[NI_Volvo_Nut]

Originally Posted by AndrewBrown

The fuel injectors don't work like diesel ones which crack open under a typical injection pressure pulse of between 1000 to 2200 bar or 14503.77 psi to 31908.3 psi pressure in mechanical diesel pump arrangements, or opened electrically in modern electronic systems at the correct timing.

the K jetronic injector feed line pressure has got to be 3.3 bar or 47.8 psi to open the injectors and once the engine is running the injectors are open continuously spraying fuel onto the back of the inlet valves all the time.
The pressure at the injector will go up depending on the throttle position to around 4.5 bar.
If the injector line was leaking you would see fuel spraying out at 3.3 bar or more and probably catching fire quickly, you would certainly smell fuel if it was leaking at the very least.

its more likely that the injector is sticking closed reducing the available fuel to the cylinder, you would notice a misfire possibly intermittent, also a leaking injector would not cause a problem while driving because of the normal fuel delivery is greater than the leak and once the engine is stopped the control pressure is reduced and the line pressure drops in the injector lines below 3.3 bar and the injectors close if you suspect a leaking injector you can test them with jam jars and the offending injector will need replacing/cleaning.

Also the cylinder that may have a leaking injector will cause the idle CO reading to go up depending on how much extra fuel its giving out at idle and if its leaking into the cylinder that would be intermittent due to the inlet valve on that cylinder either being open or closed when the engine stops and letting the fuel run in until the line pressure has gone down this might cause starting issues but may not be noticeable due to the choke effect on starting depending on how bad the leak is, normally if there is a leaky injector its the cold start injector that leaks a dribble that evaporates before the next starting up.

The cold start injector is electrically opened on starting to richen the mix like pulling the choke on a carburettor by power supplied to it during the cranking of the starter motor

The fuel pressure fed to the metering head is more than enough to cope with a small pressure drop from the lines leaking hence the huge fuel leak you might see a huge fire ball to confirm a leak while driving

If your engine has low compression and needs oil to hold the compression in the cylinder your engine needs
to be rebuilt due to worn piston rings

Hope this helps with your 740's diagnosis

Andy

Thanks for this. The issue I have mainly occurs after a long drive or a period of heavy work - towing a trailer for example, so whatever it is - it is heat related. It does have a slight misfire which I have never gotten to the bottom of, compression test is good and wet test is as expected. Because it is so intermittent I have never had a opportunity to pull a plug lead to determine which cylinder is the cause, but really until I do that I can't go much further. I did change the injectors a few years ago with set from a car with no issues which made no difference. A few years ago I restored a 740 which was off a cylinder one day returning from work which turned out to be a split injector pipe which is were I developed my theory of how the injectors work. I need to do the PCV and I want to clean the injection / air metering unit while I'm down that far so will inspect joints and pipes at that time.