BMW e36 bogging stalling idle hunting

[Ahmadbila]

Hey guys,

My 94 320i e36 is having a very common issue that apparently is not so easy to diagnose. I ve been through several forums and have been flooded by the information but could not finalize on what is wrong. I don’t want to swap parts which don’t need to be swapped as I ll be selling the car few months down the line to get an e39 but I need to have it running properly right now.

So, heres the deal. The car runs fine when the engine is cold or is below the standard operating temperature. As soon as it gets fully warmed up, it starts to bog down and while coming to a stop, the RPMs drop so low that the engine stalls most of the times. Engine also hunts RPMs randomly. Keeps on bouncing between 400 and 1000 for upto 10-20 seconds before stabilizing.

I can also hear pre-detonation/knocking when the engine is warm under low RPMs when gas pedal is pressed. The issues kind of all go away above 3-3.5k RPM but below that the car is a mess to drive, specially in traffic.

A lot of sources point out to the engine temperature sensor, apparently there are two of them, one for the temp display and the other for actual DME data collection. I am not an expert and am unaware as to which one needs to be changed and where it is located. I did recently change the spark plugs though.

It appears that as soon as the engine warms up and goes into a closed loop, the issues start. Another notable thing is that if the engine is fully warmed and is bogging, if I turn it off for like 5-10 seconds and then start again, the issues see to go away for a few minutes before coming back.

So guys, please help the man out. Summer is coming soon and the car is gonna run very bad. I need to get it fixed coming Saturday.

Thanks a lot!

[bmwdirtracer]

The very first step for this issue is to have a smoke test of the intake / crankcase vent system done, using a professional smoke machine.

Until the intake system is proven to be intact, no further diagnosis is possible.

[Ahmadbila]

Thanks for point in that direction... i ll definitely have that checked out... however, if there was a leak, would it not affect the performance at all times and temperatures? Bcuz thats not the case here...

[323i E30]

Your air/fuel ratio will be messed up by unmetered air entering the mix, when you have a vacuum leak.
I'm really not the expert here, but I think your ECU will only start paying attention to the confusing readings this causes as soon as the engine reaches operating temp. That makes the difference.

I'd start by checking for visual cracks in vacuum lines, starting with that big elbow hose between the throttle body and the MAF.
If that thing is cracked, replace it, and then still do what Chris/bmwdirtracer advises.

[bmwdirtracer]

Intake leaks manifest most at idle and lower rpms. Depending a lot on the age of the car, they can be most noticeable when the car is in closed loop, or when it's in open loop.

On an early (pre-'96) BMW, the computer gives you little diagnostic assistance, although there are some decent diagnostic programs (like BMW's DIS, or an old SnapOn MT-9500 with BMW software), which can show viable live data. If you can access one of these, that can be helpful.

However, you've made a wise statement, about not wanting to just throw parts at the car, and that's why some testing is crucial. Your symptoms point very strongly at a major intake system leak, and the only way to positively confirm or deny that possibility is by doing a smoke test of the intake system, using a professional smoke machine.

Just for you to look at, here's a link to a smoke machine that's much like the one I use almost daily:
https://www.weselltools.com/red95-00...4aAggpEALw_wcB

I'm not suggesting that you buy one, but rather that you find a good BMW/Euro shop that owns one, to do a smoke test for you.

That's absolutely where I would begin, if you brought the car to my shop, with these symptoms.

[stvsxm]

the unmetered air can certainly do that and all the suggestions about hoses and cracks etc are right on but honestly the thing sounds lean to me.... what happens when the motor thinks its cold ? it generates a longer injector pulse and richens it up and if these things still have an AAV of some sort then it gives it a bit more air to compensate and in your case when it does that the car seems to run ok... the extra fuel sems to help. what happens when it gets warm..? the pulse shortens up so less fuel and the extra air goes away and if the fuel pressure is a bit low or the injectors are filthy then the car goes lean and you get the symptoms you describe... especially the detonation. its a horses not zebras thing... certainly look at all the simple and straight forward stuff first. 90 % of the time its something easy and right there staring you in the face... but if it were mine, I would throw a fuel pressure gage on it real quick and maybe cut up the fuel filter and look inside it just to sort of rule that area out. when you took the plugs out, what did they look like ?

[bmwdirtracer]

When the engine is still in open-loop, the computer operates on stored "maps" of fuel vs. rpms and temp. The mixture is not adjusted due to the readings of the MAF or the O2 sensor(s). Therefore, the DME is essentially ignoring how much air is passing the MAF, and is unable to read the O2 sensor mixtures, because on this old a car, the O2 sensor takes a while to warm up.

The fuel pressure is going to be the same, whether the engine is cold or warm. The fuel filter will be equally clogged, no matter the temp.

Most likely culprit here is a large intake/CCV system leak. A bad MAF is next in line, especially if the car has an oiled aftermarket air filter.

If an extra confirmation is desired, unplug the MAF. This will force the engine to remain in open loop, operating on too rich a mixture. The trouble with this technique is twofold: it makes people think they have a bad MAF, when in reality they have an intake leak, and (2) earlier cars often will not run very well with MAF unplugged.

I'd still insist that a smoke test id step one. Only after confirming that this system is intact are other tests reliable.

I agree fully that everything indicates a lean-running engine....which is exactly what unmetered air causes.

[MIKYZZ4]

Generally- older vehicles, operating in(UAE) high heat conditions = cracked or torn rubber intake boots/associated vacuum hoses.
I would start by checking these obvious components for cracks,
http://www.realoem.com/bmw/enUS/show...diagId=13_1710
if a professional smoke machine is not available in your locality.