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Why is my coolant boiling?


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For some reason, every single time I drive my truck for more than a few minutes, regardless of being on the highway or sitting in traffic, when I get home I notice that my expansion bottle is boiling, yet the engine temp remains within spec. The engine coolant sensor does work. I konw for sure, because if I do not refill my expansion bottle every other time I drive, the system somehow goes dry and the temp will suddenly spike. I don't have any leaks, and no smoke coming from the exhaust, but I have to fill up the cooling system every day. The water pump and tstat have been replaced earlier this year. The fan clutch is a little weak and probably needs to be replaced, but it works well enough. Like I said, the truck stays within spec for temp, but the coolant in the expansion bottle is boiling away, often violently if its a warmer day. And I can tell the difference between when it burps air out and when it is boiling. Ive burped the cooling system, but its not worth the effort anymore since I have to refill the system every day regardless. If I don't burp it, it will blow some bubbles when I first start it. It usually takes about 20-30 miles worth of driving before the system goes dry, or dry enough to overheat. So I carry a jug of water with me, pull over, top it off, and resume my trip. It just started doing this about a week or two ago, and had no cooling issues prior to that. I also did not have to top it off before that either, I don't know what has suddenly changed.

 

I do know for a fact that the head has some cracking, since it will seep a bit of coolant out of a valve cover bolt hole or two when I remove the cover, but if I were pouring that much coolant into the cylinders I am sure it would make some performance problems and there is no smoke. And there is no water in the oil or vice versa. I am hoping to get away with not replacing the head for another month or two until I can finish my ZJ.

 

Any ideas?

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Yes, the cap is weak. The pressure cap pops off every time it starts boiling. The threads don't seat firmly at all. I hadnt thought about it pressurizing the system though, but I don't think that is it. And yeah, I still figure the cracked head as at least a partial culprit, but I would think I could expect some sort of noticeable performance difference from losing a gallon of coolant into the cylinders every time I drove.

 

It has started idling a little rough over the last few days, but the big reason for that is because I keep breaking the off one of the CCV elbows(I have a HO valve cover) and losing vacuum because it is contacting the hood pad. The pad just touches it on a good day, but I am sure the motor mounts need to be replaced and the movement of the engine and contact with the pad break it off.

 

edit: mine is an older renix MJ without the pressure cap on the radiator, just the cap on the expansion bottle. And it doesnt seal and keeps popping off.

 

I really want to upgrade to the later model XJ/MJ cooling system, but the funds arent there right now. Maybe in a few weeks after I start my new job, but for now, it HAS to work until I get the company van.

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Pressure cap? Expansion tank?

 

What year vehicle is this? If it's a 1990 or older 4.0L, that tank is not an "expansion" tank, it's an integral part of the cooling system. The cap IS a pressure cap, and if it isn't holding pressure, that's why your coolant boils.

 

How high do you fill it? It is supposed to be filled halfway when the system is cold. If you overfill it, there is NO expansion tank, so the expansion of the coolant is what's popping the cap.

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The pressure cap is supposed to pressurize the system to 16 psi which raises the boiling point to 280 degrees or so. With a bad pressure cap the coolant will boil just above 210 degrees, which is pretty close to normal even under ideal circumstances and well within specs.

 

That cap is at least part of your problem.

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Sorry about the terminology, this is my first Renix jeep and I'm still getting use to its weirdness. I always seem to call the older style system by the newer terminology for some reason, no matter how much I think about it.

 

I fill it to the top at least twice before driving. I have left it sitting idling for 30 min after filling it, and it slowly recedes. It doesnt start blowing out until it starts boiling and if I just leave it idling for a while, it doesnt seem to boil, just after driving.

 

I did not think this type system was under pressure for some reason(brainfart) but that definately makes sense that its a lack of pressure causing the boiling.

 

But the cap has never fit snugly since I got the truck, that I am sure of and this boiling issue and major loss of coolant has just started recently. And it will be running fine, and the temp will be within spec, then within less than a half mile, it will make i all the way to the red. I always drive with one eye on the temp gauge because of this.

 

I'm honestly not too concerned about it since Ive been able to drive on regularly so long as I keep the coolant topped off and carry water with me, but for some reason the troubleshooting just wasnt working in my head. I just want to make sure I don't have some major issues later down the road, at least not before I can switch trucks.

