91' MJ - Temp gauge bouncing around

[CaptainPatchy]

My recently purchased MJ has had several small issues that I'm slowly working my way through, however one has me sort of confused.  The truck has a full cluster, either swapped in or factory I'm not sure, but it all seems to work correctly, except for sometimes the temperature gauge.  Works normal after start up, however seems to be running very cold for a long time, and once it finally get's up to a normal temp (~190-210F) if I hit bumps, speed up or slow down too quickly the temp gauge will sometimes bounce around +-100 degrees!  It will usually come back, but never seems to have any problems sitting there idling.  Could this be mechanical in some way, or likely a bad ground somewhere?  The sender for these is on the back of the head, driver side correct?  I've got a thermostat housing with some sort of sender plumbed into it as well, so I don't know what's going on with it

[gogmorgo]

The sensor in the thermostat housing is for the ECU. It’s completely separate from what you see on the gauge.

The sender in the back of the head might be broken, or the wire could simply be loose. That’s what I’d check first. Maybe get a helper to wiggle the wire or watch the gauge.

As to the slow warmup, the thermostat could be stuck open, or missing completely. You’ll want to make sure it’s a 195° tstat at any rate. 

[CaptainPatchy]

1 minute ago, gogmorgo said:

The sensor in the thermostat housing is for the ECU. It’s completely separate from what you see on the gauge.

The sender in the back of the head might be broken, or the wire could simply be loose. That’s what I’d check first. Maybe get a helper to wiggle the wire or watch the gauge.

As to the slow warmup, the thermostat could be stuck open, or missing completely. You’ll want to make sure it’s a 195° tstat at any rate. 

 

Thanks, I'm going to check the thermostat this weekend while I'm going through it in more detail.  I'll have someone mess with the wire and see what's up.  Thanks

[Minuit]

Sender is on the back of the head, driver side. It is a "resistance to ground" type of sender so clean connections are especially important. Make sure the connector on top of it is clean and tight, as well as making sure there isn't a bunch of crap in the threads the sender screws into - the sender grounds directly to the head through those threads. You may also want to clean the instrument panel ground which is connected to the metal dash frame on the driver side as preventative maintenance.

 

If you do replace, I would strongly suggest replacing it with an OEM Mopar sender. The part number you need is 53005309, and as far as I'm concerned Crown does not count as an OEM part as much as they like to muddy the waters by using OEM part numbers for aftermarket parts. Aftermarket parts will result in an inconsistent and unreliable gauge reading. With the OEM sender, if all is well you shouldn't see that gauge go over the middle in anything near a normal driving condition. Every single Jeep I've ever seen, owned, or worked on with a working OEM sender has read in a consistent and repeatable way. The aftermarket ones are all over the place. For example, a Wells brand aftermarket sender I used read almost thirty degrees higher than the actual temperature - resulting in me wasting a weekend chasing an overheating problem that didn't exist.

 

Other possibilities range from a bad connection at the back of the gauge cluster to a bad gauge, or a broken wire - although I would consider these much less likely than a bad connection at the sender or a bad sender.

 

If you don't know the vehicle's history, making sure the cooling system is generally in good working order would be a good idea. A new (again, Mopar strongly suggested) 195 degree thermostat and a coolant flush would be a good idea, as well as making sure the cooling system is full. The sender is at almost the "top" of the cooling system, so if the system is underfilled and an air pocket forms inside the head, the gauge can become very inconsistent.

[CaptainPatchy]

1 hour ago, Minuit said:

Sender is on the back of the head, driver side. It is a "resistance to ground" type of sender so clean connections are especially important. Make sure the connector on top of it is clean and tight, as well as making sure there isn't a bunch of crap in the threads the sender screws into - the sender grounds directly to the head through those threads. You may also want to clean the instrument panel ground which is connected to the metal dash frame on the driver side as preventative maintenance.

