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Discussion Starter #1 (Edited)
Does the ECU use a cam or crank sensor to detect misfires?

I was told one of my cylinders, 7, is at 100psi instead of being over 200. I was talking to a indy, a good guy, and he asked if I was getting any misfire codes. He said that if my compression was that low then cylinder 7 shouldn't be firing or at least I should be getting random P0307 codes and I'm not.

He thinks the dealer's compression checks were bad and one cylinder had carbon or something on the valve that would clear as soon as the engine was started. He said that if my cylinder 7 comp is that low, actually below around 140, that I should be getting P0307s. Since I'm not, he said bring it in and he'll test it.

Here's what's weird. My compression was checked last summer. The car sat since. Whereas last summer with compression ok I needed valve stem seals to correct the oil consumption, now I need a new engine because of the low compression in cylinder 7 coupled with the valve stem seals. So if the compression is ok, I should be back to stem seals which are covered 100% instead of 60% for a new engine.

So that's my question to the group, should I be seeing random P0307 codes with compression that low in one cylinder?

Jerry
 

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Nuclear engineer
02/2012 X5 35d M57Y CPO 99K miles NOKIAN WR G3 12K miles
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17,461 Posts
Misfire detection by crankshaft acceleration.

newTIS.info said:
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Home / BMW E71 X6 xDrive50i SC / Wiring Diagrams and Functional Description / Power train / Engine electronics, quality control valve (MSV) / Misfire detection /
Smooth-running value and misfiring detection

The smooth-running values of the individual cylinders are indicated for troubleshooting purposes.

The engine must run at idle speed for at least 3 minutes to ensure that the correct values are set. Smooth idle speed can only be evaluated with the engine running at idle speed (cold or hot). An indication of the combustion quality of individual cylinders can be obtained by evaluating the crankshaft acceleration, measured at the crankshaft position/rpm sensor. An individual cylinder with poor combustion can be detected very well in this way.

Random fluctuations of the individual cylinders can only be detected by close observation of the value. The values over all cylinders are zero in the engine with theoretically uniform combustion.

An increase in the smooth-running values may be caused by various factors (e.g. misfiring, secondary air, mixture deviations, faults in fuel supply, low compression). For this reason, exact intervention limits cannot be specified.

The rotational speed (engine speed) of the engine is measured at the incremental wheel with the aid of a hall-effect sensor. Moreover, the smooth running of the engine is also monitored (misfire detection) as a measure of the engine speed.

To detect misfiring, the increment gear is divided (by the control unit) into 3 segments corresponding to the ignition interval, i.e. 3 sparks per crankshaft turn on a 6-cylinder engine and 2 sparks in 2 segments on the 4-cylinder engine. Within the control unit, the periodic duration of the individual increment gear segments is measured and statistically evaluated. For each point on the characteristic map, the maximum permissible rough running values are stored as a function of engine speed, load and engine temperature.

If these values are exceeded within a certain number of combustion cycles, the cylinders detected as faulty are stored in the fault code memory.
https://www.newtis.info/tisv2/a/en/...y-control-valve-msv/misfire-detection/X5ZxygE
 

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Finally a believer
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Wow, always wondered how on earth the ECM could tell. Thx for posting!!!
 

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Nuclear engineer
02/2012 X5 35d M57Y CPO 99K miles NOKIAN WR G3 12K miles
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17,461 Posts
Wow, always wondered how on earth the ECM could tell. Thx for posting!!!
In other engines, a ‘microphone’ may be used, similar to a knock sensor.
 

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Discussion Starter #6
Doubting the dealer, I had another compression test done on that cylinder and they measured it at 110. So there is a confirmed compression problem in 7. This guy said he heard it popping a little when he brought it into the shop. I took it out and ran it hard up a hill a few times and I have to almost imagine a very, very slight misfire. That engine seems as strong as it always was and I never babied it so I have a long history of full throttle acceleration.

Having written that above, it will most likely post a code in heavy traffic if I drive it tomorrow as planned. It had been sitting for a few years with just random starts every few months while Bang was being faught.

Thanks for the info, pretty amazing control system.

Jerry
 

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Finally a believer
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"It had been sitting for a few years with just random starts every few months while Bang was being faught."

I gotta ask, wtf is Bang being fought?
 

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Nuclear engineer
02/2012 X5 35d M57Y CPO 99K miles NOKIAN WR G3 12K miles
Joined
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17,461 Posts
"It had been sitting for a few years with just random starts every few months while Bang was being faught."

I gotta ask, wtf is Bang being fought?
N63 oil consumption class action.
 

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Discussion Starter #9
I was driving this POS again today. It still turns heads and looks and runs like a beast. I can't feel any indication of a miss.

The guy that checked the compression for me said one of their techs wants to buy it and is offering my net tax savings once I get a donation estimate. Things have changed I know, for donating cars.

I wish I had room in my garage to pull it apart myself. I've done valve stem seals before, rebuilt heads, etc. I would have to move it into the garage though, which would be the toughest part of the rebuild - getting the kids stuff out of there.

Doing it myself, I would save about $5,500. So technically, I can get a new engine with unlimited 2yr warranty for $5,500. So that's the real debate here.

Thanks,

jerry
 
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