In a bath remodel we replaced an old toilet with a new one. Now the toilet won't flush properly. Both were installed on the same 3" drain that has a p-trap. Floor is slab on grade.
The toilet acts like the vent might not be working - water rises on initial flush and then slowly falls and siphons out until it sucks air. Ran water down the roof vents but no help.
What are my options?

If you have a 3" P-Trap under the toilet you definitely should take it out, the toilet is designed to not need one.

Toilets have intregral traps built in them. Toilet cannot be double trapped and work proberly. If you have trap in floor that toilet sets on I would think that 3" opening was once a floor drain. You must remove 3" trap in slab.

Toilets have intregral traps built in them. Toilet cannot be double trapped and work proberly. If you have trap in floor that toilet sets on I would think that 3" opening was once a floor drain. You must remove 3" trap in slab.

Any explanation for why the old toilet worked fine?

You said, "Both were installed on the same 3" drain that has a p-trap"

To me this suggests that the PTRAP isn't directly under the toilet but is instead a WHOLE HOUSE TRAP (see image). Most of these were removed years ago, but if your house still has one then that is not an issue in and of itself, OK?

What does sound to be an issue could be that the whole house trap is clogged, or it could even be that the main drain line before or after the whole house trap is clogged. In order to test this I would suggest running water from a washing machine or draining a full tub of water into the main drain and see what happens at the whole house trap (should have one or two cleanouts that can be removed) or at the toilet... ;)

Back to you...

Mark

You said, "Both were installed on the same 3" drain that has a p-trap"
...

Mark

Nope. Not a whole house trap.

Any explanation for why the old toilet worked fine?

Older and newer toilets have a different design and a different water volume, that could be the reason, another could be that the p trap is now clogged, could be coincidental, sometimes happens though...
In any case you will save headaches now and in the future eliminating the ptrap, you could always take advantage of the occasion to place a backflow valve in its place

It sounds like you should use a closet auger amnd snake through the toilet... something might be hung up at the outlet of the toilet itself. If that fails to resolve the issue you may need to lift the toilet and snake the drain.

Also, I agree with Fulgur... remove trap as it can only cause problems down the road (or may even be the issue right now)!

Both were installed on the same 3" drain that has a p-trap This is against code which states that you may not double trap any fixture. The exception to this is if you had old toilets installed that didn't have internal traps.
The trap under the slab has to go. Good luck, Tom

Thank you all for the helpful answers. It is clear now what must be done.