Unusual Idle RPM Dips

[croy]

Hey all, was hoping maybe someone would be able to shed some light on an issue i've been having recently. At first I noticed a general hesitation in the car when giving the car roughly half throttle especially in the 2-3k rpm range. This seemed like a spark plug issue to me so I ordered new oe bosh plugs to swap in. Not the most knowledgeable on plug conditions so I included a picture of one for context. Doing the service I did notice that 5/6 coil packs were bosh while 1 was a delphi. Also noticed a tiny bit of oil in the spark plug cylinders so a new valve cover gasket soon. Ordered a new bosh coil for consistency and tested the car in the meantime.

The initial hesitation issue seems to have been solved but now a rough idle in both cold and warm starts has begun. Even with the new bosh coil swapped in I still notice these strange dips in the rpm range. Included a picture of them I captured in a data log (note the 3 large spikes). No error codes and I've tried to reset throttle adaptations with no luck of fixing the rpm dips. Once diving I find no issues but the occasional similar rpm dip at stops. I've only driven the car maybe ~10 miles since so maybe just the car adapting to the new plugs? Any ideas are appreciated!

[b1aze]

Manual or auto?

[croy]

Originally Posted by b1aze

Manual or auto?

Auto, sorry should've added that

[b1aze]

Originally Posted by croy

Auto, sorry should've added that

Have you checked condition of the eccentric shaft sensor? It won’t throw a CEL when it fails but your redline will be capped at 6500 rpm IIRC.

[///Makis]

Did you just get the car? Or have you owned it for a while and are just noticing the idle fluctuations now?

It could be a lot of things.

The plugs don't look great, it looks like something is caked on and crystallizing off it, but it could just be the pic.

If your vcg is leaking like the person above me mentioned your eccentric shaft sensor gasket/grommet might be compromised as well and it could be a failing eccentric shaft sensor, you can lift the sensor up and off, I think you need to squeeze the sides to get it off, and see if there's any oil underneath the sensor.

Since you have a way to log data, check the short and long term air/fuel trims. Check the numbers at idle and then hold the gas for a few seconds at 2k rpm. The numbers should always be tending toward 0 but they'll cross over and under. If you see anything out of the ordinary like one bank isn't fluctuating over and under 0 or is constantly below or above. It could be a failing O2 Sensor. I don't think this is the case since you said it only happens at idle but it's a cheap thing to check.

Someone can chime in here but I think the n52 idles on three cylinders (I think it's 3-4-5 but probably wrong on that) but it's possible one of your ignition coils is on its way out. My cars was "stumbling" during idle every now and again, and also sometimes during moderate acceleration but it was almost random. The annoying part is I would scan the car constantly to figure out what it was but it never threw a code, one very cold day it stumbled and I had also floored it to get on the high way and I got a CEL and found out cylinder 4 or 5 was misfiring and I had already changed plugs within 5k miles so I changed all the coils and the stumble was gone.


When you changed your plugs did you notice any of the coils had a tear or cut on the outside rubber exposing the metal on the inside?

I had noticed on two occasions that I had misfires on cylinders with coils that had a small tear in the sides. I don't know for sure but I think when this happens the coil can't hold and store a charge and it discharges through the spark plug tubes instead of the spark plug (I don't know if this is the case as I don't know exactly how the coils work but it seemed peculiar to me that two of the coils that failed had been torn).


The only other thing I can think of that would cause an intermittent idle fluctuation only at idle could be a small vacuum leak.

What maintenance has been done on the car?

If you can take a video of the idle fluctuation that might help, otherwise try the above and see if it helps and report back

M

[croy]

Originally Posted by ///Makis

If your vcg is leaking like the person above me mentioned your eccentric shaft sensor gasket/grommet might be compromised as well and it could be a failing eccentric shaft sensor, you can lift the sensor up and off, I think you need to squeeze the sides to get it off, and see if there's any oil underneath the sensor.

Since you have a way to log data, check the short and long term air/fuel trims. Check the numbers at idle and then hold the gas for a few seconds at 2k rpm. The numbers should always be tending toward 0 but they'll cross over and under. If you see anything out of the ordinary like one bank isn't fluctuating over and under 0 or is constantly below or above. It could be a failing O2 Sensor. I don't think this is the case since you said it only happens at idle but it's a cheap thing to check.