 

If the engine does go for some reason, I have a very nice 96 4.0 complete engine sitting here. I wanted to use it for another project, but if I have to Ill drop it in the MJ. I am however going to steal the water neck from the newer 4.0 so I can use the temp sensor port to run an electric fan and burp the system easier, and it has a brand new(maybe 10 miles) 180* tstat in it, so hopefully that will help a little too.

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Be aware to run the 96 engine you will have to swap the entire engine compartment harness and quite possilbly the dash harness as well. Also gauge cluster, speedometer sensor, fuel pump and sender and somehow splice your tail light harness into the new electrical.

 

Renix and Chrysler use completely different engine electronics.

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Be aware to run the 96 engine you will have to swap the entire engine compartment harness and quite possilbly the dash harness as well. Also fuel pump and sender and somehow splice your tail light harness into the new electrical.

 

Renix and Chrysler use completely different engine electronics.

 

 

I know they did, which is why I decided to use that engine on another project. but if I do swap it over, I will try to use the renix electronics and the Hesco TPS adapter with the HO TB. The biggest issues I see are having to extend the wires to use the front mounted coolant sensor, and my engine does not have the spot for the knock sensor, but Ive seen the swap done before. The TPS seems to be the biggest hurdle.

 

My fuel pump gets a little grumpy at times, so Ive been passively searching for a suitable replacement.

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Had the identical same problem as your having. Replaced the tank cap and problem solved. I put a radiator hose clamp on mine (around the cap) and drove it for 3 weeks until I could get a new cap.

 

 

Yeah, I am going to try a new cap for sure. Luckily theres a junkyard near me, but I have been hoping to wait to make the trip until I had some more $$ so I could make a day of it.

 

I like the hose clamp idea though! I'm going to try that first tomorrow.

 

If I can ever manage to make time to pick up that fan of yours from your brother it will help me out a lot since I plan to set it up on its own temp sensor with the HO water neck, as well as putting a switch on it so if I start getting too warm, or if I'm sitting in traffic, I can help the system out a little.

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...it has a brand new(maybe 10 miles) 180* tstat in it, so hopefully that will help a little too.

It won't help. The purpose of the thermostat is to keep the temperature UP, not down. If your cooling system can't cool down to 195, it certainly can't cool down to 180.

 

Quadratec has new pressure bottles for about $25. be sure to get the one with a new cap ... I don't think anyone has ever been able to get a seal using a new bottle with an old cap, or a new cap with an old bottle.

 

And do NOT "top off" the bottle. Liquid does not compress, so when it heats up it has to blow the lid off. The bottle should be filled to the halfway mark (there's a post inside) when cold. Do not fill it beyond that.

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...it has a brand new(maybe 10 miles) 180* tstat in it, so hopefully that will help a little too.

It won't help. The purpose of the thermostat is to keep the temperature UP, not down. If your cooling system can't cool down to 195, it certainly can't cool down to 180.

 

Quadratec has new pressure bottles for about $25. be sure to get the one with a new cap ... I don't think anyone has ever been able to get a seal using a new bottle with an old cap, or a new cap with an old bottle.

 

And do NOT "top off" the bottle. Liquid does not compress, so when it heats up it has to blow the lid off. The bottle should be filled to the halfway mark (there's a post inside) when cold. Do not fill it beyond that.

 

I know under normal circumstances it should not be filled to the top, but in the interest of not having to stop suddenly, I have been doing it temporarily. It doesnt compress, but its sure going somewhere. Really the only time I lose any coolant is if I start it briefly after filling since it burps some air out, if I leave it running, it doesnt puke anything out.

 

The reason I want to switch to the lower tsat is for the same reason, the time it takes to rise that extra 30 degrees could mean the difference between making it to work, or pulling over on the highway.

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Maybe stating it a little different will help you understand.

 

The bottle should only get filled half way so that as it heats up and the coolant expands it does not overflow. If the cap is not fitting correctly and loose the system does not work as designed and the coolant begins to boil at 210. Between the lack of pressure and high coolant level your cap is popping off. Until you get the proper fit and working coolant bottle and cap this will continue to happen.

 

You say it takes a little while to heat up and then spikes. You need to remember that when cool it takes a while to heat all that coolant to normal temperature. Once the coolant reaches 210 it does not take long at all to boil over. 70 to 210 is a bigger diff. than 210 to 280.

