Strange overheating issue.

My LR4 had a leak for most of last year. I'd keep the tank full, and occasionally put in a bottle of leak stop, which would stop the leak for 2-3 months.
Finally, last week the weather was nice, and I went ahead and did the front/rear crossover, pump and tstat job.

Job went smooth. Front crossover was rotted completely, and leak completely gone.
I bled the system, and it seemed to work completely normal for a few days.

Then, out of nowhere, the temp reading started to bounce up and down. From high to mid, every few seconds.. sometimes linger at high... then hold at mid for a bit. It was strange.
I shut it off, and then the problem went away for a bit before coming back.

What would cause this? I first thought maybe head gasket, but my vacuum bleed tool holds vacuum, and the head gasket test showed no leaks.
Maybe there was some air still trapped in the system, and found its way to the sensor?

Maybe all the leak stop from last year has left clogs of junk, and its clogging the system? I think the next step might be a very good flush, but would that even get out all the junk?

thanks

 

Has the bouncing stopped, or is it still happening? Back in the 60's I used some 'Stop-Leak' on a vehicle and was told by the mechanics that finally made the correct repair to never use the stuff as it can clog up the small passages in the engine block and radiator. So, yes, it can be causing a blockage somewhere, but at what location it will be hard to say. A good flush with something that can disolve and clean out any remnants of the stop leak may help.

Another thought - did you replace the sensor?

 

I replaced the rear crossover, and the tstat, which I believe contains the front and rear temp sensors?
I know when I removed all the plumbing, I found rubbery, silvery globs in some of the coolant pipes, which I know came from the leakstop. I didn't realize it would form large globs like that. I thought it was just silverly hairs to plug leaks.

But what is the best way to do a flush without wasting time. Because draining and filling 3-4 times is no easy process on the LR4.
Should I use a chemical flush? or one of those garden hose attachments?

 

I would vontact the mfg of the product you used and ask them what to use that will disolve it without damaging the vehicles cooling system parts.

 

I called and they recommend Rislone "Super Flush"..... granted, they make that product. But now I know what I'm doing this weekend. I've got pretty good at bleeding the system at this point.