: How to bypass the metal coolant pipe when it rusts out on a 98 DeVille

10-06-10, 12:24 PM
The metal coolant pipe that goes around the back of the engine is very hard to replace without dropping the engine. Fortunately, you don't have to replace it when it rusts out. It is easy to bypass and can be done in about 2 hours.

Materials list:
1 7 foot section of 3/4" heater hose
1 plastic 3/4x3/4x3/4 tee fitting
2 plastic 3/4x3/4 90 degree elbow fittings
9 3/4" screw type clamps
1 piece of plastic for hose insulation - I used a toolbox mat

1. Remove the battery for easier access to the overflow tank

2. Get a couple of good pairs of plyers in order to remove the factory clamps on the hoses.


These hoses are very hard to get at and it will take some time to remove the clamps. We will be replacing most of the clamps with the screw type ones.

3. Remove the hose the attaches to the bottom of the coolant tank. We will reuse this hose so try not to damage it.

4. Remove the top small hose from the coolant tank.

5. Remove the coolant tank. It is not an easy out - I had to pry it up from the fender well with a screwdriver. Don't damage the A/C system!

6. Remove the coolant tank lower hose from the metal pipe.

7. Remove the heater hose from the lower pipe. You can just cut this one off at the pipe as it needs to be a bit shorter anyway.

8. Stick a long screwdriver into the top hole of the metal pipe where the heater hose was and crank it down and up until it breaks off. If yours was rusted out as bad as mine this took about 2 minutes.

9. Now make a new distribution point using the tee and 1 elbow. You basically want this to look similar to the way it did before accept the output that went behind the engine will now go in front of it. You could do this with one tee if they made one that was the way you want it (two outputs going the same way and one output going up), but they don't. You want one side going up and two sides facing the front of the car. I attached the 90 to the tee with a 2" piece of heater hose. You want the hose as short as you can get it. Attach all connections with a screw clamp. I also used petroleum jelly to lube the connection point. You could also use brass parts but you would have to make the crossover and it would end up costing around $50 plus would require sweating the fittings. I did find a 3/4x3/4x3/4 brass tee PEX fitting with barbs in the right size but I am not sure this would work since the barbs are smaller being designed for PEX tubing.


10. Attach the heater hose to the top of this. Make sure the two outputs facing the front of the engine are turned slightly towards the right fender. This makes sure there are no clearance issues with the drive belts.

11. Attach the long heater hose to the elbow (bottom) output. Snake it on the fender side of the A/C line and through the opening that the radiator hose uses (next to the radiator hose). Just leave it hanging off the front of the car for now.

12. Attach the tank hose that you saved first to the top front connection of the tee and then to the tank.


13. Reinstall the tank.

14. Reinstall the battery.

15. Move to the other side of the car.

16. Remove the air cleaner housing assembly and pipes.

17. Disconnect the upper radiator hose to have working room.

18. I disconnected some of the wires too for even more room.

19. Remove all the old setup from the rusted pipe to the crossover housing. You can just cut everything away as you will not be reusing anything. BE CAREFULL not to cut any wires!


20. Attach a short length of hose to the remaining elbow - just enough to cover the barbs and attach to the hose outlet on the crossover.


21. Run the heater hose through the radiator hose path on this side as well. I used a piece of plastic insulation to make sure the heater hose didn't rub too much on the engine mount bracket.


22. Put the elbow in place where it will go - don't tighten yet - and cut the heater hose to length making sure it clears everything.

23. Attach the heater hose to the elbow.

24. Attach the elbow to the crossover housing.


25. Reconnect the wires, radiator hose, and air cleaner parts. I also replaced the thermostat while I was in there. While you have the upper hose off, remove the lower hose at the thermostat housing and flush the radiator while you are at it. Run water into the upper hose.

26. Put everything back together and fill the coolant sytem. I ran it for a few minutes until it started getting hot and turned off the engine. The overflow tank emptied out and then I filled it some more. It takes a little bit of this to fill the system. You'll be getting close when your new bypass hose starts to get hot and your heater starts blowing hot air.

This is a permanent solution!

12-11-10, 03:58 PM
Nice write up. Thanks.

09-02-11, 01:24 AM
I Also have a 1998 Cadillac deville .....but the only thing i would be worried about would be the fittings that you used .. plastic might melt pretty quick and easy within a year or so .. i would probably go with a copper of aluminum fitting.. but i have to say that this is by far the best article on this website .. thank you alot . Really !!.. i am having the same problem as you did and im going to try and do what you did tomorrow morning .. i will let you know how it works !!. AGAIN THANKS so much !:yup::)

09-02-11, 08:43 PM
:bigroll:im haveing a major probelm ... i did excatly what was told ... and now my car is overheating ??? .. i dont feel the new hose getting hot and i have no heat ???? PLEASE HELP

09-03-11, 07:25 PM
it worked !!!! I only paid a total amount of 15 $$ for everything i needed to do the job... WOW I never would of thaught of this .. i am very thank full ... u saved me 2,300 $$$$ thank you

09-13-11, 12:13 PM
my plastic fittings did melt !! BAD IDEA !! so i use brass fittings and no its worxx perfect .. DONT USE THE PLASTIC FITTINGS !!!!

09-13-11, 10:01 PM
Most hardware store/Lowe's/HomeDepot is only good for service to about 150 degrees or so - MAX.

When your Flapfish Sixty Special gets up to 180 degrees+ at normal temperatures, plastic plumbing fittings don't do the job.

Spend the money for brass or copper fittings.

Making sweat joints in copper plumbing T's and straight couplers is a piece of cake.......and it lasts forever. I like to use a rat tail file to create 3 or 4 grooves around the hose clamp area - extra clamping friction - just like the barb grooves in the plastic pictures, but not as deep, of course.

01-12-12, 12:06 PM
Thank you man but please put more photos of the pieces more
Will work with my Seville STS 99

01-12-12, 07:37 PM
Sorry I haven't been able to figure out how to update this post. Do NOT use the plastic parts that I specified in the build. Instead, use the below parts designed for under-hood usage. I did have a problem with the elbows and tee deforming about 6 months after the fix causing a leak. It took about a 1/2 hour to replace the junction parts with the below parts from Amazon. You can also check your local auto parts store as they may have these parts. Summit racing also has the below parts.

Tee 1 "Dayco 80684 Plastic Connectors Bag http://www.amazon.com/Dayco-80684-Plastic-Connectors-Bag/dp/B000CIZWDA/ref=sr_1_1?s=automotive&ie=UTF8&qid=1324035978&sr=1-1&tag=5336121755-20

Elbow 2 "Dayco 80674 Plastic Connectors Bag http://www.amazon.com/Dayco-80674-Plastic-Connectors-Bag/dp/B000CIZWBW/ref=sr_1_1?ie=UTF8&qid=1324035943&sr=8-1&tag=5336121755-20

97 Eldorad0
10-21-13, 06:29 PM
Mine was rusted out as well. I am simply buying more tube/piping and fabricating it tommorrow. I don't know why this is avoided. It should be pretty simple. Tubing-pipe- bender- viola.

11-22-13, 07:22 PM
I don't see a metal pipe in here ... Does it matter what model deville???

06-01-17, 09:37 PM
I don't know if you are still a member of this forum or not, but thank you dear sir, this was exactly what I needed. The metal coolant pipe in my Mom's Deville rusted out and it needed to be replaced. I looked online find a replacement part and I found this thread. I was able to buy the materials for the bypass for about $40 at Autozone. It took about 11 hours to tear down and put in the new bypass.