Long-term intermittent BMW engine start-up misfire diagnosed as an exhaust valve problem?

I have an intermittent start-up problem with my 2001 BMW Z-3, 3.0: cylinder 1 misfires on start-up for a few seconds or up to a few minutes under certain conditions.
The misfire conditions are consistent:
A) OBDII code P0313 (misfire detected with low fuel) or code PO1343 (cylinder 1 misfire with fuel cutoff). Both OBDII codes equate to BMW fault code 238.
B) after winter storage it misfires on start–up for 5-10 minutes (I’ve always added a fuel stabilizer before winter storage);
C) after a spike in humidity it misfires on start-up for a few seconds or up to a minute after a rain storm or;
D) If I don’t run the car for a 3-5 days (always stored in a garage) it misfires for a few seconds (just long enough to turn on the “check engine” light).

Otherwise, the car runs flawlessly –plenty of power – smooth as silk - no hesitation, but with the bloody check engine light on 50% of the time.
In lieu of black tape over the check engine indicator, I’ve taken the car alternately to both Minneapolis BMW dealers over the years (once if not twice a year), but the intermittent nature of the misfire and the fact that I store the car 5 months the a year has made it difficult to pin down the exact “root cause” of the problem. The dealership mechanics have swapped and/or replaced the cylinder 1 fuel injector, spark plug, cylinder coil and cable, kept it overnight and for days. The last dealership diagnosis was a defective cylinder 1 exhaust valve that could only be fixed with a $2800 valve job!
Yes, that’s right, pull the head and do a complete valve job - on all six cylinders. What am I missing here? What about the infamous BMW VANOS valve control system?
Thanks for your help and insights,
Tom in Minneapolis

I’d take a look at #1 spark plug compared to a couple others, preferably after it’s been sitting for awhile and would misfire.
If it’s wet with oil it’s a valve guide seal, wet with coolant it’s the head gasket.
Does this have one coil per plug? Try swapping coils.
Get a compression test.
I really doubt it’s a bad valve.

A leak-down test with #1 cylinder at TDC will verify if there’s a problem with the exhaust valve.

If compressed air is injected into the cylinder and air is heard coming out the exhaust pipe that verifies there’s a problem with the exhaust valve.


Lakeville, Mn

You said when it was damp out you get cylinder 1 fail. Either bad wire or coil on plug 1.

Here is an extensive troubleshooting list for cold-start misfires on the 5-Series BMW, which I believe shares the 3.0 liter M54 engine with your Z3.


The thread gives a list of 28 possible causes for a misfire, none of which is a bad valve. I highly doubt the “bad valve” diagnosis.

Can you describe exactly how the valve problem was diagnosed, and what other items were tested? Do you know if a vacuum gage was used? Sticky valves will show up as an unstable gage at idle, and this could easily cause your symptoms.

As to the cost, well, I hate to say it, but that’s part of owning a Z3. A valve job isn’t cheap on any car, a Z3 motor is even more expensive.
For one thing, you have 50% more valves than 4-bangers.
For another, the market isn’t replete with readily available aftermarket parts.
And a dealer is almost always more expensive than a private garage, although even at a private garage the Z3 will be more costly to repair than an econobox.

The diagnosis sounds like a very good possibility, but I would like to know more about the diagnosis that went into it.