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318i fires up quickly, wants to die, but works itself out.

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    318i fires up quickly, wants to die, but works itself out.

    hey everyone. After much searching and manual reading, I decided it was time to ask for a bit of help. Most of my older car experience comes from my 1986 diesel Merc and my friend's 22re equipped 4runner, but my partner acquired really nice 1984 318i about a year ago and I have been trying to work out all of the bugs. The ongoing issue we are having is that on a cold or warm start, the car fires up, idle shoots up to where I assume it should be, but when it comes back down after 5 seconds or so, it drops way too low. Most of the time it will stay running without any intervention, but its rough. after 10-15 seconds, the idle goes to 850 and the car drives fine after that. if you shut the car off and restart it immediately, even if the engine is still cool, it starts no problem. if you leave the car off for about 30 minutes even after its fully warmed up, there is still some issue starting.

    ive searched and searched for vacuum leaks and invent been able to find any.

    I suspected the cold start injector circuit, but feel like ive ruled it out. I bypassed the TTS on a cold start to make sure the injector was firing for the entire time I was cranking. Didn't seem to make a difference.

    Something I stumbled upon by accident while testing the cold start injector was that if I blipped the cold start injector with my power probe while the engine was struggling to stay running, The engine revs up and sounds very happy. I know that the injector ONLY is supposed to fire during cranking, but this gives me the idea that maybe im dealing with lean condition.

    I T'ed off the the cold start injector line with a pressure tester. I see no issues there.

    Lastly, although we didn't see the printout, it apparently passed California emissions last year before we bought it, so i'm guessing things are more or less okay while the car is at temperature.

    Thank you to anyone who can point me in the right director to solving this!

    1) Check and see if you have a 1-wire oxygen sensor. If you do, you have the early L-Jet system. Buy two coolant temperature sensors (one for the car and one for the glove box). The car will not run if the sensor goes out. You will now have a spare. The sensor is super key to fuel/air mixture and will partially fail depending on temps.
    2) Eradicate vacuum leaks - early L-Jet has absolutely no way to correct or adapt for unmetered air in the system. Running lean you say?
    3) Only use copper core spark plugs - the brain does not "understand" platinum in this case. I used to run NGK BP6ET triples back in the day.

    It is sort of useless to think about much else until these three items are known to be okay.
    Last edited by packratbimmer; 01-14-2022, 02:29 PM.