There is nothing more annoying when you're trying to buckle in a struggling two-year-old into their car seat than having the door constantly whack you from behind, pushing you into the car on top of your kid. Even if I opened the door slowly and carefully let it go, it just wouldn't stay open. I had to open up the door panel to replace the door lock actuator anyway, so while I had it open I figured I'd take a stab at fixing the part that is supposed to keep the door open: something they call the door checker. Sure, this is a relatively cheap part, but if all four are causing you grief, it would likely run you over $100 to replace them all.
I'll be doing this on one of the back doors of my 2003 Honda Civic. The door checker on other cars may be similar though. If you have similar success on another car, let me know in the comments.
removed the door panel. Once that's done, remove the three bolts holding the door checker in place. If your Civic was built in Canada, the bolt sizes may be the same as mine: 10mm for the bolts on the door and 12mm for the bolt on the car frame.
The door checker can only be pulled out through the inside of the door. So reach inside the door with one hand and pull it out.
However the plastic pieces are small enough to fit into the grooves themselves. But because of the H-shape, the two pieces can't get any closer to each other. So sand them down. The picture shows which part to sand.
You don't have to worry about sanding too much since you really don't want those parts touching anymore. The picture to the right will show how the pieces fit over one of the grooves after I sanded them down.
Now because we're pushing them closer together than they were before, the rubber blocks will be too small. So we need to put something else in there. I used cardboard. I cut small pieces, about the same size as the blocks. Depending on the thickness of the cardboard, you'll need to use one or two pieces on each side.
I won't lie to you: stuffing the plastic, rubber and cardboard back into the thing is the hardest part. If you're finding it hard, that means you're doing it right; it needs to be tight! A flat-head screw driver came in handy for me. Keep the casing down near the narrow end of the bar to make it easier to shove everything in.
Once you have everything in, put the plate back over top, but don't fold the tabs over yet. Slide it back and forth across the bar first, holding the plate firmly over the rubber and plastic so they don't fall out. You should find it extremely difficult to move it past the grooves. In fact, it should either take all of your might to do it, or if you can't move it out of the groove at all, even better. I'm serious about it taking everything you have to move it. That's important. If that's not the case for you, shove some more cardboard in there. Otherwise it'll still be too loose once you put it back on the door.
Once the tension is acceptable, fold the tabs back into place with pliers again. Now you're ready to go! Put it back in place through the inside of the door, replace the bolts, and open your door wide and watch it stay! You may want to grease up the bar again since I found that a lot of the grease came off after handling it so much. The grease will help the door slide shut when you push it shut.