Bang! then won't start

[AndyF.]

Hi, hope you find this one interesting. 93 525i w/ m50 engine, 94,000 miles manual tranny. Been to my independent mechanic twice with this issue. Car sits for about an hour after being driven. Immediately after turning the key, loud bang accompanied by the smell of burning gunpowder. 1st time it happens, car is barely drivable-feels like its running on 4 or 5 cylinders and won't idle. Mechanic finds vacuum hose(located directly beneath intake runners) connected to crankcase ventilation has popped-off a plastic fitting. He says he can't figure why it backfired to pop the hose. He says it's not the crank position sensor and the car gives no fault codes. Mchanic keeps car for 4 days and can't reproduce the problem. Fast forward 2 months-same thing cept this time, won't start at all. Mechanic finds hose popped-off AND the plastic connector has broken as well. He calls all his Beemer contacts and no one has a clue. He eventually decides that since I'm running Mobil 1 synthetic, at least 1 of the hydraulic lifters "pumps down" after sitting a while, and allows the open valve to cause a backfire. Replaces the synth with dinosuar oil (20-50.)He keeps the car for 10 days and can't reproduce the prob. 2 months later(today) happens AGAIN. This time I can see the hose is popped off the fitting. By removing oil filter cap I can reach the hose and press it back on the fitting. Problem solved.... until the next time! Any thoughts or suggestions would be helpful. Thanx

[Jon K]

My guess?

Leaky injectors and or valve being held open when not supposed to be.

You're blowing off the vacuum draw for the crank case by the idle control valve. I would check your injectors. You say that the car sits an hour or so then when you start it up it bangs. Definitely sounds like a leaky injector ontop of a hot valve... but I am not sure.

Be sure to check the intake manifold if this is happening frequently - it only takes a few backfires to melt/explode the plastic intake.

[ryan roopnarine]

you have no "problem". i've dealt with this before. the hose under the runners is connected to a <1 lb device called your idle control valve. there should be a rubber doughnut supporting the idle control valve. if there isn't, this will cause it to move, which will pull off the hose, and put strain on the connector (possibly breaking it, like it has for you). your car came equipped with hose clamps on the hose that keeps popping off. if there aren't any, it is because someone took them off. simply replace the broken fitting, put hose clamps on all of the idle control valve fittings, put one on the broken fitting's replacement (so 3 total), maybe replace the elbow that goes to the air intake boot (so that you have a snug fit). do all of this, and your problem will not re-occur. the fact that this is happening is not indicative of high/low crankcase pressure or any other "real" problem, your car came from the factory with hose clamps on those fittings. the fact that it hasn't started to blow the hoses off sooner is only a testament to your good luck.

[Jon K]

Ryan - the hose that is blowing off is not the idle control valve hose.

It's the 1/8" vacuum line coming off of the idle control valve connector (hard plastic, clips into the manifold, the ICV moving wouldnt blow this off, its adjacent to the valve). It's a backfire causing pressure in the area... the hose blowing off is the end, not the means.

[ryan roopnarine]

aahhhh, i guess i didn't read that well enough jon.

i had that hose pop off once, but that's only because i used american diameter hose on that fitting, and oil kept building up under there until it slipped off. i guess you have a pretty serious problem if it blows the fitting off.

[tnt525i]

this happened to me a while back, i tried everything to get to the bottom of it,,, guess what? my crank angle sensor was on its way out,,,,let me guess, you turn the engine, bang! it backfires,, you see and smell smoke under the hood and the hose that pops off is in the middle of the manifold, so you stick your finger in there while prying and getting burned by the oil canister to get the hose back on, and then while doing this it just pops back in right?
well i can definitely confirm that its the sensor,, cause one day it will just give up on you, like it did me (while waiting on a red light, how embarrasing) so my friend, if i were you, i'd buy a sensor and keep it in your trunk cause its gonna die soon
oh and btw do not put a hose clamp there, cause when it backfires again, it will burst the hose, its cheaper to put the hose back on than replace it, cause it will backfire again and again until the sensor eventually dies

[AndyF.]

Thanx for the replies- the hose in question is about 5/8" ID (about 15mm for you metric fans) and yes, the 2nd time it happened the plastic fitting broke from the violence of the backfire. I'll try the CPS 1st but I'm intrigued by the leaky injector theory too. By the way, I was about to replace the idle control valve cause the idle would surge when the car came to a stop. Don't know if it's related, just goin to add it to my shopping list.

[Jon K]

If you can find someone with a known working CPS that'd be a good idea to check. Also check the cam position sensor, but I would look at leaking injectors also.