Was great to find this thread when I found a leak on the top of the filter mount. I figured a new Ford part at a crazy amount of dollars but after reading this, did indeed find that the fuel heater o-ring had started leaking.

For reference, I'm working on a 92 7.3L which has the black plug with a single o-ring seal.

One question and one clarification. Question first, when I removed the heater plate and started cleaning accumulated gunk off of it the three 'pads' that hold the heater wire to the plate quickly broke loose then one of the pads broke loose from the wire. I'm figuring on simply soldering the pad back to the wire and then the pads back to the plate. Anyone see a problem doing it this way?

Second is the o-ring itself. I see lots of reference to the size(s) and the one specification that I never saw mentioned was the profile of the o-ring. The one I just removed appears to be square profile, not a round profile and before everyone quips back saying that the round o-ring will eventually stay flattened into a squarish shape, as an experienced fluidics engineer, I'm pretty familiar with o-rings and feel pretty certain this was originally a square profile o-ring though there is no guarantee that it was the oem o-ring since I purchased the truck used. Searching for replacement o-rings, I am finding that there are more that are identified as square vs the others that simply say 'o-ring'

The size appears to still be a dash 016, being 5/8 x 3/4 (.614 x .754) with a 1/16" (.070) profile width. (McMaster # 1170N27) Looking for feedback on this on anyone who has looked at this aspect of the seal.

Appreciate any feedback on this.... Doug

 

When I did mine many years ago, I ordered two o-rings from IH. Just pulled the other one out and it is definitely a round profile.
Solder it back together.

 

I am a little curious how everyone's replacement O-Rings hold up. I replaced mine and everything seemed good for about 3 years/ 60K miles....and then it started leaking again. Instead of doing the O-ring again I decided to just tap the hole and put in a plug. That was 60K miles ago and I have had no issues. I was a little worried about winter time, but even on weeks where the highs were below 0*F I didn't have any issues.

I know there has been quite a bit of debate about the design of the heater and if it really even does anything.

 

After further inspection, I have decided it indeed was a round o-ring originally and am ordering accordingly (McMaster 9464K22 for ref).

As far as the heater plate itself, further inspection reveals that the pads are not metal bu some type of insulator material and appear to have been bonded with some type of adhesive. Not knowing what they are actually made of will make selecting the appropriate adhesive to use a bit of a challenge, let alone that it must withstand heat and contact with diesel. Not ready to plug the hole yet so gonna try something. Will post once I get it figured out.

 

So installed the new -016 o-ring and it fixed the leak. Was a bit concerned as when I tightened the filter stud back down, it tried to twist the heater plate and I was afraid it would push the connector to one side too much and cause the new o-ring to leak. No leak though. I used a small piece of translucent fuel tubing to connect to the water drain so that when I need to drain it, I can tell immediately when the water has cleared out. I now have a big bag of viton o-rings. If anyone gets desperate, PM me and for $1 thru paypal, I will send a couple via first class. Only have the -016 size though.

Where the little heater wire assembly pads had broken loose from the plate I ended up using Loctite Epoxy Weld Bond from and it worked fine. It is rated to 300 degrees F and is fully compatible with diesel.

One question that occurred to me while I was doing this, the heater wire has a single connection from the plug but it ends with no hard connection to ground leaving what appears to be an open circuit (The pads are insulators which is why I couldn't follow thru with my original plan to solder them). Curious, I looked up the electrical conductivity of diesel and didn't find much info other than a few obscure comments that they actually added something to it during the refining process to make it more electrically conductive. So does anyone know if that is how it works? Does the conductivity of the diesel create the resistance needed to generate the little bit of heat desired to keep the fuel from gelling? Does the conductivity change when it's cold to allow more heat to be generated? Curious minds want to know.