11th Jan 2009, 08:57

After reading all the complaints for months I am attempting to fix the heater core problem myself. So far, removed the heater hoses from the firewall, took out the glove box and soaked up the fluid (passenger side) by removing trim and rolling back the carpet. I see now where the antifreeze has been going. The padding underneath the carpet soaks it up like a sponge. I kept topping off the antifreeze to keep the car from overheating which kept feeding the leak. So far my only conflict is two sets of instructions - one says you have to remove all the stuff under the dash and the other says no. I'll keep working on it slowly but surely. No rush. "she's" sitting in the garage. Scared to death I'm gonna deploy the air bag...

12th Jan 2009, 07:58

I have Mitsubishi Diamante 2002, and after reading all your comments, I still don't know what to do with my problem. I had heating core issues and antifreeze smell, but I didn't fix that on time, so I have now another problem. and I don't know if they are connected?

So after replacing battery, when I shift to Drive, both N and D lights are blinking. When I shift to R there is a strong jerk. Yesterday, after 100+ tries everything started to work well, but I had to disconnect battery and when I reconnected it problem is still present, both N and D lights are on in Drive position. Car doesn't accelerate well. Any thoughts what this can be, it is driving me crazy???

12th Jan 2009, 20:01

One of the real unfortunate effects of a really bad leaking heater core gasket, besides the awful smell, wet carpet, and fogging windshield drips, is a intermittent shorting out Engine Control Module. The ECM sits directly below the heater core, and the leaking antifreeze can over time wick itself inside the electronic box and short out the circuit. This will lead to the erratic engine and transmission behavior you are describing. Check some other Mitsibishi owner forums and you'll hear this sad story multiple times.

First things first is you must get the leaking heater core fixed ASAP, and while you in there, have the ECM removed and tested to see if it still functions properly. If its bad, don't pay dealer for a brand new one, BIG $$$, but check on line with salvage resellers and with local junkyards for late model Diamantes that you can buy used one.

You can fix the heater issue yourself with instruction at web site address I provided in earlier message above. Follow the step by step instructions numbered 1 thru 22. They work, I know cause I fixed my own 02 VR-X last fall with these instructions, and the problem is fixed. Cost me only $3 for the replacement gaskets from the dealer.

12th Jan 2009, 20:05

Regarding the heater core repair, follow the step by step instructions numbered 1 thru 22 at the web site address I provided in earlier message. They work perfectly - I know cause I used them to repair my own 02 VR-X last fall.

IMPORTANT CAUTION- disconnect the negative terminal of your battery before attempting any part of the repair, to prevent accidental deployment of the airbag.

17th Jan 2009, 09:55

OK, thanks. I hope you mean this one:


When it gets a little warmer, I'll pick up where I left off. I guess I'll report sometime to let others know how it goes!

19th Jan 2009, 07:52

2002 Diamante with 65,000 miles. Replaced heater core and muffler on the same service call- $1,700. Muffler rusted on the top causing loud noise during operation. Antenna mast broken. ABS/ TCL lights are on. Same problems as everyone else. If you own this car it pays to know a reputable mechanic.

20th Jan 2009, 14:37

Same issues going on with me... 98K on my '02 Diamante LS & it is in the shop for a heater core and muffler... What a pain & Money pit this is becoming!

9th Feb 2009, 07:13

Well, I'm on to step 17. Finally got the evaporator out. It was really wedged in there. Actually chipped a piece of the plastic tab but I glued it back together. Shouldn't be a problem. Now I'm going under to look up and see where the next step takes me... so far, so good. I'm following the steps on the edumunds link.

9th Feb 2009, 19:09

Bravo in getting thru step 17, removing the evaporator. Getting it back in later is even more fun! Now onto removing the four small screws from of each of the two heater core pipes that run thru the firewall. As instruction says, you'll be able to remove the black plastic hose end once screws are removed, but the other metallic pipe hose end is in too tight to remove. I actually ended up breaking out part of the white plastic duct wall that connects to the evaporator, since it was blocking access to the heater core pipes screws. The area is so small, I had difficulty getting torx wrench in that space. After that, it was easy to remove and reattach the screws.

I glued the broken ducting piece back with two part epoxy good as new (nobody sees it anyway). Careful about reinstalling the screws. The aluminum plate they go into is very thin, so it's easy to strip out. Also highly recommend using loctite medium (blue) on the threads before installing. That will help hold everything together thru vibration, thermal expansion, etc. Don't want to have to do this again!

Good luck and keep us posted. You almost halfway done.

14th Feb 2009, 09:03

Well, I got the 8 Torx screws out (couple of them were loose). When I twisted the black hose up it came right out. You're right. The metal pipe is really tight. So, now that I can replace the bezel there on the lower pipe, what is actually the "o-ring?" Your instructions are spot on.

14th Feb 2009, 20:04

OK. With a pair of channelocks I was able to wiggle the metal pipe just enough to reach inside with my finger and pull out the o-ring. Now I understand where they both go. Bought 2 new ones from the dealer for $5.36. Going to get some lock-tite tomorrow and start the reassembly process.

15th Feb 2009, 15:23

If it wasn't for the evaporator this job would be a lot easier! I was able to replace the o-rings (with locktite) and tried to move on to the evaporator but simply ran out of patience. Gonna take a break and come back later...

19th Feb 2009, 19:42

Yes, the evaporator is bit more difficult getting back in than it was taking out, but it's doable. Just keep trying different angles. You may have to bend a bracket or even the plastic ducting out of the way. Don't worry about it, can't see anything in this area anyway when you get it all back together.

To make sure that you have sealed up the two heater hoses properly, you may want to reconnect the coolant lines on the engine side of firewall and run the car for a while before reinstalling evaporator. It's much easier now to fix it now than later.

Well done, it's a good feeling to have just saved $1000!

24th Feb 2009, 15:50

I have a 2003 Diamante LS, and from reading all these comments it seems to me that I have the very same heather core problem. My windows get fogged up and I smell antifreeze, I haven't noticed anything wet on the passenger side yet, but I haven't looked real hard either. I called a repair shop and they said $1,200 to fix it. I'm pretty handy, but I'm somewhat of a novice and I don't have a lot of tools, what tools would I require to do this job?