Is converting my car to diesel economically/practically feasible?

I bought a 1993 Nissan Sentra about 3 years ago for its fuel economy. Of course, gas prices continued to climb. I keep vaguely talking about getting a diesel car so I can run bio-diesel, but I don’t know if that’ll ever happen.

I just invested some money into the car by way of new brakes, tires, and a clutch. The car has been running great, save these routine maintenance issues. So, I don’t particularly want to buy a new car right now.

What if I were to swap the engine for a diesel? I know it would be possible, but would it be worth it? I spent $2000 on this car, and I’d be willing to spend somewhere in the 2-5K range for a newer used diesel. Couldn’t I just replace the engine instead? Then it would be practically a new car, for hopefully about the same price… ?

There has to be something I haven’t considered, it can’t be that easy.

Possible but never cost-effective.
It would be cheaper to buy a regular diesel vehicle than to convert your existing vehicle to diesel.
You might save some money by importing a Nissan diesel for that platform but I don’t know if that would work in terms of them making one and regulatory issues.
I suspect there is no way for a US consumer operating a vehicle in that class to operate a diesel vehicle of that class and approximate value and achieve a cost savings.

On edit:
Your best bet for a bio-diesel car is going to be '70s to mid-80s Mercedes or Volkswagen diesel. Newer cars tend to be pickier about their fuel. Please make sure you use good fuel and adequate filtration, as a few owners of newer diesel pick-ups have wrecked quite expensive engines and/or fuel injectors on bio-diesel.

There are lots of issues few think of to deal with:

Fuel tank: Many gasoline tanks are internally galvanized (zinc plated) to prevent rust. This causes problems with diesel fuel, so diesel tanks must NOT be galvanized.

Elastomers: Fuel systems Seals, hoses, etc all must be compatable with the fluids they contact.

Vacuum: A number of the vehicles accessories will be operated by the manifold vacuum a spark engine produces, such as the power brakes, power door locks, HVAC shutters, cruise control servo, etc. Lacking manifold vacuum, diesel vehicles normally have a belt driven vacuum pump, and a warning system in case it fails (no power brakes!)

Suspension: Diesels of a given HP are much heavier than spark engines…even turbo-diesels. In addition a second starting battery is often required to crank the high compression engine over while also supplying 50-100A to the glow plugs…thus the front suspension of a diesel carries several hundred pounds more than a car with sparkplugs, and the suspension has to deal with that.

The way to perform such a conversion economically, with a decent chance of good results, is to sell the gasser and buy a diesel.

A recent visit to a plant that I used to work at has revealed to me that even diesel tanks won’t necessarily function for bio-diesel. Something about the paint not being compatible. Also FYI it’s been ages since tanks were merely galvanized. Steel fuel tanks have a special paint on the insides of them.

By “compatible” I mean the paint won’t break down and start clogging important bits in the longer term.

If you are just looking to reduce the cost of your motoring I would recommend an LPG (Liquid Propane Gas) conversion. I am currently in this process myself. I drive a 79 vw campervan, its tank holds 13.2 UK gallons (I think gallons are different elsewhere) and costs £60 to fill up on unleaded fuel. The consumption is about 28 to 35 mpg.

Now an LPG conversion you keep your engine, get a gas canister fitted usually in the spare wheel void in the boot or just behind the rear seats. the whole conversion will set you back £1000 but the price to fill up is half. LPG is £0.432 per liter here while Unleaded £0.976 per liter. You don’t even have to give up your current fuel and can run both, (but not at the same time). Cost to cost where I live you can go twice as far on LPG for the same price as unleaded petrol.

Back in 1973, the father of an old girlfriend of mine converted his Chevy work truck to Diesel. He did a very nice job. I don’t recall what it cost him, and he did 99% of the labor himself. But when it was all over, he told me in all seriousness that he would have to drive to the moon and back to make it pay off.
YMMV, but not by much I suspect.