Disel 75 ford escort van.engine starts then air cloggs fuel filters and cuts out ,

All of these problems point to a fuel delivery problem.
The fact that a fuel treatment helped, leads me to believe its a dirty fuel system. Also, the cold start problem is an enrichment problem, as there has to be a richer mixture to fire when its cold than hot.
First thing to check is the fuel filter that may be located in the fuel tank.

could just be some water in fuel line has slightly blocked your filter again -you could change it out or try to restart and get it running for a while-may do the trick

fuel level to low,air in fuel system,idle adjustment incorrect,fuel feed or return blocked,air intake/system blockage,blockage in exhaust system or faulty injectors. Hope this helps

change engine on my 1.8d van after fitting the engine now the van wont crank over

First thing I would do is check your air filter. You can pull the air filter out and try starting vehicle. Check to make sure there are no obstructions between the air filter housing and the intake. If that dont work, on your next posting mention the year of the vehicle and if its fuel injected or carburated. Good Luck

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Diesel Engines The 6.9L and 7.3L diesel engines use a one-piece spin-on fuel filter. Do not add fuel to the new fuel filter. Allow the engine to draw fuel through the filter.

1. Remove the spin-on filter by unscrewing it counterclockwise with your hands or a strap wrench.
2. Clean the filter mounting surface.
3. Coat the gasket or the replacement filter with clean diesel fuel. This helps ensure a good seal.
4. Tighten the filter by hand until the gasket touches the filter mounting surface.
5. Tighten the filter an additional 1⁄2 turn. NOTE: After changing the fuel filter, the engine will purge the trapped air as it runs. The engine may run roughly and smoke excessively until the air is cleared from the system.
   

POSS AIR FLOW METER

You are on the right track. 99% sure it is a fuel supply problem. I suspect the lift pump is a likly candiddate or an obstruction inside of the fuel tank. Chack also on the vacuum side of the pump for any loose hoses that could result in an air leak so the lift pump will draw air instsead of fuel

should be in the air intake tube or filter box

check the accelarator cable isnt stretched (rev the car from the peddle, then from under the hood and listen for any rev differences) although its just been serviced also check that its had a replacement air filter. this could be restricting air.