Copyright 2019 Discovery Owner's Association, Inc.

Coolant Replacement Procedure

The following information was provided by Joe Brown. This details the procedure to change the coolant in the Cummins ISB engine.

The Freightliner RV recreational chassis maintenance manual tells you to remove the radiator cap when the engine is cold, then run the engine until the upper hose is hot. I chose to run the engine just a short time so as not to have to drain scalding water. I chose to remove the 2" coolant hose from the front of the transmission cooler reservoir. The transmission cooler reservoir is on the curb side and can be reached by opening the LP gas compartment on the '99 36T. If your LP gas compartment is not the next to last compartment, you will have to crawl under the coach. It appears to be the low point in the cooling system, and drains the system very nicely. You'll need a container that will hold at least 10 gallons of coolant. I used a large mixing tray that I purchased at Home Depot. The overflow bottle is attached to the radiator bracket with two bolts. You can remove the bolts, dump the overflow bottle, rinse with a garden hose, and reinstall the bottle with the two bolts, without having to disconnect the overflow hoses.

After the system drains completely, push the coolant hose back onto the transmission cooler. There is no need to tighten the clamp while flushing the system. The hose will stay in place if the system isn't pressurized. Don't forget to replace the hose and tighten the clamp after the last flush, before adding coolant.

Our rig has a decal on the surge tank (the tank mounted above the radiator with the radiator fill neck and cap ). This decal has better detail than the maintenance manual for filling the system. I practiced this procedure a few times while flushing the system with fresh water, prior to filling with coolant. The practice was so I wouldn't waste too much coolant when refilling with the real stuff. Just in case your rig doesn't have a decal, I've copied it below. My comments are in parenthesis.

  1. Close the heater loop inlet gate valve at the rear of the engine block and the heater loop outlet gate valve at the inside rear left hand (street side) frame rail. (The easiest way to reach the heater loop inlet gate valve is from inside the coach. Raise the engine cover under the bed. The gate valve is slightly under the floor of the coach at the foot of the bed.)
  2. Open the petcock valves at the side of the radiator surge tank, (1) (this one's on the street side, and easily seen when standing at the rear of coach). And the heater loop outlet (1). (This one's under the coach, inside the streetside frame rail, just ahead of the heater loop outlet gate valve).
  3. Fill the surge tank. 3.5 gallons per minute max.
  4. Start the engine and run at idle for 10 seconds. Then run the engine at high idle for one minute. Then back to idle. (Let the engine continue to idle thru the rest of the fill procedure.)
  5. Open the heater loop inlet gate valve.
  6. Close the petcock valve at the heater loop outlet when it begins to flow a steady stream of coolant. Do not allow more than 1.5 quarts to escape.
  7. Open the heater loop outlet gate valve.
  8. Close the petcock at the radiator surge tank when it begins to flow a steady stream of coolant. Do not allow more than 1.5 quarts to escape.
  9. Top off (refill) the surge tank. (Replace the radiator cap.)

Steps 5 thru 8 are best performed with a helper. If you have a helper, you will not have to waste coolant. I had my son turn on the heater loop inlet gate valve from inside the bedroom, while I caught the antifreeze being purged from the heater loop outlet petcock under the coach near the frame rail at the left rear. Meanwhile, the petcock alongside the surge tank was cracked open, but I had a hose attached to it, and caught the coolant purging from it in a jug for reuse.

The book says not to let more than 1-1/2 quarts escape either petcock. This would be very difficult without a helper to open the gate valve. At least, you'd have to be 10 times faster than I to open the valve from inside the bedroom, then run thru the coach, then around to the back, crawl under, and turn off the petcock. You could easily waste a gallon or more from each petcock, and probably create a big air bubble in the system.

When our rig was new, I had to add coolant after each trip for the first 6 or 8 outings. It's probably because of the air space in the surge tank above the filler neck. The air must displace between one and two gallons of coolant. The air is eventually purged through the overflow bottle and replaced by coolant.


The DISCOVERY OWNERS ASSOCIATION, INC, was formed to promote the sharing of information and the camaraderie of fellow Discovery motorhome owners. Membership in the club is limited to owners of Fleetwood Discovery motorhomes.


Website Questions
Email Webmaster
Membership Questions
Email Membership Coordinator