Radiator overflows when running and no cap (1 Viewer)

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Greymattr

Greymattr

Joined
Aug 23, 2019
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Location
Columbus OH
So I’m a week or so out from my HG job (all new cooling system) on my 97 fzj80 with 190k. Temps have started to level out at 186-190 in town.

I’ve noticed my overflow reservoir is dropping so I opened my rad cap (while cold) to check the level and it was under pressure so it overflowed at the cap. I figured starting it would cause the level to drop, but instead it started to do the opposite and push coolant out. Is this expected? It seems weird that I can’t run the engine with the cap off without it gushing coolant.

Note: there’s no leaks anywhere at all and the motor runs amazing otherwise.

Any thoughts or insights to what it may be would be great!
 
Have you been driving it for a week and now it started or did you do it a week ago and started driving today? Because if it’s the first start up then it could be trapped air bubbles. Take a funnel to the rad and fill with distilled until it’s all buried out.

If it’s the former…well 😳
 
NLXTACY said:
Have you been driving it for a week and now it started or did you do it a week ago and started driving today? Because if it’s the first start up then it could be trapped air bubbles. Take a funnel to the rad and fill with distilled until it’s all buried out.

If it’s the former…well 😳
Click to expand...
I’ve been driving it about a week and this started to happen. I posted to a Facebook group and got some good insight, but feared for the worst. As a last ditch effort, I drained a little bit of the coolant to make room for an engine block test. When I started the engine, the coolant level did NOT rise. I let it idle to 181 and it remained level.

I’m going to keep monitoring and will update back.
 
Once it heats up look for little bubbles in the overflow tank. If its getting over pressurized from a potential leak you’ll see bubbles in the overflow.
 
Let's get back to basics. You don't even know for sure anything's wrong yet.

With the engine cold, top off the rad and make sure the overflow's ~1/2 full (doesn't need to be precise) and MARK the level with some tape or a sharpie.

Then when you start the rig, the overflow should slowly go up an inch or two as the rig warms up, the coolant expands, and the cap lets some of it bleed off. The reservoir should stay steady at that higher level as long as you're driving and the engine is warm.

Then once you stop, coolant slooowly transfers back to the rad as the engine cools. By the next morning, it should be exactly where your mark is on the overflow.

Do the above, keeping an eye on everything *without* messing with the cap or anything else for a few days (unless something's drastically wrong), and then report back.
 
Last edited:
Greymattr said:
I’ve noticed my overflow reservoir is dropping so I opened my rad cap (while cold) to check the level and it was under pressure so it overflowed at the cap. I figured starting it would cause the level to drop, but instead it started to do the opposite and push coolant out. Is this expected?
Click to expand...
That is completely normal if the rig was semi-warm (maybe an hour or so after driving).

1) The engine's not completely cool, so the system is still under some pressure. Then 2) Starting the rig causes the coolant to heat and expand again, pushing even more coolant out. (It would have gone into the overflow as designed if the cap were on and working properly.)
 
Day One of Back to basics:

Note - I have not remove the rad cap.

1. I marked the level with tape and a sharpie first thing in the morning. The engine was definitely cold from sitting overnight.
Day1-1.png


2. Ran errands for about 45 minutes. Engine at 186 degrees.
Day1-2.JPG


3. After sitting for 5 hours, engine cold to the touch.
Day1-3.png


4. At operating temp (186) after running errands for around 2 hours.
Day1-4.png



Day 2
Sat overnight, checked first thing this morning.
Day2-1.png


If all was well shouldnt it be at the marked line from yesterday morning? I think I have enough coolant because it didn't pull the entire contents of the overflow. My original concern was mostly around pressure being stored at the rad cap and the apparent usage/loss of coolant seemed more symptomatic.

Ill continue to track it over the next couple of days.
 
Last edited:
NLXTACY said:
Once it heats up look for little bubbles in the overflow tank. If its getting over pressurized from a potential leak you’ll see bubbles in the overflow.
Click to expand...
For what it's worth, over the past 24 hours I could not see any bubbles in the overflow reservoir.
 
Please keep in mind that the coolant system will self bleed any trapped air after a few drive cycles.
Again, as previously stated, with a COLD NON-RUNNING engine, the radiator should be filled to the TOP OF THE FILLER NECK, and the overflow bottle should be filled to the FULL line.
Any trapped air in the system will bleed out and the level in the overflow will fall to make up the volume.
Once the air is out of the system, the level in the overflow should rise and fall with engine temperature, but should remain consistent over time.
This assumes no leaks, a good radiator cap, and the correct orientation of the overflow bottle cap.
 
Greymattr said:
Day One of Back to basics:

Note - I have not remove the rad cap.

1. I marked the level with tape and a sharpie first thing in the morning. The engine was definitely cold from sitting overnight.
View attachment 2810326

2. Ran errands for about 45 minutes. Engine at 186 degrees.
View attachment 2810304

3. After sitting for 5 hours, engine cold to the touch.
View attachment 2810305

4. At operating temp (186) after running errands for around 2 hours.
View attachment 2810308


Day 2
Sat overnight, checked first thing this morning.
View attachment 2810310

If all was well shouldnt it be at the marked line from yesterday morning? I think I have enough coolant because it didn't pull the entire contents of the overflow. My original concern was mostly around pressure being stored at the rad cap and the apparent usage/loss of coolant seemed more symptomatic.

Ill continue to track it over the next couple of days.
Click to expand...
As others have alluded but perhaps not explicitly stated, I think you are still getting air out of the system. That's why the level is lower after the system has cooled back down. Fill it back to your mark every time and within a few cycles you shouldn't see it move after cooling.
 
UPDATE:
The morning of the second day, I topped off the coolant in the reservoir to full. Took it for a couple of drives and sure enough the level raised at operating temp, and then lowered back to the full mark at cold. Went out to check the level first thing this morning (day 3) and it is still at full. It appears there was still air trapped and at the time of posting this thread, I had not driven enough cycles. Thank you everyone for all of your input and help with this.

Before this HG job, the biggest service I've ever done was the knuckle rebuild (one time) and regular oil service. I find myself staring at my scangauge for temps, but I'm sure that'll eventually pass as the wounds of overheating heal. 😆

IMG_0993.JPG
 
I don't know - I stare at my temp gauge like it's about to tell me I won the lotto.
 
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