91 Legacy wagon- Started shaking a week after the shop worked on it?

[OldSubaru1]

My 91 Legacy wagon had what seemed like a failing fuel pump, and I took it to a local mechanic that I have gone to for years and I completely trust. The issue I had was that the car would not start normally, but when I tried to start it with my foot on the gas then it would start, and white smoke came from the tail pipe. The mechanic replaced the O2 sensor, did some work on the electrical system, replaced the AC belt, and may or may not have replaced the fuel pump among other things. Unfortunately I am relying on memory since I don't have the service paper in front of me. The car was in the shop for well over 3 weeks, and the repairs, parts and labor all came to around $1,300 in the end. I drove the car for almost a week after it was out of the shop, and then it began to shake a great deal whenever I would drive it. The engine also shut off completely once while I was attempting to park. It also did not do this before I got the previous work done. The previous work that was done is under warranty, so I took it back to the same shop, but I am not sure if the shaking is even related to what was done previously. Moreover, the car has already been back in the shop for another week and they still haven't even looked at it yet.

The car is obviously pretty old, and I am wondering if I should even keep investing in it. Currently it does run aside from the shaking, so should I possibly just go to the shop and tell them not to bother with it? I just really need to have my car, and it's been a week and they haven't even looked at it.

Also, does anyone know why the car may be shaking based on the work that was done previously? For example could the previous work somehow have led to the current shaking problem? The mechanic mentioned that they had to remove the dashboard in order to perform the previous work. I just want to know what could be causing the problem and what my best course of action would be. Any help would be greatly appreciated.

[OLsubedrivr]

Check the easy stuff 1st; plugs, wires loose or worn, pcv. air filter, fuel filter, ignition coils and vacuum hoses. These are fairly easy DIY, unless you aren't so inclined. The white smoke could be
bad, water/coolant in combustion chamber from internal HG leak. So check your radiator & overflo tank for darkish globs and your oil dipstick & cap for frothy brownish gunk. Hopefully a more seasoned member will chime in. As for investing more money, don't consider it an investment, it is an expense, if you figure things out and the car is running good again then you've eliminated a lot of future repairs. Finidng used cars in the same price range, will likely need as much money as you've spent. Good luck

[OldSubaru1]

I paid $1,000 for the car when I first purchased it used, and I was aware that it would need some work to pass inspection even though it ran well. As it turns out, it cost $2,500 to get it to pass inspection, and after that it ran fine for a little over a year until I began having the current issues. I also had to pay $600 for new tires a few months back. I probably wouldn't mind letting the shop take another look at it except that this shop gets a lot of business and it always seems to take several weeks for them to get to my car. But I don't necessarily want to take it anywhere else because it is already under warranty at this shop. However I also can't afford to be without my car for more than about another week because I need it for the holidays. I'm not made of money either, and I can't afford to keep paying for repairs especially if I am going to continue to have problems even after the repairs are made. It might be different if I had more time or if I knew the repairs would make a difference, but as you can imagine I am just a little bit of a skeptic at this point, and I'm also frustrated because I need my car and have already been without it for about a month. Hopefully this makes sense.