 

If you do replace, I would strongly suggest replacing it with an OEM Mopar sender. The part number you need is 53005309, and as far as I'm concerned Crown does not count as an OEM part as much as they like to muddy the waters by using OEM part numbers for aftermarket parts. Aftermarket parts will result in an inconsistent and unreliable gauge reading. With the OEM sender, if all is well you shouldn't see that gauge go over the middle in anything near a normal driving condition. Every single Jeep I've ever seen, owned, or worked on with a working OEM sender has read in a consistent and repeatable way. The aftermarket ones are all over the place. For example, a Wells brand aftermarket sender I used read almost thirty degrees higher than the actual temperature - resulting in me wasting a weekend chasing an overheating problem that didn't exist.

 

Other possibilities range from a bad connection at the back of the gauge cluster to a bad gauge, or a broken wire - although I would consider these much less likely than a bad connection at the sender or a bad sender.

 

If you don't know the vehicle's history, making sure the cooling system is generally in good working order would be a good idea. A new (again, Mopar strongly suggested) 195 degree thermostat and a coolant flush would be a good idea, as well as making sure the cooling system is full. The sender is at almost the "top" of the cooling system, so if the system is underfilled and an air pocket forms inside the head, the gauge can become very inconsistent.

Thanks for the tips, the coolant system looks to be pretty much all new parts.  New-ish looking water pump, Aluminum "Mishimoto" (the seller claimed) radiator, with a triple 10" electric derale fan setup with a pwm controller.  It's all pretty foreign to me, but when I checked the overflow it was empty, and the rad took almost a gallon of distilled water to fill up.  I put another 1/2 gallon in the overflow and it hasn't left spots yet, but I'll keep an close eye on it for now.  I'm gonna peel the thermostat housing open this weekend and see what's going on, and go from there.  Thanks

[Minuit]

53 minutes ago, CaptainPatchy said:

 I checked the overflow it was empty, and the rad took almost a gallon of distilled water to fill up.  I put another 1/2 gallon in the overflow and it hasn't left spots yet,

Willing to bet there's your problem right there. Air in the cooling system will make a gauge freak out every time.

[CaptainPatchy]

4 minutes ago, Minuit said:

Willing to bet there's your problem right there. Air in the cooling system will make a gauge freak out every time.

What's the best way to burp this system, if there is one?  I kept topping off the rad while it warmed up and squeezing the upper rad hose to help it along.  

[Minuit]

18 minutes ago, CaptainPatchy said:

What's the best way to burp this system, if there is one?  I kept topping off the rad while it warmed up and squeezing the upper rad hose to help it along.  

That's about all you need to do on a late model system. Just keep filling it and squeezing the hose until it won't take any more. That's all I do and I've never had any problems. Once the radiator is full, fill the overflow tank to the full line. The system will eventually self-bleed if there is coolant in the overflow tank to replace escaping air, but it's a good idea to get as much as you can out now.

 

If I had to guess (as someone who's been there and done that) whoever put that radiator in just filled it until it wouldn't take more coolant and didn't bother to squeeze the hose or let the engine come up to temp with the radiator cap off to allow the air to escape. You might want to look over any other work the previous owner did and post anything that looks odd or questionable up here. I'm not accusing the previous owner of being incompetent, but if there are no leaks and they just didn't fill the cooling system up completely, any other work they did is suspect.

[gogmorgo]

I’ll second the coolant level being a pretty major player here. In my experience the system only holds about three gallons so being down by half that is a lot.

I’d also caution adding straight distilled water to an empty overflow tank. Repeatedly blowing and changing rad hoses on the side of a Saskatchewan highway at -20°F before factoring in the howling wind isn’t something I’d wish on anyone else. That’s also a lesson in proper container labelling, but we’re getting off topic. You should be fine for now, especially this time of year, while you try to sort out any other cooling system issues but seeing as you’re in the Midwest you’ll definitely want to make sure you’ve got a good antifreeze mix in there come September. Having a proper mix is also beneficial for corrosion resistance, and general cooling system longevity. It also makes it easier to find and identify leaks, as the coolant leaves residue where water just evaporates.
 

But yes, I’ve always just filled everything as best as I can, fired up the engine with the rad cap off, and slowly added coolant as necessary. I tell myself having the front wheels higher than the rear by 6” or so is helpful if safe and convenient but I typically fill them however the Jeep was parked without issues. Assuming the system doesn’t leak and the rad cap is in good shape, any air left over will collect at the cap and then get pushed out the overflow and vent to atmo as the system warms up and pressurizes.

