Misfire on cylinder four, five and six

ERICKRAYOS

MEMBER
2004 BMW M3
3.2L
6 CYL
2WD
MANUAL
160,000 MILES

During cold start it will run okay with some dip here and there on the RPM but once it is warmed up misfires happens.

Replaced the coils, checked the plugs and they are good.

When the misfire is not happening the car drives fine but the moment it starts misfiring I get this smoke up my tail pipe, it kind of looks white but doesn't smell like coolant, it smells like fuel to me and others, plus I don't see any change on coolant level. (I am catless as well).

but whenever the misfire is not happening during cold start drive the white smoke is non existent besides the little smoke from condensation during early morning starts.

Another thing, when it starts misfiring if I turn off the car, it will have a very hard time turning back on, like cranking it to about ten seconds or so before it starts up again.
Checked voltage output on torque app when the car is on and it is at 14 volts.

What else could be the cause?

5 Replies

KASEKENNY

MECHANIC
18,907 POSTS

Hi,

We need to start with a compression check because your engine is a straight six which means it does not have the traditional separate banks. Because of the smoke and hard starting, I am thinking you have low compression on those three cylinders.

We need to eliminate that before moving to much further.

Here is a guide that walks you through how to do this. Let me know what the results are and we can go from there.

https://www.2carpros.com/articles/how-to-test-engine-compression

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Tuesday, June 4th, 2019 AT 9:01 PM

ERICKRAYOS

MEMBER
18 POSTS

The hard start happens after the misfire starts happening.

If I let the car sit for a bit after turning it off the car will start fine without misfire then starts misfiring again after five to ten minutes.

Someone from another website suggest it may be a bad pre o2 sensor on bank 2, since bank 2 is cylinder 4, 5, 6 and if the o2 sensor was bad it will give a misfire as well and hard starts if no start at all as it was mentioned there. Is this true as well? I have a extra o2 sensor laying around and will try installing that tomorrow.

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Tuesday, June 4th, 2019 AT 9:07 PM

KASEKENNY

MECHANIC
18,907 POSTS

I apologize, I missed the runs okay at first, part. I focused in on the smoke and hard starting.

If it runs okay at first and then starts missing that is normally a sensor issue. The fact that you have smoke and it smells more like fuel means it is the unburned fuel from the misfire. It may look white but more than likely it is a grayish color. This is fuel.

Based on all this, an o2 sensor would make sense because it is one of the only sensors that effects an entire bank. Also, the engine is not looking at o2 sensor data after a cold start. The PCM is only using the temperature sensors and baro to determine best parameters to run the engine until it gets to operating temperature. Then it starts looking at o2 sensor data to determine the amount of fuel to add to achieve the optimum air/fuel mixture based on how the exhaust is coming out. If the o2 sensor is incorrectly saying the mixture is lean, then the PCM will add more more fuel and this will cause a misfire.

If you have a scan tool that will confirm this. We just need to see what the o2 sensor is reading.

That is great that you may have another sensor but what I would suggest is swap it with the other upstream sensor. Then your misfire should move to 1,2,3. If it does then you know that is the issue. If not, we have something else.

https://www.2carpros.com/articles/engine-black-smoke

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Wednesday, June 5th, 2019 AT 7:51 AM

ERICKRAYOS

MEMBER
18 POSTS

I would switch the sensors up but unfortunately the sensors has unique plugs with them so they only fit in one slot so the plug for bank 2 pre catalytic converter sensor is different than the bank 1. I am going to switch the sensor later on today and see if that fixes it.

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Wednesday, June 5th, 2019 AT 8:28 AM

KASEKENNY