Thermostat showing hot just a few seconds then back to normal?

[Johnny Glenn]

Thanks for looking in - I've got a 4.0 that is pegging max temp for just a few seconds then going back to normal. There is some coolant around themostat housing but I don't see a leak as of yet. Also there is a bearing noise coming from pulley area and I've replaced Idler and Tensioner - could it be a water pump problem or hopefully thermostat? - any way to check? Thanks.

 

[RickM]

there is a bearing noise coming from pulley area- could it be a water pump problem or hopefully thermostat? - any way to check? Thanks.

I'm not thinking the bearing noise is coming from your thermostat

 

[Johnny Glenn]

Thanks Rick for the helpful advice - the bearing noise could be just about any pulley on the belt drive and may not have anything to do with the overheating problem - just trying to figure out a way to test potential problems rather than part swapping. Can the water pump when it's going bad make a bearing sound?

 

[thepotroast]

Could be a bad WP, or fan clutch. My entire cooling system was replaced last year (radiator, WP, thermostat, coolant temp sensor, fan clutch) because I was having overheating issues.

Since that overhaul, I had the coolant temp sensor go bad on me again, and it was due to the same scenario you're describing, that the coolant gauge would read hot, then go back down to normal immediately.

Just start with a new thermostat, that's where I would check. You shouldn't have ANY coolant around your thermostat housing if it's not cracked and properly sealed. Your water pump does have a bearing inside, so that could be your issue.

Start with the easy stuff first. A thermostat should be under 10 bucks, and a coolant temp sensor is about 25 bucks. When you replace the thermostat, it's a good idea to also replace the large rubber o-ring that seals the thermostat housing together. If you're having seepage, could be a bad o-ring at that housing, or worse...a cracked thermostat housing...they are just plastic, and crack easy if they are overtightened or overheat. They aren't the cheapest piece of plastic either....

 

[Johnny Glenn]

Thanks - I'll give that a try. Where is the coolant temp sensor - is that the (sensor?) that sits behind the thermostat housing? I changed the tensioner the other day and removed water hose and so some fliud got into the area but it seems like there may be a possible leak - doesn't seem like the therm housing but the plastic piece under and to the rear of it - the piece the (coolant sensor?) plugs into. What is that called? Thanks again.

 

[RickM]

Johnny,
I'm glad you have a sense of humor. Yes, a water pump bearing can get pretty loud when on it's way out. Used to be any Toyota you saw with over 50k miles had a distinctive rattle form the water pump bearing......I don't know why but that sound sticks in my head.

 

[Johnny Glenn]

Thanks - is there a test for the WP - I've heard of a weep hole but don't know if that applies now?

 

[abarkl]

I had the same symptom of the engine heating-up for just a few seconds and I couldn't believe it would be the thermostat sticking until I replaced it!

 

[Johnny Glenn]

Thermostat on the way and I hope it's that easy??!! The bearing noise has me a little concerned about the WP but I'll know soon enough. Is there a check for a faulty WP?

 

[pet575]

I would also check (and possibly replace) the coolant bottle fill cap. I had another vehicle that would constantly overheat unless the AC was turned on to kick on a cooling fan on the radiator. Replaced the thermostat, all hoses, water pump, and was completely stumped that it still did it. Replaced the cap and everything was right again. All the other stuff was a complete waste of time and money.

Cheap option if the other cheaper stuff doesn't work.

 

[RickM]

Here's some fundamental testing you can do: http://www.ehow.com/how_4829065_test-water-pump.html