Volvo Owners Club Forum - View Single Post

petey · Dec 10th, 2018, 20:46

More or less, the battery's only real job is to power the starter motor.
The starter motor's only job in the world is to crank the engine at the required speed. I haven't got the instrumentation to prove it from your video, but it certainly sounds like both are making a fair crack of things and your problem probably lies outside of those two.

At 5°C, the glow plugs should be lighting up. The glow plugs only do one thing, but they do have sort of two jobs to do. Firstly, they give an incandescent surface to light the fuel in an environment where the metal of the cylinder walls and head could be at very low temperatures, therefore allowing the engine to start. For sure, cranking of the engine will warm things up, but not terribly quickly; the indicator that a glow plug is not all it could be is a long cranking period as the compression of gas process (how a compression ignition engine works after all) warms things up.
The second thing they do is continue to provide an environment for stable combustion after the engine has started. This only works for a short time as it's hardly effective to use a short length of very thick wire (for that's what a glow plug is more or less) when there's a gert big fire going on inside the cylinder, but for a short period, it works well at reducing nasty by-products of incomplete combustion.

So I would first look for issues with the glow plug and their actuation. SiRobb has videos far more descriptive than ever I can be here and they're well worth a view. VIDA may also report issues and really should always be your first port of call, not least because it's nice and clean and less exposed than leaning under the bonnet in December.

If all there is fine, then the next thing to consider is how the injectors work:- the common rail is brought up to a pressure by the high pressure pump during cranking. Then the injection time and duration is calculated and then an injector opened. If one of the injectors is letting by, it will drop the pressure in the rail and the cue to open the injector may never come because the rail pressure is always too low. The engine will crank until the battery's flat (or you've fried the earth connection, whichever happens first) but it won't start.

If the injector is borderline, it'll crank for a while, and if the rail pressure happens to creep over the threshold, suddenly the engine will start. It might well run mostly fine after that as the cranking speed is 200-400 rpm and idle speed is well over double that and hence the high pressure pump can now keep the pressure up.
Again SiRobb has excellent videos on how you measure how much fluid leaks past the injectors, but you do need a running engine to test this.

There's several dozen other things that can cause issues, but work methodically and don't just change things randomly, and you'll quickly get to the root of the problem. All of what I've mention above can be tested without disconnecting anything (except the leak-off pipes) if you have VIDA and a voltmeter. No need for sporadic changes that might introduce as many problems as you find.

Dec 10th, 2018, 20:46	#7
petey Master Member Last Online: Sep 11th, 2023 13:10 Join Date: May 2009 Location: Chippenham	More or less, the battery's only real job is to power the starter motor. The starter motor's only job in the world is to crank the engine at the required speed. I haven't got the instrumentation to prove it from your video, but it certainly sounds like both are making a fair crack of things and your problem probably lies outside of those two. At 5°C, the glow plugs should be lighting up. The glow plugs only do one thing, but they do have sort of two jobs to do. Firstly, they give an incandescent surface to light the fuel in an environment where the metal of the cylinder walls and head could be at very low temperatures, therefore allowing the engine to start. For sure, cranking of the engine will warm things up, but not terribly quickly; the indicator that a glow plug is not all it could be is a long cranking period as the compression of gas process (how a compression ignition engine works after all) warms things up. The second thing they do is continue to provide an environment for stable combustion after the engine has started. This only works for a short time as it's hardly effective to use a short length of very thick wire (for that's what a glow plug is more or less) when there's a gert big fire going on inside the cylinder, but for a short period, it works well at reducing nasty by-products of incomplete combustion. So I would first look for issues with the glow plug and their actuation. SiRobb has videos far more descriptive than ever I can be here and they're well worth a view. VIDA may also report issues and really should always be your first port of call, not least because it's nice and clean and less exposed than leaning under the bonnet in December. If all there is fine, then the next thing to consider is how the injectors work:- the common rail is brought up to a pressure by the high pressure pump during cranking. Then the injection time and duration is calculated and then an injector opened. If one of the injectors is letting by, it will drop the pressure in the rail and the cue to open the injector may never come because the rail pressure is always too low. The engine will crank until the battery's flat (or you've fried the earth connection, whichever happens first) but it won't start. If the injector is borderline, it'll crank for a while, and if the rail pressure happens to creep over the threshold, suddenly the engine will start. It might well run mostly fine after that as the cranking speed is 200-400 rpm and idle speed is well over double that and hence the high pressure pump can now keep the pressure up. Again SiRobb has excellent videos on how you measure how much fluid leaks past the injectors, but you do need a running engine to test this. There's several dozen other things that can cause issues, but work methodically and don't just change things randomly, and you'll quickly get to the root of the problem. All of what I've mention above can be tested without disconnecting anything (except the leak-off pipes) if you have VIDA and a voltmeter. No need for sporadic changes that might introduce as many problems as you find.