Help! 2007 pathfinder- overheating/weak at start up

[A-aron55]

155000k miles SE 4x4 -radiator replaced in 2012 (Thank god- before I even knew SMOD was a thing!)
Son’s car now he was sitting in a parking lot for an hour- overheated- took it to a mechanic and changed fan clutch and thermostat- also recommended check for water pump and blown head gasket but he didn’t have the pressure test to do it. Took it to another mechanic and did a pressure test that was negative, he also put on a radiator clamp that he said was missing.
My concern and need for advice. Since then the temperature gauge goes from 25- 50% and I never remember it doing that before, is this normal with a new thermostat? Also since overheating Issue a month ago and pressure test when I first crank it the first 15 seconds it is real “weak” and sounds like it’s gonna die (battery replaced 2 months ago and it is fine). Additionally there appears to be a small rattling noise in right front of vehicles (alternator?) that goes away after 1 minute of running- thanks in advance for ANYONE that can help

[Rockwood]

I drive an R50, but i know R51's have had some issues with timing chain tensioners at higher mileages.. So you may want to see if the mechanic thinks that may be the cause of the noise at startup.

If the engine now cycles between 25 and 50 percent on the gauge, but does not overheat, I think I would try another thermostat. However, make sure there is no air in the system, and that the radiator cap pressurizes properly.

[Mike W.]

I don't know about Nissans, but many cars thermostats weaken with age, so a new one will cause it to run hotter. As long as it just goes to a different point and stays there, I think you're ok there.

Cranking for 15 seconds? That's an awfully long time. Mine starts noticeably slower than my BMWs of similar vintage but it's still probably only 3 to 4 seconds before it starts. Check the battery terminals and maybe even clean them as you can't see resistance, but it might be the starter, what's the mech say? It's not using coolant is it?

Rattle? No clue.

[A-aron55]

Thanks so much! It cranks fine but the first start of morning after initial start it gets “weak” for about 5 seconds (rattles a little and then feels like it’s gonna die) and then after that it’s fine. As day goes no problems after initial start- and I live in Texas so cold is not a factor. There’s also a rattling noise in right front area that disappears after a minute

[A-aron55]

I drive an R50, but i know R51's have had some issues with timing chain tensioners at higher mileages.. So you may want to see if the mechanic thinks that may be the cause of the noise at startup.

If the engine now cycles between 25 and 50 percent on the gauge, but does not overheat, I think I would try another thermostat. However, make sure there is no air in the system, and that the radiator cap pressurizes properly.

The Nissan technician said I had a vacuum leak near throttle body (repaired it free!), thermostat and temp difference was ok and within range. He said rattling noise at initial crank was an aluminum heat shield and not to worry. They of course recommend valve gasket cover replacement with spark plug ($800) and timing chain (nothing wrong with chain just it has 155k miles and recommended ($2300)- told him thanks for time and correcting leaks but I’ll hold off on 3100 dollar- he nodded in agreement- thanks!!!