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E36 (1991 - 1999)
The E36 chassis 3-Series BMW was a huge hit among driving enthusiasts from the first moment the car hit the pavement. The E36 won numerous awards over the years it was produced and is still a favorite of many BMW enthusiasts to this day! -- View the E36 Wiki

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Old 02-21-2013, 02:17 PM
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drivinfaster drivinfaster is offline
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meh...went back and revisited some of my scribbles in my ep notes...and yes, an over advanced ignition timing *will* cause issues, but, from what i remember and notes refreshed, that this would be more of an issue with carb/tbi systems (since they do not usually have knock sensors, for one) and is usually going to be upwards of 40degrees or more total ignition advance.

this parameter (i would expect) would be under heavy throttle and not so much part throttle, but plausible.

from what i read, though, the vanos is about 12 degrees total variation (6+/6-). i do not know what the 'base timing' spec is, as it is not adjustable, and i do not see how that would play as a factor in this particular case anyway.

so if the vanos starts to adjust the cam timing @ 2500-3000rpm (depending upon which source your heart desires), even @ 1800rpm the vanos should still be @ 'base spec', and not opening, or, if i understood correctly from one site, that it is actually retarded @ idle and advances up through to advanced.

that being said, there should be additional factors pointing to vanos failure, such as poor fuel economy (again, not stated, but plausible), noisy operation (ditto), to something as simple as poor fuel quality.

without disconnecting the vanos oil supply (*not* a good idea at all), i know of no other way to confirm this. not sure if a vacuum measurement would be helpful or not, either.

also, i do agree that NOx is a bit higher than average, and maybe *that* right there is the 'smoking gun' of a failing-but-not-enough-to-trigger-alarm symptom.

been out of ep for so long that a lot of this stuff is getting too muddled in my memory banks.

still, i want to know what could be the issue with the CO2 and O2 falling off from their previous marks. it's an auto, so i do not think that the wrong gear suspicion i had was accurate, but something went very wrong somewhere...

also, i aparently got the air pump valve on my 96 confused for an egr apparently is an airpump valve, but takes an 'egr' gasket... silly me....

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Old 02-21-2013, 06:39 PM
granlund granlund is offline
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Electronic fuel injection engines usually run on , or near the ignition advance limit, so an incorrect cylinder gas mixture (from incorrect cam overlap) for a given rpm, throttle, temperature, can indeed cause knock.

The VANOS has 12 degrees of advance from base, so it's +12/0, not +6/-6. That is, the intake cam has 228 deg duration and closes at the same crank phase as the non-VANOS 240 deg cam when the VANOS is not activated. The VANOS intake cam will have 12 degrees less overlap than the non-VANOS. With 12 degrees VANOS advance, the intake valve will open at the same time as the non-VANOS engine's and thus have the same overlap.

The VANOS can, as I understand it, advance just above idle.

One way of testing the operation of the VANOS actuator is to disconnect the electric plug and force the low-overlap condition. Performance should be significantly reduced up to ~3500 rpm, and since the VANOS operation is not monitored, there will not be a Check Engine Light.
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Old 02-22-2013, 09:36 PM
ali.allo ali.allo is offline
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Mein Auto: E36 1996 328i convertible
waw I didn't see all those replies !

drivinfaster: yes the fuel system has been done at a shop and the car is auto trans.

The second test, on a sunny dry day, after fuel filter and injectors cleanup, was when I blast it on the highway so the cat was really warm but the engine as well so I guess it is why the Nox peak up!

I ve done the 3rd test (+ after spark plugs, and air filter) on a rainy day and the smog shop made me wait for about 1/4 hour before passing so I guess the cat was cold again (drove it just 3 miles, the tires were wet, exhaust was doing white smoke)

Do you think that could explain the increased o2 values ?
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Old 02-23-2013, 04:08 AM
granlund granlund is offline
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On an otherwise functioning car, NOx doesn't increase or decrease if you drive the car hard or gentle. It's not really correlated to engine coolant temperature, but instead combustion peak temperature which is on the order of 10x higher under a very short duration. Combustion peak temperature is a function of compression ratio, ignition advance and EGR (among others).

If the catalytic converter is failing, you should have better values at lower speed, or idle, than at higher speed. The reason is that less exhaust gasses pass through the converter and more of them have time to be converted. Since your HC and NOx go the other way, I don't believe you have this problem.

How handy are you fixing cars?
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