Cold starting

[Clan]

Originally Posted by globetrotter

Just established that there is no spark at the plugs whilst cranking a short time but comes on after a while and starts, it's starting to narrow what could be at fault, what tells the plugs to start firing?
Steve

The ignition ECU establishes the sparks as soon as it gets signals from the flywheel sensor . It then initiates the Fuel ECU if all is ok '
I would suspect the flywheel sensor at the moment , the lead can deteriorate near the sensor after 20 years or so ...

[Laird Scooby]

Originally Posted by globetrotter

Just established that there is no spark at the plugs whilst cranking a short time but comes on after a while and starts, it's starting to narrow what could be at fault, what tells the plugs to start firing?
Steve

The CPS (Crankshaft Position Sensor - mounted on top of the bell housing) tells the plugs to start firing - BUT - as Tony pointed out above, the way the ignition system is set up on the later cars, it takes at least 2 turns of the crankshaft before it generates a spark.
This also prevents the fuel pump running but that is over-ridden by the starter signal to the fuel ECU which switches the pump on while cranking.

In other words, if your car is turning the engine over twice before it fires, that's normal on a late one.

Might be worth checking the condition of the cables to the CPS though, they're exposed to a lot of heat at the back of the engine bay and tend to go brittle with age. Could be giving a problem.

[TonyS9]

Yes so it sparks eventually, that is normal, but if its still not starting when sparks are going then either the spark is not getting to the plug or there is not enough fuel.

It can seem like an age for TDC to come up, and it will vary from 1 turn of the crank to next to nothing. BTW the hunting you get from the starter is just starter revolutions, its much lower geared, maybe needs something like 50 turns to 1 crank turn.

I think it should start the pump when the crank is turning, but it may not start the injectors until the ignition is fired (After TDC detected), not sure.

Anyway doesn't definitely sound like you have a real fault. Just maybe thicker oil and long 940 starting more noticeable.

[bob12]

Flip the coin .... as has been well noted here over many years I just love cold weather starting.

My '90 745 with LH2.4/EKZ116 which is very temperamental starting on warm days or when left for 20 mins after driving (although I have the knack of starting it quickly) is currently starting 'on the button'. In fact it doesn't noticeably even turn over ... it just starts/fires-up after a second or so of turning the key!!

Bob

[TonyS9]

Does the 740 not use hall triggering on the cam/disti? Hall triggers are typically 4 per revolution, which with 1/2 crank speed should be twice as fast as crank trigger 1 per revolution?

[Laird Scooby]

Originally Posted by TonyS9

Does the 740 not use hall triggering on the cam/disti? Hall triggers are typically 4 per revolution, which with 1/2 crank speed should be twice as fast as crank trigger 1 per revolution?

Depends when it was made. Early ones had the block mounted dizzy with Hall Effect trigger, later models had the cam mounted dizzy with the CPS in the top of the bell housing.
With Bobs being a 1990, it has the later system.

[Clan]

Originally Posted by bob12

Flip the coin .... as has been well noted here over many years I just love cold weather starting.

My '90 745 with LH2.4/EKZ116 which is very temperamental starting on warm days or when left for 20 mins after driving (although I have the knack of starting it quickly) is currently starting 'on the button'. In fact it doesn't noticeably even turn over ... it just starts/fires-up after a second or so of turning the key!!

Bob

Check your fuel pressure at rest , it should hold 3 bar for 20 minutes that is the volvo specification , if it leaks away that will be your problem , the heat from the engine vaporises the fuel in the line if there is no rest pressure ... It is starting on the button now as it is cold weather now so the heat rapidly vanishes when you turn off .

[bob12]

Originally Posted by Clan

Check your fuel pressure at rest , it should hold 3 bar for 20 minutes that is the volvo specification , if it leaks away that will be your problem , the heat from the engine vaporises the fuel in the line if there is no rest pressure ... It is starting on the button now as it is cold weather now so the heat rapidly vanishes when you turn off .

Thanks Clan - when the Summer comes I shall re-visit fuel pressure at the rail/valve and lines .... again!!! Bob

[Clan]

Originally Posted by bob12

Thanks Clan - when the Summer comes I shall re-visit fuel pressure at the rail/valve and lines .... again!!! Bob

it will either be leaking back through the one way valve in the end of the pump or less likely through the pressure control valve on the fuel rail back to the tank ..

[bob12]

Thanks again .... well, I replaced the pv on the rail years ago to no effect ..... BUT I have never thought of or considered the/a valve at the pump in the cradle slung underneath! Got a spare one of those!! Thanks ... Summertime awaits. Bob