 

You say the head is cracked. Your symptoms or effects from the crack will vary depending on where the crack is. You may not get oil and water mixed. The compression in the cylinders is way higher than the coolant pressure. Maybe when the motor heats up the crack opens and allows pressure to enter the cooling system.

 

Without thorough testing and maybe tear down you will not know the definite cause and corrective measure but if you continue as you are it may cause more damage that is way worse.

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Maybe stating it a little different will help you understand.

 

The bottle should only get filled half way so that as it heats up and the coolant expands it does not overflow. If the cap is not fitting correctly and loose the system does not work as designed and the coolant begins to boil at 210. Between the lack of pressure and high coolant level your cap is popping off. Until you get the proper fit and working coolant bottle and cap this will continue to happen.

 

You say it takes a little while to heat up and then spikes. You need to remember that when cool it takes a while to heat all that coolant to normal temperature. Once the coolant reaches 210 it does not take long at all to boil over. 70 to 210 is a bigger diff. than 210 to 280.

 

You say the head is cracked. Your symptoms or effects from the crack will vary depending on where the crack is. You may not get oil and water mixed. The compression in the cylinders is way higher than the coolant pressure. Maybe when the motor heats up the crack opens and allows pressure to enter the cooling system.

 

Without thorough testing and maybe tear down you will not know the definite cause and corrective measure but if you continue as you are it may cause more damage that is way worse.

 

I understand what youre saying, and if it were working properly, I would not fill it so. The temp does not spike until I have been driving for a while and have managed to(i am assuming I am burning it off, because while driving it does remain within spec and below 210) go wherever it goes. The cap will pop off whenever there is pressure, but it won't erupt out unless it gets really hot, or I drive for a little while. Whatever is happening to the coolant, it is receding enough that by the time it reaches the boiling point, the overflow bottle is already at halfway or less. The temp spikes suddenly after it has reached a critical point in its consumption of coolant and can't cool anymore. By this time( about 20-30 miles) it takes significantly more water than if I were to just top it off before I left the house. The times that it blows out are when I make a trip and forget to top it off before I leave, and decide to top it off wherever I'm at before I head back home, while the engine is still warm. Then it will overflow, but as long as I fill it while its cold, by the time it reaches the boiling point the level is within spec. It does take it a time to start boiling. I can drive to the store and back and it would be ok, but if I hit the drive through, I can expect boiling, especially if the coolant is getting low, but not low enough that it spikes.

 

I konw the head is cracked, but like you said it could be cracked elsewhere, but I won't know without a tear down, and REALLY don't want to do that right now since Ive got too much other to worry about. It drives, and thats the best I can hope for right now. It just seems weird how it is acting. I am definitely going to address the cap sealing, but I have a feeling I am still going to be losing an inordinate amount of coolant with no obvious signs of where its going. Thats what really puzzles me, under most circumstances the coolant stays inside the system or engine, and the occasion that it really boils and erupts is very few and far between so long as I give it the right attention here and there. The only place I can figure all that coolant is going is into the cylinders, but the engine isnt running so.

 

Regardless, the cap is about all the work I can manage on it at least over the next week or two, so hopefully that solves the major issues.

 

I may have caused the cracks in the head that I saw. When I swapped out valve covers there were some...issues...with the grommets and screws and not paying attention I overtorqued a few. After that I got a little seepage of coolant from a bolt hole or two, but nothing would leak once the bolt was installed fully. But it would not surprise me at all if there were more cracks in the head. This engine is pushing 200k and was not well cared for by its previous owner(s).

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I had the issue on my XJ with a new aftermarket bottle and cap. I ended up wrapping the bottles threads with pipe tape to help it seal till i could get an OEM bottle and cap.

 

i had a neighbor go through it too but he just put a hose clamp around his cap and left it that way.

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What it could be if you head or head gasket is broken or not sealing properly, is that exhaust gases from you combustion chamber could be getting into your coolant on the exhaust stroke, and the exhaust gases would escape through the expansion bottle because that is the highest point in the system. I have seen this before on a 2.5L TJ.

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What it could be if you head or head gasket is broken or not sealing properly, is that exhaust gases from you combustion chamber could be getting into your coolant on the exhaust stroke, and the exhaust gases would escape through the expansion bottle because that is the highest point in the system. I have seen this before on a 2.5L TJ.

