the way you can check for vacuum leaks is to have some sort of spray (carb cleaner, brake cleaner, small propane can) and spray this on the parts that may be leaking. If the engine revs up....due to the gas fumes burning...then you know you have a leak.

Some people use water....but whatever works works

It could be the temperature sensor for the ECU, easy to test but somewhat difficult to get at.
If you insist on replacing the pre-heat tube make sure the air-box thermostat is working or you could cause premature failure of the AMM.
Good luck. Dan
--
85 245 M46 192K, 92 245 A70 119K

You are probably running too lean until the car warms up. Check for vacuum leaks (additional air entry). The hiccups may be backfire?
--
1980 245 Canadian B21A with SU carb and M46 trans

Start with the cheap stuff first:
1. Make sure the pre-heater air hose from the exhaust manifold to
air intake (I believe it feeds into the air filter box) is not
broken or obstructed. They often need replacement because they
are exposed to the elements (rain ...) under the engine.
2. If that doesn't fix the problem, make sure there are no leaks
in the vacume hoses.
3. Check the wires from the water temp and thermal temp guage and
make sure they are not loose, disconnected, or broken.
4. Clean or replace the PVC and Flame Guard.

You need to maintain these items on an 85 240 anyway. After that,
the price tag goes up, and it's best to find a experienced 240
mechanic. It's getting harder to find people who can work on these
old volvos, but if you can find one, they are worth it. Good luck.