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Discussion Starter #1
This is not the first time I've posted this issue on here but I'm still scratching my head. I have a 2007 with 140000 on it. At 135000 I had the timing chain, radiator, thermostat, temp sensor, and radiator cap replaced. Before the work was done it ran fine (other then the noise from the works timing chain). Water temp was good and steady and plenty of heat came out of the vents. Since the service has been done, the vehicle still runs fine but my temperature gauge fluctuates. I can run the vehicle for about 20 minutes or so and then gauge rises to normal operating temp and holds pretty steady, then it starts to climb up to about 1/2 way, then to the hot side of things and back down again. It does this non stop as long as I'm driving it. I've bled the system numerous times and have gone through 2 thermostats. I actually tested the first one I put in and it tested fine. I get loads of heat out of the vents at idle and while driving. This problem did not exist before the timing chain was replaced so I don't believe head gaskets or water pump are the issue. It this point I'm scratching my head. I have no idea what it could be. Unless my bleeding procedure is incorrect. Also I've noticed that no matter how long I drive it, my bottom radiator hose is always cold. This tells me that the coolint isn't getting circulated the way it should. I don't know. I'm stumped on this one. Any help would be greatly appreciated.
 

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Discussion Starter #3
Some 1st gens (to include mine) had a bouncy temp gauge...replacing the sensor probe fixed it.
I did that. Still bouncing. Plus it didn't do it before the timing chain was done. 🤷‍♂️. I've bled it numerous times but I'm thinking maybe a vacuum machine is needed at this point. Not sure. Thank you.
 

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Check the sensor that triggers the cooling fan...and the fan itself. Even older cars with failing fan clutches would experience temp fluctuations at first before final failure. However since you observe the lower coolant hose stays cool....could to point to poor water coolant flow....I'd think both top and bottom hoses temperature should be pretty close but the bottom hose cooler if any ......might be a good idea to check the radiator out. I'd get one of those OBD2 scanner guages and plug it into your OBD port....set one of the parameters to the water temp before doing anything....https://www.scangauge.com/products/scangauge-ii/
 

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Did you burp the system with a radiator funnel? Or did you just use the bleeder valve? If you didn't use the funnel try that. It's honestly the most efficient way to fully bleed the system. Also maybe try another sensor? I know they are pretty cheap and you could have gotten a faulty one. Most importantly look at your heads, might have a slow leaking head gasket (witch in my opinion it probably is). Be sure to get under your Xterra and look at the back of the drivers side head. Very common place for a leaky head gasket. Hope this helps and good luck!
 

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Discussion Starter #8
Did you burp the system with a radiator funnel? Or did you just use the bleeder valve? If you didn't use the funnel try that. It's honestly the most efficient way to fully bleed the system. Also maybe try another sensor? I know they are pretty cheap and you could have gotten a faulty one. Most importantly look at your heads, might have a slow leaking head gasket (witch in my opinion it probably is). Be sure to get under your Xterra and look at the back of the drivers side head. Very common place for a leaky head gasket. Hope this helps and good luck!
I used a funnel in the rad each time I have bled the system. And the radiator itself is brand new, along with the sensor (although it could be bad) and thermostat. I thought maybe it definitely could be a head gasket but it doesn't smoke at all when it's first started (I had a Ford the would pour white smoke when the gaskets went) and the oil isn't milky. What gets me is this problem didn't exist before my timing chain was replaced. It was completely normal. It only started after the work was done. And the vehicle runs fine. I've noticed if I turn the heater on full tilt the gauge does read more normal while it's being driven. Also my bottom rad hose is always cold. This tells me that for some reason the coolant isn't circulated well. Just my thoughts but at this point I have no idea.
 

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Mine had a small leak at the back of the drivers side head and I didn't have any white smoke either. You could always pull out the thermostat and run it that way to see if it works. Have you ever changed the water pump?
 

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Discussion Starter #10
No the water pump is the one it came with as far as I know. I thought about running it w/o the thermostat too. I'm hoping it's not head gaskets. Not sure what the cost of that is but I'm sure it's more than I want to spend.
 

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I've seen this many times, the thermostat is to blame. The best way I can describe it is; the coolant gets hot but the thermostat doesn't respond (Open) it gets hotter still (So your gauge reads hotter than normal) and then the thermostat wakes up and opens, the coolant which in relative terms is quite cool rushes through the now open thermo' and circulates the engine as it passes the Temp sensor the now much cooler coolant rapidly drops the gauge to below normal, after that it will behave it's self. Try another brand of Thermostat and try to get one with the toggle valve.
 

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Discussion Starter #12
I've replaced 2 thermostats in the vehicle so far. Both thermostats happened to be motorad Brad parts. I could be this brand just doesn't work all that well. I did test one of the parts by tossing it in some boiling water just to test it and it did open but it took a long while. It had to sit in the water for a minute or so before it started to move. I do know that the lower rad hose still feels as if the engine hasn't been running at all. I could drive that vehicle for 4 hours and it would still feel cold (which to me is not right). I thought about taking the guts out of one of the old termos I have and just running it open to see if it still acts up. I've talked to Nissan a couple of times about it and not one person there said head gaskets are to blame, but I've had ppl tell me (including the shop that did the timing chain) that that COULD be what the problem was. They did pressure test the system and said it held pressure, which to me, rules out head gaskets. Not to mention I've had a vehicle with bad gaskets and it did everything in the book except for overheat or have the fluctuating gauge. Thermostats are pretty cheap for that year and not to hard to change, what do you recommend for a brand?
 
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