Toyota4runner SR5 - zero compression in one

Did you drive the vehicle there ? Run ok ? Have misfire's ? Zero compression on all cylindes or just one or two ? What make , model an year vehicle ? Engine size ?

Run a compression check on all the cylinders for excessive leakage. Check the timing marks with a timing light to see if the timing is correct. Test the map sensor and the oxygen sensors for proper operation.

Check your compression in each cylinder.
Check your fuel pump pressure.

Determine what cylinder is misfiring and do a compression test on that cylinder. Compare it with a cylinder that is not misfiring. You should have no more than 20% difference between the good cylinder and the bad cylinder. If it is low on compression, suspect a burn valve or bad valve seat... and it will require cylinder head repair. If it is good on compression, then look for a faulty injector or some other reason for the misfire on the dead cylinder.

Zero compression could be a lot of things, All major.
Piston or ring problem, intake or exhaust valve not seating or burnt.Or problem with valves not opening at all.And so on. You need to have it checked by a good shop you trust.

popping out of the tail pipe is usually a tell tail sign of a timing issue. Pull the timing cover and hand crank the engine. If the marks on the crank and cam line up check vacuum. If needle fluctuates roughly 2in/hg take note(possible burned valves) Inspect the plugs for signs of carbon fouling(fuel injector issue). if the plugs show good burning with a yellowish white check compression. If compression is good check cylinder leak down. if cylinder leak down is less than 30%. check exhaust back pressure.

303 is a misfire on the number 3 cylinder, 432 is a bad catalytic converter or O2 sensor, 456 is a minor evaporator leak (Most likely the gas cap wasn't screwed on tight). Do a compression check on the motor. Most likely you'll find zero compression or low compression on 1 or more of the cylinders. I suspect your timing belt broke. Unfortunately this means you be replacing the timing belt but you will also have to have the head fixed and replace all of the valves. If the vehicle has a Distributor and not a coil pack you can remove the distributor cap and crank the motor over, if the rotor doesn't turn it is you timing belt, if it does the timing belt is good. Some vehicle also have a plastic cover over the front timing belt pulley that can be removed to check the belt. You maybe able to remove this and check it that way.

injecters are cloged

If you have a 0 reading from cylinder 2 that means there is no compression from it and you need an enfine rebuild.It seems that there had been damage to the engine.Have it hooked up to a computer and check diagnostic code.Overheating of an engine can be very costly and damaging, rememeber that always.

I think you could be right. Leaking valves could create this same problem as well.