How To: RSX Heater Pipe Fix

Note: This applies to all K20 engines, so all models of the RSX, 8th gen Civic Si, and also the K24 platform, as well as K20 / K24 Frankenstein builds. Some part numbers may differ, but they all look the same despite this.

Underneath your intake manifold is the heater pipe. This pipe, as its name implies, carries warm coolant from the thermostat housing port to the heater control valve. 

The Heater Pipe

This pipe is known to leak and be hard to find because it’s tucked out of sight. The design of the pipe is also suboptimal in my opinion. The key elements are a flanged end with a nitrile rubber o-ring, some mounting holes, and the other end of the pipe that’s connected to a rubber hose. 

This part has the o-ring on the end, and is under the intake manifold.

The previously mentioned o-ring is seated in a flanged part at the end of the heater pipe, and pushed into the plastic rear thermostat housing. If the housing isn’t lubricated before seating the seal, it can catch on the wall of the housing and stretch the seal, ruining it. It’s also very important to use both bolts to secure the heater pipe because they locate the seal in the exact orientation it needs to be in to properly seal. Also, the pipe can rust and sometimes compromise the seal. 

This is the other end of the pipe, which connects to the heater control valve via a hose.

When I removed my heater pipe, I treated it for rust in my ultrasonic cleaner with a bath of evaporust for a few minutes. It can be done without an ultrasonic cleaner, but the temperature and agitation elements speed up the rust removal process. I washed it off afterwards, wiped it with alcohol, and painted it with a rust converting rubberized paint. It’s almost like a light undercoat. 

Aftermarket suppliers of seals sometimes differ in quality and effectiveness. I was pleased to find out that FelPro’s part works perfectly fine in lieu of a Honda alternative. I have no issue recommending either. The only thing I do not recommend is trying to use a general purpose o-ring for this. The size needs to be perfect to create that seal reliably.

I was missing a bolt to mount the pipe, and I noticed the service manual and the parts websites are incorrect about the size. They state it’s a “6x12 flange bolt”, or rather an M6x12mm 1.00 thread pitch bolt. Instead, I found both mounting bolts were M8x16mm 1.25mm thread pitch on my K20A2 head. 

These are the required parts numbers:

• FelPro Water Outlet Gasket: FPG 36022 (this is what most parts stores will have)
• Honda/Acura Water Outlet Gasket: 91315-PNA-003 (Same as above, but Acura and Honda dealerships have these in stock sometimes. They also fit on the K24)
• Honda/Acura Heater Pipe: 19510-PRB-A00
• 2x M8x16mm 1.25mm thread pitch bolts
• 2x M8 washers
• Some kind of lubricant that's safe for rubber. I use RV rubber seal conditioner.

Installation Instructions

0. You should probably empty the coolant before you do this. If you haven't changed it in a while, just take it to an autoparts store to be recycled and pick up more Asian vehicle blue coolant. If not, put it in a clean basin by opening the drain at the bottom of your radiator. 

1. You have to remove the intake manifold to access this pipe. I've tried removing it without taking off the manifold and it doesn't work. It's kind of a pain to remove, but with the right combination of ratchet extensions and u-joint adapters, along with wrenches you can get it. The middle bolt on the bottom requires a few 1/4" extensions and a u-joint adapter if you want it to be easier and not scratch your intake manifold since 3/8" extensions are too wide. A magnetic part retriever can help pull out bolts so you don't lose them, and put the bolts back in. Keep in mind if your car is stock, you might have to get at those bolts from under the car, as well as the support for the intake manifold. I use an RBC intake manifold and don't use the support to make it easier.

2. Remove the bolts from the heater pipe. They're labeled as 26 and 27 in the drawing below. 27 is actually a stud, but I replaced it with a bolt like from the part number list above. Technically I also replaced my intake manifold studs and nut with bolts from eBay, and I love how much easier it is to remove and install, but that's for another article.

3. Remove the hose from the other end of the pipe, and the hose on the middle of the pipe. It's not shown in the picture for some reason, but I have mine blocked off, and no coolant routed to my throttle body.

4. Articulate the heater pipe and get it loose by wiggling it around, and then lift up the intake manifold to navigate the pipe out of the engine bay. 

5. Check the status of the pipe. If it doesn't look good, you can do rust treatment and paint the outside with a more rugged paint to protect it from rust. If it does look good, slip the o-ring on the flanged end, stretching it as little as possible.

6. Lubricate the inside of the plastic thermostat housing by spraying some of the RV seal conditioner on your finger and wiping it around the inside circumference. Then lubricate the outside of the seal the same way. This is so it slips in evenly, and doesn't slip off the pipe and stretch out and become useless.

7. Seat it, and put the bolts in when it looks properly seated, and reattach the hose to the opposite end of the heater pipe. Technically, the best way to make sure everything is correct is using a radiator pressure tester to ensure it holds pressure up to the 16 PSI that your coolant system will see regularly. If you don't have one, that's okay, but it's really helpful.

8. Put the intake manifold back on, and put the coolant back in, check for leaks, bleed the air from the coolant, and you're done.

The pipe is 14, the o-ring is 23.

I should add there's another solution for tucking the engine bay and race purposes. TracTuff, makers of race-ready K series parts has an adapter with an o-ring on the pipe side, and either -6 or -10 AN threads or even 5/8" hose barb on the other. It requires removal of the thermostat housing (5 in the drawing above) and you have to drill a hole in it and run safety wire through. There's instructions on the purchase page.

TracTuff K-Series Thermostat Port Adapter

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