Learn How To Plastic Weld! – Repairing A Classic Saab 900 Turbo Coolant Reservoir!
September 26, 2020
This is what a cracked coolant reservoir from a 1991 Saab 900 Turbo looks like. Not just a little cracked like I was hoping, but REALLY cracked. How do you fix something like this? Some kind of glue or epoxy? While JB Weld might do the trick, we have had a lot more success with something called plastic welding. Never heard of it? Well then, let me show you what it is and how it works.
This is an 80-Watt plastic welding kit, also known as an 80-Watt soldering iron with a special tip. Admittedly it comes with a small wire brush, a cheap stand, some plastic samples and some fine metal screen mesh which are pretty specific to repairing plastic. This particular one came from Harbor Freight, the warehouse of really cheap tools. You can use a soldering iron instead, but make sure that it is in the 80-100-Watt range, you will be waiting FOREVER for the plastic to melt with anything lower.
I bought this last summer to repair the gas tank in our other Saab 900, and at the same time I picked up a package of plastic welding rods, just to make sure that I would be covered for anything that I needed to do. Unfortunately, gas tanks aren’t made out of PVC, PP or ABS. Do you know what they are made out of? A type of plastic called HDPE which is known for its resistance to chemicals, specifically to fuel. Well, that was a bummer. However, after a little searching on this magical thing called the internet, I discovered that milk jugs are also made out of HDPE, leaving me with a readily available source of welding plastic for things like fuel tanks and reservoirs.
The first thing to do is let the iron heat up completely. I am not going to force you to watch that part, it was pretty boring really, but an episode of PBS Space Time was there to keep me going.
Next, find the crack you need to repair. Take the edge of the hot end and melt a vee shaped groove along the entire crack. You want to get about three quarters of the way thru the plastic, not all the way. It may take you a couple of passes to get there, but take your time, and let the heat do the work. Do not try to force the tip in, all that will do is bend and possibly break it.
Time for a quick side note that really should have been the first point of the entire video. DO NOT BREATHE IN MELTING PLASTIC VAPOR. Work in an open space, with a facemask to filter anything that you might breathe in. This stuff is toxic, and you don’t want to mess with it. You can’t see it in the video, but my garage door is wide open here. Yes, it’s was a cold and stormy night, this is Oregon after all.
Once you have carved your channel into the plastic, it is time to go back over the edges and flatten them into the groove. You wont be able to fill it in all the way, you are just flattening everything back out. This is the point when you grab the strips of milk jug that you cut up and start melting it into the remaining gap. Make sure to heat up the existing plastic as well as the milk jug, you are trying to bond them into a watertight seal that will not immediately break open again. Go ahead and use extra, this is not something that winds up being pretty at the end of the day, it is something that is extremely functional, and far cheaper than buying a new one (sixty five dollars in this case).
At this point I take the wire brush and scrape off any of the plastic that didn’t actually bond to the tank. If it looks like I still need to fill in with some more plastic I will do that as well and then smooth it out again to get it ready for the next step.
Next, we want to add some strength into the cracked area to keep it from coming apart again. Take your metal mesh and cut a piece large enough to completely cover the crack that you just patched. If you have run out of the mesh that comes with the kit, aluminum door screen works really well for this. At first, I work my way around the mesh and get it heated up to the point that it starts sticking into the plastic. When that begins to happen, you need to start working from area to area, only heating up one section at a time. Get one corner hot and stuck into the plastic, then hold it in place with something else, in my case a pair of children’s scissors. Once that section has cooled a bit move to the next area, and just work your way around. If you try to keep the whole thing hot and do it all at once, you are fighting the tendency of the mesh to pop back out of the melted plastic. Just do it in sections, and you will eventually reach the point when the entire thing is encased. I like to go over it at that point and scrape off any excess that isn’t bonded to the tank.
Something like this is what you should be left with. I still have a ton of cracks to fill, but if that was my only crack, I would be able to leak test at this point and put it back in. However, I am not that lucky so let’s speed this up and get it finished.
It can be pretty easy to fall into the trap of thinking that used cars just aren’t worth it. Something is always breaking down or broken when you buy it and it is constantly costing you money. While that can be true, especially with a project car like this, a new car isn’t really going to save you money and may cost you more in the long run. Between a car payment and full coverage insurance you can wind up paying anywhere from $200 to $1000 a month just to have a vehicle. If you cannot work on your own vehicle, then you can argue that an older car can cost you just as much by taking it to a mechanic. However, why aren’t you working on your own car? The internet is a treasure trove of information on how to repair just about anything. We will soon have up some videos on replacing shocks, front brakes and how to tell if you have a vacuum leak. This is not knowledge I was born with, some I learned from my dad but much of it I taught myself by reading or watching a video and then trying.
As you should be able to tell from our channel, we refuse to be part of a disposable society. If something can be fixed, it should be fixed. Someone once told me that poor people can’t afford to buy cheap crap, and that is true. Don’t bury yourself, but buy the best quality that you can, when you can. Read reviews, find out what can happen with the purchases you are planning and when it inevitably breaks learn to fix it. Educate yourself, constantly and always. Ignorance is not an excuse in this day and age, as a society we have never had more easy access to information than we do right now.