Sludge in coolant reservoir?

Tiny
ARIANADANIELLE5345
  • MEMBER
  • 2002 JEEP GRAND CHEROKEE LAREDO
  • 4.0L
  • AUTOMATIC
  • 111,111 MILES
There’s thick black sludge in coolant reservoir that doesn’t smell like oil. Opened radiator cap and coolant is fine so oil on top of water or anything. Car never overheats but oil pressure is closer to high on the instrument cluster gauge.
Images (Click to make bigger)
Thumb
Thumb
Wednesday, September 10th, 2025 AT 7:07 PM

21 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
So what happens is the transmission cooler inside the radiator goes bad and pushed transmission fluid inside of the engines cooling system, please go over this guide to see what I am talking about:

https://www.2carpros.com/articles/transmission-fluid-in-radiator-coolant-reservoir

You will need to replace the radiator and flush the cooling system.

Was this
answer
helpful?
Yes
No
Thursday, September 11th, 2025 AT 10:18 AM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
And the cooler?
Was this
answer
helpful?
Yes
No
Friday, September 12th, 2025 AT 3:15 PM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
Or did you mean replace cooler and flush cooling system?
Was this
answer
helpful?
Yes
No
Friday, September 12th, 2025 AT 3:16 PM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
Or replace radiator and cooler?
Was this
answer
helpful?
Yes
No
Friday, September 12th, 2025 AT 10:27 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
The cooler is inside the radiator, so just replace the radiator.
Was this
answer
helpful?
Yes
No
Sunday, September 14th, 2025 AT 10:25 AM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
It isn’t inside on this one though. It has a radiator and a separate transmission cooler.
Was this
answer
helpful?
Yes
No
Wednesday, September 17th, 2025 AT 9:35 AM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
Here’s a pic. At least I’m assuming that’s what that is.
Image (Click to make bigger)
Thumb
Was this
answer
helpful?
Yes
No
Wednesday, September 17th, 2025 AT 9:42 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
Hmm, does it have transmission cooling lines going into the radiator?
Was this
answer
helpful?
Yes
No
Wednesday, September 17th, 2025 AT 9:45 AM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
Lol never mind it’s A/C condenser.
Was this
answer
helpful?
Yes
No
Wednesday, September 17th, 2025 AT 9:45 AM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
She said she just had the radiator replaced not too long ago so I’m thinking that maybe they didn’t change the reservoir and that’s why the radiator wasn’t that dirty but the reservoir is full of gunk.
Was this
answer
helpful?
Yes
No
Wednesday, September 17th, 2025 AT 9:47 AM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
Can I submerge the radiator and put my compressor on the transmission cooler line and check for bubbles to see if it’s leaking internally? Or would that not work or could that possibly mess up the trans cooler part?
Was this
answer
helpful?
Yes
No
Wednesday, September 17th, 2025 AT 9:56 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
That could be that is was left over from the pervious repair, you cannot submerge the radiator and look for leak, sorry.
Was this
answer
helpful?
Yes
No
Wednesday, September 17th, 2025 AT 4:37 PM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
How do I flush the system?
Was this
answer
helpful?
Yes
No
Thursday, September 18th, 2025 AT 6:41 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
Dump and fill several times is your best bet, here is a guide to help you:

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

Once the system is empty, pour more hot water through it to help get the sludge out. Also, before you begin try to use a turkey baster to suck as much as you can off of the top. Let us know how it goes.
Was this
answer
helpful?
Yes
No
Friday, September 19th, 2025 AT 7:42 AM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
Do I need to flush the transmission also?
Was this
answer
helpful?
Yes
No
Friday, September 19th, 2025 AT 1:49 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
That would be a good idea as well, yes. Good call
Was this
answer
helpful?
Yes
No
Friday, September 19th, 2025 AT 4:58 PM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
A trans flush won’t damage it since it has a lot mileage on it?
Was this
answer
helpful?
Yes
No
Sunday, September 28th, 2025 AT 7:57 AM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
And the trans fluid isn’t pink, so does that mean there’s no coolant in it? Looks dirty but not pink.
Image (Click to make bigger)
Thumb
Was this
answer
helpful?
Yes
No
Sunday, September 28th, 2025 AT 7:58 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
It looks like it needs to be changed, it is tough to tell if there is coolant in there.
Was this
answer
helpful?
Yes
No
Sunday, September 28th, 2025 AT 10:59 AM
Tiny
ARIANADANIELLE5345
  • MEMBER
  • 393 POSTS
I tried to ask a question about an oil pump and It was there for like a day but now it’s gone. Should I redo it? Or is it just my browser or something
Was this
answer
helpful?
Yes
No
Tuesday, October 7th, 2025 AT 6:04 PM

Please login or register to post a reply.

Related Coolant Reservoir Content

Coolant Reservoir Replacement
VIDEO
Coolant Reservoir Replacement
Instructional repair video
Transmission Fluid in Radiator Coolant Reservoir
GUIDE
Transmission Fluid in Radiator Coolant Reservoir
All automatic transmission vehicles are equipped with an transmission fluid oil cooler, when transmission fluid is discovered inside the coolant reservoir or the radiator it...
VIEW MORE