Did a search but couldn't find an answer.

Yesterday my engine temp spiked higher than I've seen before and I noticed a few things.
- Engine temp is high when at low speeds (low ram airflow) and low RPM (low fan speeds)
- Running the heater would quickly bring it down so I don't think it's a pump issue. (driving with the heat going full blast when it's 100* and super humid sucks)
- I do not smell collant at all nor see any sign of leaking and radiator is full.
- collant level was a little low when the engine was cold so I added coolant.

The first 2 items seem to indicate an issue with not enough airflow through the raidator. The main fan seems OK. But the electric on the front or the radiators I'm not sure about. The fuss to the fan is ok. How can I tell if it is coming on when it should? Or suggestions to test if it is working at all?

Does revving the engine (fast idle) bring down the temp?

First, check the radiator exterior for blockage. If the fins have a lot of dried mud clogging them, obviously it won't work as efficiently.

Check the fan clutch. When you're in the middle of having overheating problems, shut the engine off, pop the hood, and try to rotate the engine fan by hand. It should be very hard to turn. If it slips easily, then the clutch may need to be replaced.

search again.... plenty of talk about overheating lately, such as THIS . Check coolant. Wash exterior of radiator (from front and back) with hose (loosen radiator bolts and tilt back to get back side). Replace thermastat. Flush radiator. Replace radiator. Good luck.

Try replacing the radiator cap...that's all it would take to cause those symptoms...and its cheaper and easier than the other options, so if it doesn't work, its no big deal.

If you do all of the other things first, and it ends up being the cap...you'll feel sheepish.



Essentially, the cap loses its ability to hold pressure...the cooling system depends upon pressure to keep the coolant from boiling (Like a pressure cooker...).

If the coolant can boil, such as at slow fan or vehical speeds, some of it will evaporate, lowering the level.

If you endure the stiffling heat and so forth, and run the heat when the temp gauge spikes, and reduce the temperature...it stops boiling....and the temp returns to more normal ranges.

As this is what he is describing, the cap's seal is a reasonable thing to question.

I had a similar problem about a year and a half ago. Driving upto my sister's house Xmas day of all days. Just going up I91, looked down and let loose with the expletives. Got off the highway to a Mobil station, let it cool off, checked the cap and fluids. everything seemed fine but just to be safe I turned around. It never got over the H but it was way higher than normal.

Now for the fun part..get to the dealer a couple days later. They have it all day while I'm at work. I get back there and am given some BS "its a sensor/seal/valve on the thermostat". Sorry can recall the exact term they used. I got assurnaces that the cooling system was working fine and was essentially a gague problem. Of course they wanted to replace it ( prob $2-300.00 no doubt) and I said I gotta get back to ya blah blah blah.

Havent had a problem with it since!!! Either something was just loose or got repositioned properly when the techs were futzing with it. Best part, not a dime outta my pocket..warranty work BABY!!!!!!!!!!!!!!

I'm having the same issue with an 04 Xterra, 50K miles......revving the engine DOES bring the temp back down! So, what's the solution???

Why would this just happen now? last summer no problems.

Thanks!

your radiator is prbably rusted from the inside. Check to make sure everything else is correct and working(rad cap, thermo, fan clutch, airpockets). If everything is kosher, drain your coolant. If there is any rust in the fluid, swap the radiator.
This happened to me. I changed everything and in the end it was the main part that everyone told me wasnt a problem.