[BeatCJ]

16 hours ago, Minuit said:

If you do replace, I would strongly suggest replacing it with an OEM Mopar sender. The part number you need is 53005309, and as far as I'm concerned Crown does not count as an OEM part as much as they like to muddy the waters by using OEM part numbers for aftermarket parts.

 

Where would you find an OEM Sender? I am gathering parts for the swap to a full gauge cluster.

[Eagle]

16 hours ago, CaptainPatchy said:

What's the best way to burp this system, if there is one?  I kept topping off the rad while it warmed up and squeezing the upper rad hose to help it along.  

 

It burps itself. That's what the overflow bottle does. Be sure the bottle is 1/3 or so full when cold.

[CaptainPatchy]

20 hours ago, Minuit said:

If I had to guess (as someone who's been there and done that) whoever put that radiator in just filled it until it wouldn't take more coolant and didn't bother to squeeze the hose or let the engine come up to temp with the radiator cap off to allow the air to escape. You might want to look over any other work the previous owner did and post anything that looks odd or questionable up here. I'm not accusing the previous owner of being incompetent, but if there are no leaks and they just didn't fill the cooling system up completely, any other work they did is suspect.

 

My thoughts exactly, I'm going to go through everything with a fine tooth comb this weekend to make sure it's all up to snuff.  FWIW, the jeep has been running fantastic all week otherwise

6 hours ago, gogmorgo said:

I’ll second the coolant level being a pretty major player here. In my experience the system only holds about three gallons so being down by half that is a lot.

I’d also caution adding straight distilled water to an empty overflow tank. Repeatedly blowing and changing rad hoses on the side of a Saskatchewan highway at -20°F before factoring in the howling wind isn’t something I’d wish on anyone else. That’s also a lesson in proper container labelling, but we’re getting off topic. You should be fine for now, especially this time of year, while you try to sort out any other cooling system issues but seeing as you’re in the Midwest you’ll definitely want to make sure you’ve got a good antifreeze mix in there come September. Having a proper mix is also beneficial for corrosion resistance, and general cooling system longevity. It also makes it easier to find and identify leaks, as the coolant leaves residue where water just evaporates.
 

But yes, I’ve always just filled everything as best as I can, fired up the engine with the rad cap off, and slowly added coolant as necessary. I tell myself having the front wheels higher than the rear by 6” or so is helpful if safe and convenient but I typically fill them however the Jeep was parked without issues. Assuming the system doesn’t leak and the rad cap is in good shape, any air left over will collect at the cap and then get pushed out the overflow and vent to atmo as the system warms up and pressurizes.

 

I just happened to have distilled water handy, and didn't think it would take as much as it did.  Like you said, living in the midwest, I understand the importance of not running straight water all too well haha.  Thanks though

4 hours ago, Eagle said:

 

It burps itself. That's what the overflow bottle does. Be sure the bottle is 1/3 or so full when cold.

 

Thanks

[CaptainPatchy]

Made sure the system was still full today, and solved the wandering temp gauge.  The connection was so loose I could easily pull it off.  I gently squeezed it with channel locks and reinstalled the much tighter sensor wire.  No more bouncing needle!  

[Eagle]

Make sure you get some real coolant into the system. At this time of year you don't have to worry about freezing, but coolant also has anti-rust additives in it.

 

[Tex06]

On 5/22/2020 at 1:37 PM, BeatCJ said:

 

Where would you find an OEM Sender? I am gathering parts for the swap to a full gauge cluster.

I pulled one from a JY. Finding OEM was difficult for me at the time (a couple years back), I think I only saw 1 or 2 on eBay.

[CaptainPatchy]

On 5/23/2020 at 11:17 PM, Eagle said:

Make sure you get some real coolant into the system. At this time of year you don't have to worry about freezing, but coolant also has anti-rust additives in it.

 

Absolutely, this was just a temp fix to make sure everything else in the cooling system was ship-shape.  I'll keep driving it for another week or so and if I don't notice any leaks or the overflow dropping, I'll drain the whole system and fill with some 50/50.