In Reply to: starting characteristics of my 3.5l posted by Scottie on January 30, 2001 at 14:10:01:
My current ride is a nice little 535ia with a great, tight engine. 120k miles. On previous 3.0 (e12) and 3.5l sixes I have owned the starting was characterised by a quick blip of the starter and then the motronic would take over and get the motor going.
My current 3.5l takes a 3 to 5 seconds to "catch" if I don't throttle it. If I don't give it any throttle, it will catch slowly, firing weakly, and then slowly it will increase in rpms until it is idling. Sometimes, with no throttle, it will stall on the 1st try. If I give it some throttle, it will catch quickly and then the motronic will keep it going. Sometimes the rpms will drop way low and then the motronic will juice it to get the rpms back up and keep things running.
It does this whether it is cold or hot.
Once running, the motor runs and idles great.
I'm not complaining if this is the characteristic of this particular motor, but I would like to fix it if I can. What could be causing this starting issue?
no vacuum leaks
good fuel with redline cleaner
otherwise in tip top shape
It sounds like the fuel pressure in the rail may be dropping off after sitting. Once you start cranking, the pressure comes back to a nominal value and stays there while running.
There have been posts in the past about going to Home Depot and obtaining the bits to make up a fuel pressure rig for about $15. Tap into the line between the fuel rail and the regulator with it engine running, note the pressure and then shutdown the engine. The pressure should stay up near the nominal value that was present when the engine was running. If it bleeds off, something is probably not right. Unfortunately I don't have a clue whether the fuel pump has an internal check valve or if there is one in the line between the pump and fuel rail.
Does anyone else have this answer???
Richard '83 533i