2003-2011 [Subscribe to Daily Digest] |
On my '07 I had a few similar instances (nothing I would call "banging" but rough start with short period of off balance, rough running, on one occaision only there was initial puff of blue smoke). This only occurred when I ran the car cold for only a minute or so and shutdown for the night (i.e., moving it from one parking space to another). Problem seemed to be solved by the '07 recall/software tweek. That notice was vague enough that its hard to tell exactly what was done and while it only covered '07 models it didn't rule out the possibility that a similar problem could exist in earlier models. A dlr would presumably have access to that top secret information and might be able to figure out if related.
The "out of balance" sounds like a cylinder is not firing. As already mentioned, blue smoke is oil and perhaps running cold briefly when engine metal is not fully "heat expanded" allows some oil to seep into the chamber on cold shutdown whereas if fully warmed the same oil would drain away from the now tight spaces before cool down and loosening of gaps between mating surfaces.
Also, (just complete speculation here) if there was gas sitting on top of the pistons all night perhaps it dissolved/loosened some accumulated oil deposits and those ignited on startup.
Keep in mind that electicity flows slower in cold temps and while I would expect all cylinders to get power at the same time regardless of temp perhaps one didn't get the juice in time as you cranked the car, a cylinder flooded, and it took a while to recover.
Also (more speculation) if the engine "pulls" a mis-fired piston down the cylinder instead of it being "pushed" from normal ignition above the piston -- maybe the different forces involved would allow some unusual rocking of piston and temporary loss of normal oil ring control thereby introducing a bit of oil from below into the chamber until it fires. A cold cylinder and piston will have "looser" fit and allow more rocking.
Whatever the reason, it appears some unique "off normal" condition allows a bit of oil into the chamber (which will also interfere with smooth running) but it doesn't seem like the normal "oil control" problems because AFAIK those are rather consistent and only tend to get worse over time (i.e., valve seals, rings, gaskets etc).
Because its not good for any engine to be run briefly w/o full warmup, its a good practice anyway to avoid lots of short trips, particularly in the cold, and engine seems happier when shut down warm vs cold. If you only have the problem after short trips then the easy fix is to avoid the short trips when its really cold. Of course, any time you see blue smoke its a good idea to monitor it to see if its a one time thing or whether its regular and potentially the more common wear related oil control issues.
Thus, the behvior has been seen before, by myself, and others...so it could be a "feature" but that doesn't rule out the possibility of a specific problem w/ your car.
Pat
'07 2.0T SS
'87 900T (RIP)
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