Purpose of this Posting:
This posting is intended to show the investigation/analysis of the radiator transmission cooler fittings connection for one failure on 2003 Acura MDX. From reading MDXers.org I have been made aware of the issue that has shown up recently for cars of this age/mileage, I was able to diagnosis it once I saw the radiator overflow tank. I just wanted to add more information to the collective knowledge.
Acura MDX (Nighthawk Black Pearl, Base, 2003, 93K miles)
Naperville, IL (Suburb of Chicago, rust-belt)
Radiator Coolant factory installed
Symptoms (November 2011):
• Transmission slippage in all gears that progressively got worse over two days.
• RPMs not matching transmission gear and vehicle speed.
• Found “Strawberry Milkshake” in the radiator overflow tank.
Failure of transmission cooler inside of radiator.
Tow car to home, remove and replace radiator.
Analysis of the failure:
• The radiator is a top to bottom coolant flow. The automatic transmission fluid flow is side to side inside the transmission cooler which is inside of the bottom tank of the radiator, immersed in coolant.
• The failure/leakage was internal to the radiator. All external hoses and fittings were still attached when the radiator was removed.
• The inlet and outlet fitings for the transmission cooler (TC) were easily broken off with a little pressure . The radiator and the TC line fittings are shown below:
• The aluminum body of the radiator (core) was removed from the bottom (tank) by prying up all the aluminum tabs that crimp the two pieces together. There is a gasket between the core and the tank.
• Below is a picture of the bottom black radiator tank (coolant) and the aluminum internal transmission cooler (automatic transmission fluid).
• Below is one of the female sections of the threaded fittings on the internal transmission cooler. The other section is very similar to the pictured one.
• The size of the O-rings compared to the outer flange.
• The amount of corrosion still left after initial cleanup.
• The outer diameter of the inner connection is just slightly larger than the hole in the radiator tank. Since the inside is not able to slide completely into the hole, the outer flange holding the O-ring is held up about 1/32’’ away from the inner wall of the radiator tank. The O-ring is then mainly responsible for keeping the coolant out of the connection.
• Below are the male sections of the threaded fittings.
• The amount of corrosion on the outer flange.
• The amount of the corrosion on the threaded portion (white hard crusty, lime scale??).
• The red/orange thread lock material seen on only 1 fitting.
• That the inner aluminum tubing and O-rings were in good condition
At this point, I thought the outer tubing, hex nut, and circular flange were all one piece made of aluminum. However, the corrosion between the hex nut section and the circular flange/washer just did not look right to me. It showed flakiness that is normally seen only in rusted steel. I was able to separate it into 3 parts (as shown below):
1. Aluminum tubing with hex nut section.
2. One rusted steel washer in the middle.
3. One rusty steel washer on the outside.
Conjecture on what went wrong with the connection:
• Corrosion around the threads weakened the connection.
• The corrosion could have come from two places.
• Water (and salt, being in the rust belt) can get between the steel washers and the tank and eventually enter the threaded area. There are no O-rings on the outside.
• Coolant might have slipped past the internal large O-rings and also entered the threaded area.
• The internals to the transmission cooler looked good, The end of the cooler fitting had no corrosion and a tight small O-ring.
• The threaded connection eventually starts leaking internal to the radiator.
• Due to the higher fluid pressure, the ATF is mainly forced into the coolant.
• The two steel washers on the outside of the radiator experienced very different corrosion. The washer in the middle that touched the aluminum on one side had major corrosion that might have been caused by galvanic corrosion due to the dissimilar metals (steel and aluminum). The washer closest to the radiator tank was in much better condition.
• The amount of threads that are engaged between the fittings and the transmission cooler core is also a concern. It looks like only 2 rows were ever engaged looking at the fitting. I was able to count at least 5 rows of threads when the corrosion was scraped away. Looking at the threads in the core, it was unclear how many were actually engaged.
• Could the outside washers have prevented the threads from fully being more fully seated?
• Why are the O-rings that seal out the coolant larger than their containing flange?
Summary: The connection of the tube fitting to the transmission cooler had the following issues:
1. No corrosion protection on the steel washers.
2. Potential for galvanic corrosion with steel and aluminum washers placed side-by-side.
3. Threaded mechanism with potential issues.
4. Internal O-rings with a significantly different diameter afterwards.
The reliability of the new radiator relies mainly upon the manufacturer. The only thing I was able to add, was to spray paint the exposed transmission line fitting hardware in an attempt to prevent future corrosion. It is unclear how many of the issues that I had with the previous radiator still appear in the design of the new one. It is also unclear to me, if this could have been prevented with a more timely coolant change.