreeze or something else?

Had a similar problem with the Z20 I installed in my Galant supersaloon. the vane in the front cover was eroded causing the flow of water to be less than necessary but enough for circulation and the fan was cooling the slow flow too much. I changed the front cover and problem solved.

I am having a bit of the same problem with my 720. This is what I have done and what I am planning on doing.

I replaced the water pump because it was leaking and since replacing that and the OEM thermostat the truck just will not warm up! When I replaced the coolant I mixed it myself and I am wondering if I used too much antifreeze and not enough water. I am told the OEM thermostat is 180 deg. When the radiator cap is off and the engine warm I can see the coolant moving across the top. I can pinch the top coolant hose and feel back pressure and the flow stops. It seems that my 25 minute drive to work should be enough to get the system hot but it just gets warm. I am going to try and drain a little of the coolant and replace with some distilled water to bring the ratio of pure coolant to water down.

Anyone have any other thoughts?

An update. Today I picked up a new gasket and took the thermostat housing back off. Problem is I underestimated my own stupidity. Apparently, when I was tightening the bolts before, the thermostat slipped down the housing just a tad and was allowing coolant to bypass it. There was just a tiny gap, maybe less than 1/8", but that was enough.

I took the housing completely off, marked the top where the little jiggle valve goes, cleaned it well, took it inside and "glued" the thermostat in place with the RTV then glued the gasket on top of that. I let it sit inside until it had fully cured, locking the thermostat in place. Once it had set, it wasn't going anywhere and I was able to install it without worrying about it falling out of place again.

Started it up, within 5 minutes of driving for the first time ever, the gauge was on the half way mark and I had glorious, glorious HEAT!

Apparently the design of this makes it easy to make this mistake as the old thermostat installed by a previous owner had the same problem. If you're having overcooling problems, this would be the first thing I'd check. Hope this helps somebody.

Thanks to everyone for your help and ideas.

Be careful with a 195 degree stat. These trucks with aluminum heads and general lack of quality in the head design are prone to warping and blowing. Personal opinion would be to stick a 180 back in it, but then get an aftermarket gauge so you can tell what the exact temp is... don't trust the stock one.

Then again, if you have heat and you're happy... maybe just leave it until it warms up again in the spring.

ben1 wrote:An update. Today I picked up a new gasket and took the thermostat housing back off. Problem is I underestimated my own stupidity. Apparently, when I was tightening the bolts before, the thermostat slipped down the housing just a tad and was allowing coolant to bypass it. There was just a tiny gap, maybe less than 1/8", but that was enough.

Started it up, within 5 minutes of driving for the first time ever, the gauge was on the half way mark and I had glorious, glorious HEAT!

Thanks to everyone for your help and ideas.

Awesome! I'm really glad you took the time to report your discovery. Waaaaaayyy too many folks stop in here looking for help, only to never reply as to whether or not the suggestions solved the problem. So, I really think it's cool that you told us how you solved the problem. I hope you stick around and help some more!

Yes thank you so much for returning and posting your answer! I had the same problem and fixed it tonight.

You can see the gap at the top!


You can see how the thermostat sat too low!


Thank you. I will be especially thankful tomorrow morning on the way to work when I can feel my feet!

Exactly how mine looked, glad that solved your problem too.

Thanks for the tip on the 195, I don't drive that much maybe once a week, so I'll wait till spring to swap it, don't want to look at another thermostat for quite a while after this ordeal!

fastboatman29212 wrote:
Awesome! I'm really glad you took the time to report your discovery. Waaaaaayyy too many folks stop in here looking for help, only to never reply as to whether or not the suggestions solved the problem. So, I really think it's cool that you told us how you solved the problem. I hope you stick around and help some more!

I never got to the temp sender this weekend because I figured I'd start with the ball joint job... And it turns out the lowers I got from Summit Racing don't fit. The mounting holes are too close together, by a good 10mm... Can anyone recommend a part number for lower ball joints that are known to fit? Thanks!

(I did manage to replace the stabilizer bushings, tension rod bushings and tie rods, will post pics once it is all said and done.)

Cool...haha..

I have the same problem with my 720/Z24 1986. I have a SW. mechanical gage for water temp in it and the sender is between the thermostat and the head in the intake manifold. mine never gets above 165 deg. summer or winter. the heat is ok but definitely not going to run you out of the cab.
I had the same problem with a mitz. mighty max. the 195 deg stat never opened up summer or winter. like the Z24 Nissan both motors have a HUGE bypass hose somewhere around a inch ID. I often wondered if the by pass hose was bypassing so much coolant that the motor never warmed up enough to open the thermostat. The only way I could get the Mitz. up to say 180 deg. even with a 195 deg stat in it was to block off a portion of the radiator with cardboard in the winter or it would not even clear the windshield. I had a SW. mechanical gage in that truck also so I would know exactly what the temp was running.