Previa runs terrible after tune up

My 93 previa recently went in for brakes, shocks, and tune up. 320,000 miles original owner. Afterwards it ran rough. The o2 sensor needed replacing. When I got it back it ran rough. Mechanic assured me it just needed a little time. It has the following issues:
Left front would shudder like an out of balance tire between 50-70. Replaced tire.
Now it hesitates and stutters at 70. Feels like its choking out. Won’t climb hills without shifting a lot. No pick up. Gets 12 miles to the gallon instead of 20. First three minutes of driving are okay, after that it rides terrible. I have him looking at it now. He swears it is just because it’s old.
Your thoughts?

Well, it is old but that’s not a reason for it to run badly.
Did he use an OEM o2 sensor or some generic one?

Generic. Light is still on. Throwing the signal of too much and not enough, if that makes sense.

I would take it to someone else but I have no money for another evaluation.

Hate to say it but that O2 sensor could be a problem.
Does he still have the old one? It didn’t run like crap with that one in, did it?
If it ran okay, swap it back in to see what that does.

It was burned up. The bolt snapped when he took it off. I was suspecting it was a problem but I’m not a mechanic. To be fair, the plugs were so used that they disintegrated upon removal. I have no idea the last time the sensor was replaced.

I appreciate you taking time RemcoW. Thanks

The o2 sensor is an integral part. Very often using generic sensor can be a crap shoot.
Regardless, he should be able to tell why it runs like crap. There are plenty of cars that old that run just fine.

There could be other factors in play as well. For instance, him having to wrestle the plugs out could mean those threads are hosed up and aren’t sealing well. Things do happen with an old car so don’t immediately accuse him.
Again, he should be able to tell what the actual problem is so then you can decide what’s what.

Good luck with it.

Thanks. I did wonder if the spark plugs were loose or if the throttle wasn’t adjusted properly. Being the original owner I kinda feel like I really understand the car and how it feels. I should hear soon.

With a 320k miles car the first step in a tune-up is running a compression test on the engine to assure that the engine is worth tuning up.

There’s a lot of things it could be. Assuming the compression is still at least even and tolerable, maybe an aged plug wire is giving up due to being wrestled off during the spark plug change. If those are the original wires then it’s a miracle they lasted this long.

If the ignition timing was checked and adjusted improperly by dialing in too much retard by not following the underhood decal an engine can run sluggish, buck, and suffer poor fuel economy.

An air leak on the intake due to a dislodged or cracked vacuum hose could also cause a problem like this.

Ah, an air leak. He did call and say the number three is at 120 lbs. it is a blackened spark which leads him to believe that the injector is stuck open. I will ask about vacuum hose though.
He checked timing. I’ll ask about the retard.

Did he use NGK or Denso plugs (properly gapped)? Previa’s, like many Toyotas, are persnickety about ignition parts. Also, was the cap and rotor replaced? How old are the spark plug wires?

Did he check or replace the spark plug wires? That could account for a lot of the problems but with the mileage issue, the O2 will do that and can cause a rich or lean condition.

All parts were replaced. Platinum plugs, no chincing on that. I always make that clear. But definitely good ideas.

A 120 PSI is not good and the blackened plug could be due to poor combustion.

The general rule of thumb is 20 X the compression ratio at sea level. This can vary a little depending on altitude, barometric pressure, engine wear, and so on.

I owned one of these for years. This sounds just like how they run with bad spark plug wires. New or not I think the problem lies there.

2.4 4 cyl motor, fuel injection. Should be fairly simple tuneup. O2 sensor is on exh manifold. Is it hard to reach?

Yes, anything to do with the engine is hard to do!

@CurlyMike - Were the parts used OEM parts or aftermarket? As I mentioned in my first post, these vans are persnickety about ignition parts. I owned one for a long time and used only OEM wires, cap and rotor and I used the Iridium Denso plugs. The platinum Denso’s are fine as well since they are spec for this van (as well as NGK) but using other brands of plugs, wires or cap & rotor could lead to the trouble you are having.

I agree that OEM ignition parts almost always work best. Anything else should be considered an experiment. Be willing to remove them and install OEM if the experimental results are unsatisfactory.