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Temperature gauge fluctuating with RPM and hills


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How I have my water system routed. Under the thermostat is a small bypass that goes straight to the pump intake. There is no heater valve so my core always has water flowing. Radiator is from a Fiero works great, skinny aluminum type with plastic tanks.


It's always ran fine until about last month the gauge starts fluctuating. Normal operation for me is right in the middle of the gauge. Since last month, I will be fine around town when I cruise doing 30MPH but when I do 70-80MPH and the motor is wound up the temperature drops nearly into the blue "cold" section on the gauge. As soon as I come to a stop light and idle, the temp goes back to the center as it should. It never reads hot like I'm losing water.

I can now notice that when I climb an extended hill the temperature drops damn near the cold mark. When I go down extended hills, it reads in the center. Sometimes on level surfaces at speed it will read cold and then go back to reading normal for a few minutes (usually when I hit a bump).

I have an overflow bottle that I keep half full so the radiator can draw water back in when it cools. The first time I noticed it getting cold I pulled over because I know gauges will read cold if there isn't any water. I popped the cap while it was hot and noticed FOAM at the top of the radiator. I thought the pump may be cavitating from lack of water so I topped it off and it didn't take more than maybe half a quart. I suspect the foam was acting like a low water scenario and the gauge reads colder than usual. It was about an inch layer of foam and you could feel that it was cooler on that part of the radiator.

I did find a little drip on one of the hoses to the pump intake so I tightened the clamps and now it doesn't drip. As I said in the last paragraph, I popped the cap when hot. I've never worried about getting sprayed with water because it's never hissed. My father thinks that's peculiar that it isn't building pressure. I don't leak water though so i figured it never gets hot enough to make lots of pressure. I can do a pressure test next time I'm at fathers shop, it read just fine before but then again I didn't have this problem before.

I've jacked up the car to get the air bubbles to the fill neck. I don't lose water when running or when parked. I don't overheat. Maybe something simple like the sensor or the thermostat getting stuck? I will try to get a video tomorrow

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Replace the thermostat. 

If a 521is going down a hill, and the engine has the throttle closed, and the heater is flowing water, the heater has enough cooling capacity to draw engine temp down below normal.  There is also a significant heat transfer from the water heated intake manifold out of the cooling system.  

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I will go ahead and order one to see what happens. Just an FYI, I am not running manifold heat, I don't have a head with the ports for it anyway. Obviously when sitting at a long light the temp may very slightly go above the middle operation temp. line and while cruising on a cool day I sit just a tiny bit below the line.

These are BIG changes though going down a hill. Normal temp climbing a hill. High RPM cruising it will be low, low RPM cruising it will be normal/not as bad. Idle it will be normal. Pull out of gear and coast down a big hill and it will be normal. And when I say low I mean like this, some times even a little lower!

Which is why I am bothered when its 100º ambient doing 70MPH. If it's the thermostat getting stuck then I guess the Fiero radiator works GREAT and I should be happy about that! I worry though that it's a false reading or something.

Think I should stay genuine Nissan thermostat? I once heard that cheap brand thermostat failures was like.... above 50%

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I would consider any where in the grey area OK.   How big is the engine in a Fiero?  If the radiator will keep a V-6 cool (Do Fiero's have a V-6) it will easily over cool a L-16.


In Oregon, I would suggest drilling the head for the intake manifold water passages.  In Texas, not so much.


By the way, what hills are you talking about in Texas?  On a clear day, you could probably see the back of your own head.

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I live on the biggest hill ever to exist in Texas.

You should see people handle it during the winter when we have ice for the first time in a few years. Everybody freaks out and starts buying guns, ammo, gold, water, and MRE's. End of the world, dude.

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Judging by my fuel economy the motor likes being on the line or just a needle above it. On a road trip in the SUMMER 26-27MPG reality and 30-31MPG odometer. Winter fuel blends take a shit on my truck and I get like 18-21MPG reality on long trips.

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Warm engines are more fuel efficient.   When the engine is cold, gasoline liquid does not fully vaporize, and liquid gasoline does not burn.  This effect is made worse by not having a heated intake manifold in the winter.

Did anybody mention you need a good thermostat?


I do not know if they use a E-10 gasoline mix of fuel in the winter in Texas, but if they do, ethanol does have less chemical energy than gasoline. 



That is a Texas hill?  So much for everything being bigger in Texas.  I think there is more elevation change on my 8 acres of property than that "hill"

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THIS is a hill son!!






Over a 2,000ft difference in elevation in 6 miles. 2 180 degree turns.

Cabbage Hill in Eastern Oregon.

Been over it too many times to count.

Oregon 1 Texas 0

Unlike Oregon, Texas does not cut through its, granted bigger, hills to make the road level. :fu:


That is one reason why I am happy to have shitty low gears. 1: I can crawl in traffic without an fits, like slower than a walking pace 2: I can get my fat ass up a big Texas hill 3: I'd like to think I will tow or haul heavy stuff but that's doubtful.


Shitty part: It's like I'm on a speedway when I get on the highway. Taking H190 offers here! Will part from my 4.80 pumpkin at any time! :thumbup:


I know one of you's made a thread considering the idea of putting the high/low case into a rear wheel drive unit or something. Don't you lie to me.

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The triple bolt? Most L20Bs have the same two bolt and easier to find and swap if you strip or break a bolt in them. The '79 and up L20Bs and the 810s had them although I have a '79 620 motor and it doesn't.

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