M

Thanks for the replies guys. The car has been in the family for the last year roughly and has had basic maintenance but I don't know much about the previous history only that it's had one previous owner who seemed to take it to BMW for service. Not sure what is up with the plugs but will check on the condition of the new in a few months. I did check the eccentric shaft sensor gasket and it seems to still be good as well as the sensor. I did notice the hinge on one of the plugs seemed to not hold in the connector very well so I did order a new plug as a precaution. Since then however, I haven't been able to replicate the usual hesitation over the last few days, maybe I got lucky and its just the plug connector . Will keep monitoring to see if the issue resurfaces and update if I notice anything.

[bmw1racer]

I had a sort of a simlar idle problem... I read that if you're going to replace a coil pack, you might as well replace all of them and that's what I did. And since I was at it, I replaced the plugs as well.

Also, since I had the plastic cover off, I checked the connector to the eccentric shaft sensor and it had some oil in it, so I cleaned the connector and its mate with electrical contact cleaner and applied some dielectric grease.

Idle has been fine ever since (two months ago, knock on wood).

[///Makis]

Originally Posted by croy

Thanks for the replies guys. The car has been in the family for the last year roughly and has had basic maintenance but I don't know much about the previous history only that it's had one previous owner who seemed to take it to BMW for service. Not sure what is up with the plugs but will check on the condition of the new in a few months. I did check the eccentric shaft sensor gasket and it seems to still be good as well as the sensor. I did notice the hinge on one of the plugs seemed to not hold in the connector very well so I did order a new plug as a precaution. Since then however, I haven't been able to replicate the usual hesitation over the last few days, maybe I got lucky and its just the plug connector . Will keep monitoring to see if the issue resurfaces and update if I notice anything.

You mean the hinge on the coil pack / ignition coil?

Yeah one or two of mine don't operate correctly, sometimes they don't spit the connector out when you lift the hinge or don't pull it in when you close it down.

You can just use your other hand to assist it. So long as it is seated in on the coil pack with the hinge down and doesn't move around or have a lot of play, I think it should be fine.


Does your car always idle in the 800s? Or was your ac on?

Also which spark plug tube had the oil in it? Was it the one with seriously fouled spark plug?

[croy]

Originally Posted by ///Makis

Does your car always idle in the 800s? Or was your ac on?

Also which spark plug tube had the oil in it? Was it the one with seriously fouled spark plug?

Yeah the car had A/C on as I usually run with the AC. Just checked and without the AC on the car idles 650-670 rpm roughly.

I believe it was cylinders 3,4, and 5 while 1,2, and 6 were perfectly dry leading me to believe that it's the little gasket in between them that has started to fail. A bit concerning but all of the plugs had a similar fouling to them. I'll try to throw in a pic of the plugs in 1-6 order and what cylinder 5 looked like with the oil

[///Makis]

Originally Posted by croy

Yeah the car had A/C on as I usually run with the AC. Just checked and without the AC on the car idles 650-670 rpm roughly.

I believe it was cylinders 3,4, and 5 while 1,2, and 6 were perfectly dry leading me to believe that it's the little gasket in between them that has started to fail. A bit concerning but all of the plugs had a similar fouling to them. I'll try to throw in a pic of the plugs in 1-6 order and what cylinder 5 looked like with the oil

Yeah it's time to change the vcg and spark plug gaskets and ESS gasket and valvetronic motor gasket.

I think the new spark plugs are made my NGK not Bosch, so I'm wondering how old those are.

Maybe it's the pic by cylinder 6 looks like it has a huge gap whereas cylinder 3 looks almost new with the exception of dirty.

[Almuliman]

Originally Posted by ///Makis

I think the new spark plugs are made my NGK not Bosch, so I'm wondering how old those are.

FWIW, I just got NGKs from FCP Euro as a match for my 128 – "BMW LZFR6AP-11GS".

[///Makis]

Originally Posted by Almuliman

FWIW, I just got NGKs from FCP Euro as a match for my 128 – "BMW LZFR6AP-11GS".

Did it help at all with your idle problem?

Mine started doing this idle problem I had, it's nothing bad like a failed plug or coil but it just jumps every now and again when stopped

[Almuliman]

Originally Posted by ///Makis

Did it help at all with your idle problem?

Mine started doing this idle problem I had, it's nothing bad like a failed plug or coil but it just jumps every now and again when stopped

No idle problem here I was just chiming in with what FCP has as a match for our cars. I'm doing plugs because there's no record of them having been done and my 128 is at almost 74K miles, so it's prevention and peace of mind.