Erratic Temp readings - b4 and after water pump replacement

Thanks for any assistance in advance.

I have a 1994 Nissan Sentra LE 1.6 ltr

I was driving to go camping with about 500lbs on my roof rack. We were heading up a very steep incline and I saw a sign that said, "Turn off your AC for the next 5 miles" ... I didn't have my AC on, but glanced at the temp gauge and it was pegged well above the "H" to the top of the dial.

We pulled over and popped the hood... the radiator fluid was bubbling and low... so I filled it up with fluid, and waited about a half hour till it cooled down. We proceeded.

The gauge read between C --->H for about 3 miles, then started climbing to H rapidly... we pulled over again.

There was fluid leaking out of the bottom of the water pump, so figured that was the issue.

I pulled into a Kragens Auto parts the next day and bought a water pump, replaced it, and the leaking stopped.... but the gauge still goes from between C --->H to H rapidly after about 8 miles. If I keep driving it goes back down to between C --->H but every few minutes rises again to H.

I have not tested it without the load, we just returned yesterday, but was wondering if air in the engine, or something else might cause the gauge to bounce between C and H so frequently.

Thank you.

Also... checked thermostat, and there wasn't one... had the car for 10 years without it, so I don't think that is the current issue.

I've never worked on that engine before but there are two air relief plugs, one at the rear of the engine above the flywheel and one on top near the front of the engine. Your issue can be a result of not having a thermostat. A missing thermostat causes a decrease in efficiency of the radiator. The coolant moves much faster without a thermostat which limits the time the radiator has to transfer the heat to the air. The hot coolant is just recirculate into the system. The hotter the coolant the less heat it can remove from the engine.
First suggestion is to install a new thermostat and bleed the system. Second is to have your radiator checked. If it has not be replaced before, there is a good chance that the passages may have built up corrosion and deposits, restricting flow. Flushing the radiator may help but will not remove all of the build up. A radiator shop can boil it out and will be better than a flush but it will still leave deposits.

Thanks 91STT

I forgot to mention that I did replace the radiator about 3 weeks ago. It had sprung a leak. I was planning on replacing the thermostat, but the parts store I went to on vacation did not have the thermostat or the gasket, so I figured I would do that sometime this week.

Thanks for the tip about the air relief plugs!

Update

I drove the car on my normal everyday commute (14 miles), and in the 13th mile the temp reading went all the way to "H" with no abnormal load in the car.

Another thing I noticed that on the drive home I put on the AC for the last 2 or 3 miles and it was working erratically.

I have ordered a new thermostat, and will install that tonight, but I am wondering if the erratic AC is revealing anything... any help will be appreciated.

try bleeding the system

Verify that your radiator fans are working (They should come on right after you turn on the A/C). If the fans aren't working, you can sometimes get by w/o them once you get up to speed (relying on the wind vice the fans); but then if you load the engine down (uphill, or turning on A/C) then you really need the fans to keep things cool. And definitely put the right thermostat in; check to see if its the 170deg; I think the parts stores are listing 180deg thermostats when the factory-installed were 170.

I installed a thermostat and bled the system and everything seemed ok for a while, then the brake light and battery lights went on when I turned on the AC...

The belt for the Alternator was gone when I checked under the hood after my last test drive :frown: thus the "brakelight and batterylight" problems.

I replaced all 3 belts on Sunday and will test again today...