Gave it an oil change and replaced the gasket where the lower radiator hose goes into the block via the thermostat. I mixed the coolant myself so pretty sure I didn’t mix water with 50/50. Drove it around and checked the coolant and oil level and looks good. Spent all day city driving and the thermostat is right where it should be. Then on the freeway the thermostat drops to 1/4. WTF? Been like this for a week. Never really gets up to temp after an hour of driving and the heater is luke warm air. After driving an hour on the freeway getting on surface streets it will warm up to the proper temperature. Thing is it’s intermittant. Just tonight it was at temp on the highway then all of the sudden dropped from 1/2 to 1/4 on the gauge and the air from the heater goes from hot to warm.
I’ve never heard of a problem like this. What could it be?
You goofed around the area where the thermostat lives, my guess is it’s the thermostat. You have the typical description of a faulty thermostat. (Or one that is impinged by an improperly installed gasket).
WAG, the coolant needs to be topped off.
The temperature gauge fluctuating and no heat in the cab are a result of air bubbles in the cooling system.
As soon at the car is totally cold (ie hasn’t been driven for a few hours), take the radiator cap off and see if it’s full, if it isn’t top it off. Also, make sure the overflow tank is filled up to the proper line.
If their both full, check to see if your car has any oddities for bleeding the cooling system. I believe some need to be parked with one end facing uphill or downhill or you need to be squeezing the upper hose etc. I know I usually start the car with the radiator cap off whenever I’m messing with the coolant so that it starts circulating and hopefully the air can escape and I can keep adding right away.
I’d start with that, it’s easy to check. After that, the next thing you’re probably going to have to look at the thermostat and see if there’s something wrong with it.
Not always.
If the coolant level is low enough, the sending unit may be in air, not coolant, and thus register a low reading. I don’t know about this Jeep, but this was a common answer on Click and Clack (where the majority of my car knowledge comes from).
I don’t know about the industry at large, but I know Ford has been moving towards cylinder head temperature sensors (CHTs) instead of coolant temperature sensors. they’re meant to do the same job as coolant temp sensors when everything is working normally, but in a loss-of-coolant situation they’ll signal trouble really quickly.
I learned it on my own one day. Same situation, car appeared to never come up to temp (Ford Bronco) and couldn’t get heat in the cab. When I checked, it was low on coolant. Topped it off and all was well, until it leaked back out, but that’s another issue.
It does sound like the thermostat is stuck open. At highway speeds so much air is blowing through the radiator it dissipates way more heat than the engine is capable of producing with full coolant flow, leaving little for cabin heat. When air is trapped in the system you’d get higher temperatures and more cabin heat at highway speeds due to the higher engine revs increasing water pump speed, which helps force the water past the air bubbles, compared to stop and go city driving with lots of stops/idling. This was an issue in my Passat with the heater core. Not only was it prone to trapping air since it’s a cooling system that requires vacuum filling, but the heater core is at the high point in the system and has high restiance/poor flow. Similarly that made it prone to clogging over time with debris. If air or debris was clogging it, high engine revs were the only way to get more heat.
I had a similar situation. Additionally, if I turned on my heat, it blew cold air. Googling the combination of problems along with make and model, it was mentioned that an unattached vacuum hose could be the culprit. Popped the hood and reattached the hose.
I’m guessing that hose was controlling one temperature mixing door. Though I’m surprised it fails to cold air.
In my haste I forgot to add in that one of the other reasons I’d start here is because even if it is the temp sensor or t-stat you’ll have to drain (or partially) drain the coolant to replace them so you might as well start with the easy thing…make sure it’s full.
Checking for resistance on the temp sensor as the engine heats, replacing the sensor, replacing the t-stat, making sure the dash gauge works and anything else are all a lot more work than just opening the cover and saying ‘oh, it’s low, lets fill it up and take it for a test drive’.
The most likely explanation is that the cooling system is not full, and that this happened because the system was opened to do the repair. While a faulty thermostat is not impossible, it’s rare for a thermostat problem to be intermittent.
I have a special tool that allows me to evacuate the air out of an empty (or nearly empty) cooling system, then install the coolant with it being sucked in by the vacuum I created. It works a treat, and no bleeding is needed. Without such a tool, it can sometimes be very difficult to drain and refill a system without leaving air pockets.
I’m gonna guess thermostat, and I’m also going to guess that as part of your engine work you flushed a whole bunch of crud out of the cooling system of your 12-year-old Jeep and it’s gunking up the themostat (you can pull out one of the hoses and see if the thing is clogged with old crud, but it’s the very first thing a mechanic would check.) It’s normal for an older cooling system to suck up coolant as it makes its way through all the tiny fins of the system. The coolant can only go two other places, out on the ground, which you can spot by looking underneath where you’ve been parked, which is usually a leaky hose, or out the tailpipe as white smoke on startup, which is a very very bad thing indeed because your coolant is mixing with your oil and your head gasket is likely gone. I’d replace the thermostat because it’s cheap, easy and probably the problem, but sure, top up your coolant again and look for leaks.
I agree with Joey, water needs to be topped off. Your thermostat closed while you were filling with hot water and you assumed it was full. I usually run with the radiator cap off until hot water starts circulating, then I know its full. You may have a bleeder you can use.
