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A Stumbling Subaru

Q. Two questions, if you please. This is a 4-wd, 1.8 L, standard, carburetor, engine with 130,000. 20,000 miles back a rebuilt carburetor was installed. It actually gave the car more power, but at times it misses the mark. This usually happens after it's been driven a while, then parked, then driven again before the engine cools downs. It runs rough for a while then it will run smoothly again.

At other times it can happen while I'm riding down a long country road. The engine will begin to decelerate and slow down enough for me to have to pull over. Sometimes it will die, and sometimes I can keep it running by pressing the accelerator to the floor, as if it is flooded. The rest of the time the car runs great. Any ideas?

The other question is about body rust. I've heard of a product that can be applied directly to a spot of rust that will halt the process. Do you know what this product is and where it's available? I've searched the web and found nothing.

Thanks much.

A. You actually have two separate problems here and are unrelated to each other.

Let's look at the hot restart first since it's a more common problem. When you park and shut off the engine the engine temperature will rise about 40 - 50 degrees. This is quite normal and it's because the cooling system is no longer running and not taking heat away from the engine. When this happens the fuel in the carburetor gets hot as well, and begins to boil. When the fuel boils, it turns into a vapor and you have what is known as vapor lock. Since the fuel has to be in a liquid state to flow through the carburetor, if you try to start it to soon it will do exactly as you describe. Once the carburetor cools down, the fuel turns back into a liquid and the engine will run normally.

Auto manufacturers have dealt with this problem in two ways, one was to install small carburetor cooling fans to cool off the carburetor. They would come on at a certain temperature and the key off, run for a set time and turn off. This cooled off the carburetor and prevented vapor lock. Another method was to install a heat shield between the engine and carburetor to keep engine heat away from carburetor. If your car has a cooling fan, you may want to take it in to have it checked to see if it's operating properly. More than likely there was a heat shield under the carburetor. After time they deteriorate and rot away. The tendency is for the guy who replaces the carburetor to just throw it away and not worry about replacing it. If this is the case, have him replace it and that will take care of the problem.

Now, the other problem is not quite so common, In fact it's quite the opposite of your other problem. It sounds like the carburetor is actually getting frozen. Not frozen in the sense that something is locking up, frozen in the sense that it is getting so cold, ice is forming inside of it.

Inside the carburetor is a device called a venturi. What this does is speed up the air flow and atomizes the fuel more efficiently. As the cool air is brought in, the wind chill factor kicks in. This is the same thing that happens in nature, it's 30 degrees out and the wind chill factor makes it feel like it's 10 degrees. It will bring the temperature down so low, ice forms in the venturi and as the ice builds up, it closes it off, slowly killing the engine. When it finally shuts down, the carburetor will warm up, melt the ice and will start and run normally again.

What the manufacturers have done to prevent this is this; there is a heat collector around one of the exhaust manifolds. Connected to this is a large heat tube that goes into the air cleaner horn. Inside the air cleaner horn is a vacuum motor and an air temperature sensor (ATS). When the ATS senses the incoming air is too cold, it will close of the air coming in through the air horn and draw in air heated by the exhaust manifold through the heat tube.

The most common thing that happens is the heat tube gets brittle, falls apart and gets thrown away without being replaced. Not so common, but distinct possibilities are a bad vacuum motor or bad ATS. This is easy enough to check with a hand vacuum pump. Just unplug the hose from the motor and apply a vacuum, the flap should open and close freely. If not, you have a bad motor or a jammed flap. There should be a vacuum present at the hose where it connects to the motor. If not, check for a broken hose or something not connected properly. To check the ATS, start the car and look into the air horn. The flap should be closed and slowly open as the incoming air temperature rises. If it doesn't then you could have a bad ATS.

As far as the body rust goes, there is no miracle product that will stop your car from rusting. Rust forms when oxygen atoms in the air combines with the iron atoms in the steel of your car forming iron oxide. The only way to prevent rust is to keep the oxygen away from the steel in the first place. Once rust starts, it's very difficult, if not impossible to stop since there is no rust preventative that will stick to the rust.

Fender wells rusting are the most common rust problem areas. In fact, some manufacturers had such a problem with rust that they were paying to have the affected components repaired or replace. I know one manufacturer that actually bought back cars from customers because rust was such a major problem. I had one such car where the rust was so bad, I actually pulled one of the front struts out of the car with just my hands.

The only thing you can do is repair the rusted area and apply an undercoat or rust preventive to keep it from rusting again. Cars today are built using a steel that is more rust resistant and will go a lot longer before it begins to rust.

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