Coolant Reservoir Tank Cap Question 1990 4.0

[AnotherOldJeepGuy]

I just swapped out my coolant reservoir.  I think this might be the 4th time over the years!  My question is about the cap.  I adding in a picture of the old split cap, as well as pictures of the old very bloated tank and the new one.  The old tank swelled up a couple years ago, I think this was a result of overheating (which was due I believe to insufficient coolant because the folks that swapped in a new radiator didn't burp it well., but I managed to get it burped and the overheating stopped).

 

Anyway my real question is about the cap.  The split open open one has an obvious pressure release, but the new cap doesn't particularly look like it does.  The new tank is Dorman 603-300 purchased a few years ago when I knew I'd need it sooner or later.  The last time I replaced this tank, it had the same black cap but I didn't use it, opting to reuse the one from the one before that, the one that has now split.

 

Is this new cap also designed to let off excess pressure but just doesn't look like it?  If the answer is NO, can anyone direct me to when I can get one like my old one?  I do see that every Dorman 603-300 I can find online now has this same black cap.

 

Oh, my other question is, that is the metal threaded orifice/port on the top of the tank used for?

 

 

 

 

 

[fiatslug87]

That black cap is fine. That’s what my ‘90 had when I purchased it new. I think the one that split is a different aftermarket one.

I’ve seen some ‘87s in the junk yard that had a bracket attached to that threaded piece instead of the strap.

[NC Tom]

FWIW: Cruiser54 recommended this, "Get a 703-1396 cap from Napa or an equivalent. From a Volvo."

I did, and it works really well. Holds tight.

[89 MJ]

3 hours ago, AnotherOldJeepGuy said:

Is this new cap also designed to let off excess pressure but just doesn't look like it?  If the answer is NO, can anyone direct me to when I can get one like my old one?  I do see that every Dorman 603-300 I can find online now has this same black cap.

 

I ordered the Dorman one and used the cap a few years ago. My tank let go, it wasn't the cap that let go from the pressure. That could've just been a bad tank though.

[AnotherOldJeepGuy]

I attempted to determine if the cap was hitting the hood with a propped  up piece of masking tap on the cap and closing the hood.  I don't think it is touching, I am pretty sure the old one did a little because of the bloated tank.  I read that the Volvo cap is lower profile, but it's also closing in on $20 at this point, so my plan is to monitor for a little while and if no leaks are seen, leave it alone.  If I see leaking I might go ahead and get the Volvo cap and see what happens.  If I learn anything useful, I'll post it.

 

I saw on a different forum that someone lowered the tray a little for clearance to the hood, I have wondered about this myself but have not pursued it, yet 

[pizzaman09]

The best combo I found was the doorman tank with the original black cap.  The cap with the brown vent just wouldn't hold pressure for me for the three different tanks I went through.

[hgeranium]

This thread was very conveniently timed because my expansion tank just busted. Good to hear that the Dorman tank and cap do OK. Going to keep the original style cap as a spare just in case. 

[fiatslug87]

There's also this option, price has gone up significantly but it will be the last tank you will ever need.

https://macsradiatorshop.com/product/macs-1987-1990-jeep-cherokee-aluminum-coolant-tank/

[watchamakalit]

9 hours ago, fiatslug87 said:

There's also this option, price has gone up significantly but it will be the last tank you will ever need.

https://macsradiatorshop.com/product/macs-1987-1990-jeep-cherokee-aluminum-coolant-tank/

X2

Mac's tank is a great solution. 

[coolwind57]

When I bought my truck, I had looked into the Mac's tank and just couldn't pull the trigger with that high price.  I decided to convert to an open system, which to me simplified dealing with the system in my tiny brain.  Instead of replacing the radiator with a later one with the filler neck, I kept "closed radiator" and installed an aluminum filler neck adapter in my upper radiator hose.   Pulled a junkyard recovery bottle from a later XJ and made it all happen lickety-split.

 

Might be something to consider.  No real issues with that critical purging of air from the system.  Takes out that failure-prone bottle/cap from the equation.  Give you a modern-type coolant system that may be easier to troubleshoot.   

 

I remember being quite surprised that Jeep had used a closed system--this was the only vehicle I'd ever bought from that era that had such a system.

 

   

[fiatslug87]

Using the Mac’s tank essentially turns it to an open system.

[coolwind57]

20 minutes ago, fiatslug87 said:

Using the Mac’s tank essentially turns it to an open system.

Oh, I was thinking that the Mac's tank was a replacement pressure bottle.  My bad.  It's been several years since I did my conversion.  And I'm old.  The water neck adapter on the top radiator hose is still a much cheaper option.    

[PIKE]

1 hour ago, fiatslug87 said:

Using the Mac’s tank essentially turns it to an open system.

The expansion bottle from a 2.5 liter works very nicely with the Macs tank. It bolts right in, the mounting holes are already there.

 

https://www.quadratec.com/products/51216_02.htm?gad_source=1&gad_campaignid=19472278524&gbraid=0AAAAAD_wpjimqWgguCgVpB6d83jQ2KQ0l&gclid=CjwKCAjwi4PHBhA-EiwAnjTHuSHNute531deQ-asKqcdWeqKsHb1cH5bsOR2Dl948PO8905E3VziIhoC_M4QAvD_BwE

[fiatslug87]

Then you need this cap for the 2.5L overflow bottle.

https://www.amazon.com/gp/aw/d/B000C5G8CM?psc=1&ref=ppx_pop_mob_b_asin_title

[PIKE]

38 minutes ago, fiatslug87 said:

Then you need this cap for the 2.5L overflow bottle.

https://www.amazon.com/gp/aw/d/B000C5G8CM?psc=1&ref=ppx_pop_mob_b_asin_title

There was also a short round tube that would go with that under the cap originally, I haven't been able to find one. I think it was to prevent coolant from sloshing out the cap since it is vented.  Number 9

 

[AnotherOldJeepGuy]

On 10/3/2025 at 8:21 PM, fiatslug87 said:

There's also this option, price has gone up significantly but it will be the last tank you will ever need.

https://macsradiatorshop.com/product/macs-1987-1990-jeep-cherokee-aluminum-coolant-tank/

Yeah, I'll be holding off on this one, way too pricey for me!  And I kinda like to be able to do a 2-second level check.  I have already purchased the next new tank, I generally keep one of these in the garage ready for replacement.

[AnotherOldJeepGuy]

On 10/3/2025 at 6:13 AM, pizzaman09 said:

The cap with the brown vent just wouldn't hold pressure for me for the three different tanks I went through.

I think mine was a constant source of a little leak as well, but I was never really sure it was the cap's fault since because of the tank bloating, I think the brown part touched the inside of the hood.  Either way, the new unbloated tank w/ new black cap has been working leak free, so I guess I'll keep it. 

[hgeranium]

9 hours ago, AnotherOldJeepGuy said:

I think mine was a constant source of a little leak as well, but I was never really sure it was the cap's fault since because of the tank bloating, I think the brown part touched the inside of the hood.  Either way, the new unbloated tank w/ new black cap has been working leak free, so I guess I'll keep it. 

I must’ve gotten unlucky with my parts. That new black cap instantly started leaking from all around when it got pressurized.. Reusing the brown one made it airtight again.