Crank Timing Sensor

Next time it doesn't start or all the time until you get this corrected, slow down your starting process a bit. Turn the key on to run but don't crank, watch the dash, look for the check lamps to go through self check, then when they are finished doing their thing and go off begin cranking with your eye on the tach. If it doesn't fire up and you see nothing on the tach or below 200 rpm this would cause your starting problem.

 

One of two things, too slow of cranking rpm for the crank sensor to read or failing crank sensor. The ecm will not fire the injectors without seeing rpm, the same rpm signal that is missing for your tach to read.

 

Hopefully you're near the end of your mystery, I would pretty much bet you found your problem. Good luck to ya!

 

Another thing you stated, is that you have to re prime the fuel system. This would indicate a leak somewhere that's causing you to lose the prime. Check the filter heads for any cracks or leaks. It's common on volvos and won't throw a code.

 

the ecm injects fuel into the cylinders by timing of the cam position sensor.

the crank position sensor tells the ecm where #1 top dead center is. so that the ecm knows that it's injecting fuel into the right cylinders according to the cam position sensor.

the two sensors help keep the ecm properly timed for fuel injection. into the appropriate cylinders.

typically, the camshaft has 3 notches. and makes 2 revolutions for 1 revolution of the crankshaft. which only has one notch. the ecm uses those notches for timing of the injectors.

if the crank didn't have a sensor. the ecm would know to inject fuel by the cam sensor but it wouldn't know which cylinder to inject. the ecm knows the firing order. but it needs to know when and where.

unplug one or the other and the engine won't start. once the engine is started, it'll run without the crank but not the cam. least that's what happened on my isx motor when a road call was trying to figure out my electrical problem.

as for your tach. i could be wrong but i think it's part of the cam position sensor.

 

There is an check valve on your water and fuel separator, if that's bad it also has a hard time starting, I had the same symptoms as use describing, turned out to be the cam sensor was my problem. On Volvo engines depending on what model and year you have there are two sensors they both work off each other, just in case one goes bad, cam sensor and the flywheel sensor a.k.a. speed sensor.