Weird sortof overheating issue. Super annoying..

89 XJ, closed cooling system. New stuff:

Water pump
Radiator
Thermostat
Coolant bottle

After replacing my old rad with a nice shiney 2 core (row?) aluminum one from Performance Radiator, and thermostat with a MOPAR one, I have an odd issue. When the jeep warms up, it'll go past the middle (100c) mark, half way between that and the red zone, stop, then quickly move back to slightly left of the middle mark.

It won't stay there for long.

After sitting there for a few seconds, it does the same thing. Creeps back up to in between, then moves back to the normal position. Over and over. Some times it'll be weird and just stay to the right of the middle mark, and won't go back to the left.

What's causing this? It almost sounds like the thermostat is bad. Could it be air in the system? I've tried bleeding air out of the back of the head where the temperature sender is, and I've also got a later model thermostat housing and I've unscrewed the temperature sensor there - pure coolant comes out, with no air.

What's going on?

Edit: Also, it's happening at both freeway speeds and in town. I have a manual override for the electric fans (taurus install) so it's on all the time while testing this.

Tstat sticking. Get a Stant premium version. Probably all of $6.

winterbeater said:

Tstat sticking. Get a Stant premium version. Probably all of $6.

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Positive? I wonder if my OEM one carries a warranty. I'd rather stay MOPAR than go aftermarket on the thermostat..

mattbred said:

Positive? I wonder if my OEM one carries a warranty. I'd rather stay MOPAR than go aftermarket on the thermostat..

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MOPAR thermostat as in "lowest bidder".

Stant Premium.

So you're telling me a brand new factory thermostat is junk and should be traded for an aftermarket? That's not right and you know it.

I bet it's probably still air in the system.

mattbred said:

So you're telling me a brand new factory thermostat is junk and should be traded for an aftermarket? That's not right and you know it.

I bet it's probably still air in the system.

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Yea I understand what you are saying but trust me and the other guys, Stant is much better.

mattbred said:

So you're telling me a brand new factory thermostat is junk and should be traded for an aftermarket? That's not right and you know it.

I bet it's probably still air in the system.

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What we are trying to tell you is that there are better quality thermostats available than what you would buy from the dealership.

joe_peters said:

What we are trying to tell you is that there are better quality thermostats available than what you would buy from the dealership.

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Sure, I understand this, but honestly, would a factory OEM thermostat really allow the temps to rise almost to overheating before opening? Doesn't that seem a little far fetched?

Maybe I'm just irked about all the thermostats I've tried in this thing. My last one (with two drilled holes) allowed the engine to cool off all the way down to nothing when coasting down the ski hills in winter - that's why I wanted to try OEM, and at $15 CDN I really want to make it work.

Also the Stant Premium is listed at $11 before tax.

Just try it, what do you have to lose?

bigalpha said:

Just try it, what do you have to lose?

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Money and frusteration.

With the "Super Stant Premium" do I need to drill the two holes? On the website it says it has some sort of notch that lets coolant flow all the time to steady temperatures.

mattbred said:

Money and frusteration.

With the "Super Stant Premium" do I need to drill the two holes? On the website it says it has some sort of notch that lets coolant flow all the time to steady temperatures.

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The holes simply make evacuating all the air in the system easier.

mattbred said:

Positive? I wonder if my OEM one carries a warranty. I'd rather stay MOPAR than go aftermarket on the thermostat..

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Just out of curiosity, who do you think manufactures that MOPAR Thermostat???assgas:

Do you think Chrysler / Mopar / Jeep has a Thermostat Factory?

Nah, $1 says the MOPAR part comes from China.

Upgrade to one from Taiwan - It'll work 200 times better!

joe_peters said:

The holes simply make evacuating all the air in the system easier.

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Yeah, I know, but I was just wondering if the notch in the superstant does the same thing? Maybe all I need is a single hole at the top now, since the top lets the air out and the bottom lets water displace it - if the notch is letting coolant flow all the time, that would make the extra hole on the bottom redundant and letting it cool down too much in winter time mountain descents.

Just out of curiosity, who do you think manufactures that MOPAR Thermostat???assgas:

Do you think Chrysler / Mopar / Jeep has a Thermostat Factory?

Nah, $1 says the MOPAR part comes from China.

Upgrade to one from Taiwan - It'll work 200 times better!

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I would assume that Jeep outsourced their thermostat contract to a company that produces at least somewhat quality products. Maybe I'm wrong - it is Chrysler after all.

Well I tried one last feeble attempt to remove any air in the system - there was none. You guys got me to buy a stant premium so I'll see how it works.

I'm going to drill a 1/16" hole on the top and bottom of the flange and see how it works. Here's hoping it does

Okay the new Super Stat works perfectly, thanks guys!

I don't know for sure about the thermostats, but I know that the radiator caps on current and recent Mopars (can it still be called that or should it be Mofiats) were made in India. OEM parts nowadays are purchased from the cheapest supplier that can convince the customer (OEM) that they come close to meeting the requirements that the original request for quote is based on. If it says the warranty is for 12k miles or one year, MOST of the parts are supposed to pass the test. As meager as it is, this plan doesn't even always work. Then the supplier has to pay Chrysler (or whomever) money back. But this occurs only after EXCESSIVE warranty returns.

Chrysler could have probably got Stant premiums for $3 a piece, but using a $1 unit saves them a couple of million dollars a year.

mattbred said:

Okay the new Super Stat works perfectly, thanks guys!

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Feels good, right?

Sad commentary on OEM, but there it is.

Very sad but also very true, and I have bought brand new thermostats and installed them just to find out they are bad. Returned them and had to buy new gasket, what a crock of sh*t. They should replace the gasket for free since they sold a defective part. Sorry for the rant, I'm done.

^ It's a good idea to test t-stats first. Hold it (not bare handed) in a pot of water (not boiling yet) on the stove along with an accurate thermometer. Watch the thermometer to check the temp. when/if it opens. You'd be surprised at how many open late or not at all.