 

 

I don't think that is it since it should start bubbling immediately and it does not until it heats up. When the head gasket went on my ZJ 4.0 a few years ago that is exactly what happened though. That engine is what I have on hand now.

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Often a crack in a head, block, manifold will contract and be sealed when cold. As the part warms up the crack will expand and then leak.

 

Ding ding ding, I think we have a winner. I tried the hose clamp method and took a trip up to the local wendys drive thru to test. First thing I noticed was that the engine temp rose slower and stayed more stable. It stayed around 190-200 and did not reach 210 until I was almost back home. Also previously, I could watch the temp gauge move depending on traffic and speed I was moving. But when I got home, the bottle was bubbling like crazy, although the engine temp was still right where it should be. So I guess the exhaust is pressurizing the cooling system after all, although it takes some time to do it. Not the answer I wanted, but at least its an answer. Now to figure out if its a crack or the head gasket. My guess is a crack since it does take some time before it starts doing it. When my ZJ blew its head gasket, the expansion bottle would start bubbling almost immediately at idle. But hopefully no major failures occur in the next week since I can't do much about it now.

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A blown head gasket does not necessarily put coolant in the combustion chamber. The only way to identify a head gasket leak for sure would be a leak down test. If you lose pressure in the cylinder, then check the coolant bottle for bubbles.

true about check bubble from the coolant bottle and sorry for my rusty knowledge... its been an while to get those memory out of my mind.

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Well, after running some errands today, I think I am back to the loss of pressure idea. The clamp on the cap seems to help the system be more stable and run cooler in general, but I don't think it can hold the full pressure. Without the clamp I can pop the cap off without unscrewing it.

 

My reasoning behind this is I just got back from running a few short errands nearby and the truck stayed nice and cool the whole time staying around 190-200. I made 2 stops a few miles apart, and both of which I was around 200 or so when I stopped. No boiling at all. After my last stop on my way home, I got caught in the same crappy construction clusterfook that has been causing all sorts of trouble in this area for months. Sitting there, I reached 210 where it held steady, and when I got home a few blocks away, it was boiling away. So I guess it really isnt holding the proper pressure to raise the boiling point high enough. So long as I am moving, the cooling system does its job and keeps the engine in a good range below the tstat threshold, but sitting with little air flow seems to get the coolant right to the boiling point. Guess its time to look into a new bottle and/or cap. Beats head work though!

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Well, after running some errands today, I think I am back to the loss of pressure idea. The clamp on the cap seems to help the system be more stable and run cooler in general, but I don't think it can hold the full pressure. Without the clamp I can pop the cap off without unscrewing it.

 

My reasoning behind this is I just got back from running a few short errands nearby and the truck stayed nice and cool the whole time staying around 190-200. I made 2 stops a few miles apart, and both of which I was around 200 or so when I stopped. No boiling at all. After my last stop on my way home, I got caught in the same crappy construction clusterfook that has been causing all sorts of trouble in this area for months. Sitting there, I reached 210 where it held steady, and when I got home a few blocks away, it was boiling away. So I guess it really isnt holding the proper pressure to raise the boiling point high enough. So long as I am moving, the cooling system does its job and keeps the engine in a good range below the tstat threshold, but sitting with little air flow seems to get the coolant right to the boiling point. Guess its time to look into a new bottle and/or cap. Beats head work though!

 

or maybe both? i heard that bottle tend will crack someday so better get metal bottle? correct me if I'm wrong?

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Now that you have solved the overflow problem, once you replace the bottle and cap you can work on the cooling problem in slow / stopped traffic.

 

Earlier you said that you replaced the water pump and thermostat and the fan clutch being weak but did not mention the condition of the radiator or the lower radiator hose. If the radiator is clogged or dirty it can restrict the flow and as posted in another thread if there is no spring in the hose it can sometimes collapse restricting flow.

 

I have an 88 with the 4.0, 5 spd, 4wd, and a/c (not working but connected) and when I replaced my radiator I put in a 2 row core. Mine also has an additional electric fan that was rewired to a toggle switch and had to be on all the time. I reconnected the wiring to the fan sensor so it works automatically. When I hit stop n go traffic in construction or rush hour in 85+ heat my temp gauge dos not go up and the electric fan has not come on yet.

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