Quote:
Originally Posted by NI_Volvo_Nut
I have a 740 with similar symptoms however always starts. I suspect a leak on the injection pipes but in my 20 years of ownership I haven't checked them properly lol
TBF I only came up with the theory recently after actually sitting down and working the system out, it seems to work very similar to a diesel in that it needs pressure to make the injectors "pop" therefore a slight leak allows pressure to drop in the line thus not allowing that injector to fire until pressure is regained. MY other theory is a leaking injector which is washing the cylinder walls and taking away compression until the oil is replaced.
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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