Install: GDT Cylinder 4 Cooling Mod


I picked up this piece a couple of months ago. I was planning on installing when I did my timing belt but that’s kind of on the backburner until the fall semester is over. 

If you aren’t familiar, this is the GetADomTune (GDT) cylinder 4 coolant mod.  It is supposed to introduce reverse cooling to the EJ engine to help mitigate cylinder #4 ringland failures. I’m not super worried about #4 issues with the equal length header equipped EJ207 (I’m more worried about the rod bearings) but since this piece is pretty cheap I decided to go ahead and order it. 

I’m not going to pretend that I know anything about cooling systems, I don’t, but Dom explains how he came up with this piece in a couple of his youtube videos. 

Also, I bought the wrong kit. It’s technically for a GR/GV WRX/STI but it just has a longer hose. If it was too long for me to use then I would just trim it. 


The installation wasn’t too bad. The instructions are pretty straightforward, however, the hardest part is realizing that you, in fact, don’t have a 12mm hex socket in your toolbox and you just drained all your coolant out of your car.

Once you locate a 12mm hex socket, at exorbitant prices no less, the next hardest part is actually cracking the 12mm hex plug loose. Good luck to you on that endeavor. TIP: everything would be a lot easier if you disconnected the steering link, but I was too lazy to do that. 

First thing is first; drain the coolant! I used some scrap hose I had lying around to direct this coolant into my drain pan. It is slow but it is far less messy.

Drain the coolant!

Also, I use the blue coolant that Subaru uses in 08+ Subarus. However, I use either Pentofrost A3 or Peak OE Blue coolant as the OEM stuff is like $30/gallon. 

While the coolant was draining, I went ahead and pulled the TMIC off. I actually hadn’t pulled this thing off in some time. 

Then it was time to look at my janky heater hose setup. I had cut and bypassed my heater core when I first bought the car. The heat barely worked at all or it would go in an out and I was afraid that it was clogged up (and it was) so I bypassed it to prevent it from blowing out on me. 

Eventually, I would get around to cleaning it out and then I hooked it back up. It was going to replace the heater hoses with brand new OEM hoses but I couldn’t for the life of me figure out the part numbers for those hoses (I have them now). Now I am thinking about installing my future AOS in the area of the washer tank and tapping into those heater hoses for the AOS. I don’t know exactly how I am going to hook up the AOS until I actually do it so I am going to hold off on replacing those hoses.

The GDT Coolant Mod connects to the most outboard heater hose.

Now the fun begins!

I recommend attacking this plug from the bottom of the car. There just isn’t enough room up top to make any movement. 

And definitely use some sort of penetrating oil first, preferably right after you remove the TMIC to give it some time to work.

From underneath, I used a 6-inch and a 3-inch extension along with a 2-foot 3/8-inch breaker bar that cleared the axle but didn’t hit the firewall. 

Once the plug is out, it was time to clean the threads and prep the threaded end of the hose with Permatex gray. 

The cure time is 12-24 hours so be sure to plan ahead. Also, a stubby 1-inch wrench or stubby adjustable wrench would be super helpful here. I used a normal adjustable wrench and it took quite some time to get the hose fully seated.

After the hose is thread into the cylinder head, figure out where the tee lines up and cut your existing heater hose. Since mine was already cut, and the tee happened to land in that location, I just went with what I had.

I really don’t like these hose clamps. I think I am going to replace them at some point. 

Now we wait 12 to 24 hours for the permatex to cure before refilling the system with coolant.

Once the Permatex has cured, it time to refill the coolant system. I recommend this spill-free funnel by Lisle. It attaches to the expansion tank and in my opinion, it makes burping the system a lot easier.

My preferred method is to fill up the funnel, then squeeze the upper radiator hose until the level draws down then I fill the funnel back up again with coolant. Repeat this process until the coolant level no longer drops and the large bubbles disappear.

Then check again for leaks.

If there are no leaks, it’s time to start the car and let it run until at least thermostat opens (I check and see if the lower radiator hose is warm). Once the thermostat opens and the bubbles are gone, then you can insert the plug into the funnel and put the cap on the expansion tank. 

No more bubbles.

Using the method above, I have never had an issue with air pockets. 

Check again for leaks.

No leaks were found, it’s time to put the car completely back together and go for a test drive.

Quick Review: I honestly have no idea if this part does anything. My car seems to be running smoother but I also changed the oil at the same time so who knows for sure. I will probably send some logs to my tuner and have him take a look and make sure I don’t need any adjustments. In my opinion this is similar to an AOS or any other prevention modification.

The quality was great. It’s most likely the highest quality component in my heater system. Even though I bought the wrong one it fit perfectly. 



  1. Jesse Waddell says:

    Can this be done to a 2014 FA20DIT motor?
    I have a forester that is horrible with knock and the ECU pulls all the timing.


    1. A.A.Smith says:

      I’m not familiar with the FA engine family. From what I recall, the FAs are supposed to have a better knock detection strategy over the EJs. Of your FXT is modified, then I would have your tuner look at your logs.


Leave a Comment

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s