Turning the ignition-on with sensors disconnected - a bad idea?

garrett.fell · #1 03-14-2014, 09:41 AM

So I am wrapping up my valve cover, front timing cover, valley pan and hose-change project. Before I put the intake and fuel injection back-on - I figured I'd pressure test the system to make sure everything is cool first.

From memory (when I changed a hose a couple years ago) - part of the procedure to refill the cooling system was to allow an electric pump to circulate fluid. This suggests I need to turn-on the ignition.

This just sounds like a bad idea - will it unleash the BMW demons and set all kinds of "conditions" in memory because it cannot see ANY of the sensors which are disconnected?

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racingbmwm3 · #2 03-14-2014, 10:43 AM

you'll get a check engine light. as long as you have a way to clear the faults when you are done don't worry about it. conditions only get set while the engine is running.

garrett.fell · #3 03-14-2014, 10:49 AM

I've got a standard OBD2 code reader (harborfreight top of the line from 3 years ago or so). It's allowed me to check and clear the generic codes I've had pop up over the years. Those were mostly misfires and the occasional "it took longer than expected for the coolant to warm-up". (Which usually
coincided with a sudden drop in outside temperature)

I was more worried about some other conditions that it might set that would require a BMW specific scanner

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racingbmwm3 · #4 03-14-2014, 11:05 AM

even while running, the adaptations don't start getting adjusted by the car until the the O2 sensors are at operating temperature and the system is in closed loop status. And then they are adaptation values, so they'll always change to keep the system at equilibrium. if you leave something super important unplugged you'll know right away because the car won't start or it will run very rough (limp mode). Then when you correct the problem, it will run normal, just with a fault code. To the DME it's the same as if that part failed.

Now, having said all that. You don't need to bleed the system to pressure test it. If it holds pressure and nothing is dripping, yay! If something is leaking, you've just spent a bunch of time bleeding the system and topping it off only to have to possibly open it back up again. Good thinking on checking the repairs before completely reassembling though. It is always lame having to dismantle everything after finding out a hidden hose clamp wasn't tightened all the way or a bolt was left out...

garrett.fell · #5 03-14-2014, 11:18 AM

Maybe I am thinking about this too much / too hard?

I was thinking that maybe the coolant wouldn't get
to all of the places it needed to go and that would make pressure testing the system more
difficult as it's be filled-up with air rather than coolant!

I am trying to avoid double-work if I can!!!

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racingbmwm3 · #6 03-14-2014, 11:33 AM

if the system is tight it will hold air, you are right about it being harder with just air as you have to pump more in there. pumping a bunch of air won't cause it to get trapped as you already have air in the system (unless you are working in a giant hypobaric chamber?) I'd do an initial check with just air to verify it holds some pressure, maybe up to 5 psi? If you have any major leaks, it won't ever build up pressure. Next I'd go ahead and fill up the system and test again. 10-15psi and watch the pressure and look for dripping. If it passes that test, go ahead and now bleed the system to get the rest of the air out. You can test one more time at this point if you like, but probably unnecessary. Your final test is really after the cooling system is warmed up.

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