My 1990 240 has developed a starting problem lately that I can't figure out. When sitting for a few hours (or overnight) engine will only sometimes turn over and catch when keeping on the starter for way too long. Once running it runs just fine and if shut off will start again no problem if not too much time has passed so I'm thinking that it's not a fuel problem. Changed plugs, wires and rotor/cap. What could be happening here that only is an issue when the car has been sitting? What difference should that make? Outside temperature is not cold right now, but could cold start injector be the culprit? Should I be looking at coolant temp sensor (although gauge seems to work okay)? Any suggestions or advice really appreciated. Thanks.

 

If your engine takes a long time to catch when cold - you might have a residual fuel pressure problem. The check valve in the fuel pump or the pressure regulator may be allowing the fuel pressure to drop to 0. If so it will take more than a few seconds for the engine to fire when first starting after sitting for a while.
You can test this with a fuel pressure gauge - there will be x (some) fuel pressure in the system when you try to start the car after sitting for x time.
If a gauge is not possible/feasible - then before you start the car (when you know the problem is going to happen)
Cycle the key to the on position for 2 seconds, 2 or three times. this will turn the fuel pump on for 2 seconds each time and pressurize the fuel system. If the engine starts right up after that
you now know why - there's no residual fuel pressure left to be able for the engine to fire - so the pressure is either leaking from the check valve (in the fuel pump ) or the pressure regulator - Most commonly it would be the fuel pump check valve that is causing the problem.

 

Hoonk,

Thanks for the quick reply. What your are saying sounds very logical. I'll get on it today and (fingers crossed) will get back to you with some positive results.