Car continues to overheat but only at high speed

[Jonny]

I’ve been struggling with constant cooling issues on a 2011 Nissan Sentra 2.0 with about 75k. One year ago radiator leaked so I swapped it out. A few months after, it overheated again so I took it to a shop where they replaced the thermostats. Replacement thermostats failed after about 2 months, the mechanic replaced again and did a power flush. On the way home from the mechanic the car overheated and the bottom radiator hose stayed cold, not even warm. The mechanic mentioned large amounts of deposits and corrosion. I decided to take a look for myself so I removed the thermostats drained the coolant, filled with prestone flush and water, drove for 3 days, drained and continued to fill and drain until I had cycled 30 gallons of distilled water through the system. I then pulled the radiator out of the car and flushed with a garden hose. I replaced both thermostats and radiator cap with new Nissan parts then filled the system with coolant. I used a spill free funnel to bleed the air out of the system which took alot of work before I got good heat out of the heater core. I also had to continuously squeeze the heater core return line because it was cold and felt empty. I went for a drive and all was well for about 30 minutes then the heat started to climb to 230 and the heater started getting cool. I returned home and purchased a vacuum coolant fill tool because the system was acting like there was still air trapped and refilled the coolant. Every time I drain and fill or flush something alot of deposits and sediment can be found in the discharge, even after 30 gallons, lol. The sediment has rather large pieces like 1/8 inch in diameter like an unground pepper corn. I recently did a coolant flush on a 20 year old f350 and it had alot less sediment then this little 5 year old car. I pressure tested the system with one of those bicycle pump testers and it held good pressure. I left it on for about 30 minutes a few times and it would only drop maybe 1 or 2 psi during that time. Also the vacuum fill tool tests pressure, I noticed no drop in pressure during 30 minutes with this tool. I don’t see any loss of coolant although I am draining and filling alot. I set the coolant right at max, drive for an hour or so and it rises maybe a half inch and falls upon cool.

I’ve been driving it for a few days now and here is what I have noticed. I’ve let the car idle for 3 hours, twice, and the hottest it got was 213 but mostly stayed around 206, fans kicked on or went high at 210 with good heater output. FYI, the thermostat is fully open at 203 and the second thermostat is fully open at 226. I then drove around town for an hour, twice and again the hottest I saw was about 213. Next day I idled the car until it reached 200 then proceeded to drive on the freeway. 20 minutes of 60 mph the car stayed mostly around 213, I then sped up to 80 and after a few minutes at 80 mph the temp rose to 230. I cranked the heater and then pulled off the freeway and drove around town which cooled the car back down to 213. I’ve repeated this a few times and I’ve concluded I can drive around all day and stay under 220 but when I tried to go 70 or 80 mph the temp will slowly raise to 230. I also notice the heater is a little intermittent. After bleeding I can get good heat but when the temps rise to the 230 range the heat drops to luke warm at best and will get a little warmer under acceleration. When the heater starts to drop I can pull over and continuously squeeze the heater core return line and eventually it will get hot. Any ideas? Thank you all ahead of time.

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[Jonny]

I’m thinking my next move is to pull the water pump and access the physical condition. I don’t hear any noises. It’s not to difficult of a job, I just have to remove the alternator and belt. If it looks good and doesn;t wobble I would just reinstall. After this step I would next probably replace the radiator but I want to avoid just throwing parts at it.

[Mike]

I’m concerned that you’re still getting debris after so much flushing.

It’s possible that a previous owner used a “leak stop” compound to deal with a leak issue. Or maybe you have a coolant hose that is breaking down internally and generating those black particles you’re finding.

[Jonny]

I checked out the water pump it was good. I attached a picture of my 5 gallon bucket after blowing the heater core out with compressed air. Massive amounts of deposits and this picture is not even all of it. I’m a little worried about replacing the radiator only to have it plug again. Draining and filling the coolant today resulted in very few deposits., barely even noticeable.

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[Jerry Marshall]

That looks like stop leak maybe?

[Jonny]

Get this, I finished my heater core flush and then went for a test drive to check the heat. The heat was great, I could almost bake cookies in the car. Whats strange is I was able to drive for a solid hour at 80 sometimes even 90 and the car stayed at a steady 195. I could not get the car to overheat. I have no idea why a heater core would cause the engine to overheat, maybe it was so plugged up it was blocking flow to the rest of the system. This car does not have a heater control valve so maybe the heater core always flows and the system is calibrated based on that. Or maybe since one of the thermostats sits in the housing both heater hoses flow into, that was effecting thermostat operation. I’m going to go for a few more drives and see how it continues to operate.

[Author]

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