Help troubleshooting a "EVAP large leak" (code 254) on a S52 BMW

Anybody familiar with how the emissions system does the test for evap leaks for S52 BMW's and how to troubleshoot?  Car is a 2000 Z3 MCoupe.

I replaced the gas cap, and I verified the purge valve and shutoff valves actuate when energized and that the charcoal canister isn't plugged.  I did as much of a visual inspection as I could, but man there is no access to the filler neck area or the top of the tank.  I have not yet looked at the fuel pump area through the access panel in the cabin.

I am mainly curious about how the system works.  Later versions used a leak detection pump to pressurize the system and measure leak down, but if the docs I'm finding on line are correct, my car doesn't have an LDP.  I'm guessing it uses engine vacuum to pull a vacuum on the system and measure leak down, but struggling to find any detailed info on it.

Thanks

Anybody dealt with this before?

Difficult as I find it to suggest going outside the GRM Hive for advice, try the General BMW Mechanical Help subforum over at Bimmerforums. There a bunch of pro techs in there on a daily basis - I'm sure one of them can assist.

In general:

Vehicles of that age, other than Chrysler, don't use a leak detection pump.  The vent solenoid will close, the purge solenoid will open for a period of time, pull a small vacuum in the tank, then the purge valve will close and the computer will watch how quickly the vacuum changes in the tank, based off the fuel tank pressure sensor. A large leak, if memory serves, is the amount of vacuum loss you would get with an over .080" hole, but that spec could be my lie.

Cracked hose, vent solenoid that doesn't seal, tank pressure sensor that doesn't read properly, hole in the cannister, loose or cracked gasket on the gas cap, etc.  A smoke machine is the answer for finding an evap leak most of the time.

02Pilot:  Thanks, I had not noticed that sub group on that forum.  I had posted in the z3 section and wasn't getting a lot of info applicable to the S52.

Streetwiseguy:  Yeah, that's my basic understanding too.  Mainly just trying to learn in detail how it works for my general knowledge and maybe help in troubleshooting.

My large evap leak on an M54 was a brittle hose right at the nipple of <er, some part>. It looked fine until I touched it and it fell apart. It was cracked on the backside where you couldn’t see it. 

Streetwiseguy said:

In general:

Vehicles of that age, other than Chrysler, don't use a leak detection pump.  The vent solenoid will close, the purge solenoid will open for a period of time, pull a small vacuum in the tank, then the purge valve will close and the computer will watch how quickly the vacuum changes in the tank, based off the fuel tank pressure sensor. A large leak, if memory serves, is the amount of vacuum loss you would get with an over .080" hole, but that spec could be my lie.

Cracked hose, vent solenoid that doesn't seal, tank pressure sensor that doesn't read properly, hole in the cannister, loose or cracked gasket on the gas cap, etc.  A smoke machine is the answer for finding an evap leak most of the time.

Basically, yeah all of this. Without a smoke machine you will be chasing your tail looking for an evap leak.

Yep. Very common on the E36. When my ex had her '97 M3 I was this >< close to buying a diagnostic smoke machine as that car would throw an EVAP code if you looked at it wrong.  Owning an R53, I'm sure I'll need one eventually. Fortunately, the prices have come down a bit in 15 years, but they still are not cheap.  

The cheap, kick-the-can-down-the-road option is to just get a PEAKE reader and reset it. Usually stays off for a week or two. My ex- could reset that code in seconds due to how much practice she got.

I misspoke. I wasnt suggesting buying a smoke machine for personal use, unless you find a good deal on craigslist or something. I was more meaning this is a time when paying a shop with the proper tools for diagnosis is worth it. When i worked at the ford dealership we had a $50 flat rate for diagnosis. It was a good way to cars in the door. If something became a major pain and it was going to be more than $50 we called and got approval. If you can find a shop with a similar policy I think it would be money well spent in this case.

In reply to gearheadmb :

Not disagreeing, but sometimes finding a good shop willing to do that diagnostic work can be a task. 

My temptation to buy the machine myself was born from frustration. When the code showed up, she used to take the car to her local BMW dealer who would basically blow it off saying the gas cap was bad and/or she was filling the gas tank with the car running (which she never does).  The car was still under the emissions warranty back then which probably meant BMW corporate wasn't paying out enough to cover the diagnostic and repair time for EVAP issues.

small update... I bought a more expensive ($200) machine from Autool that should arrive next week.  They seem like a legit company with a full line of products and not just something someone cobbled together from paint cans and gas grill regulators.  So hopefully that means they thought through making it safe for evap use.

I went ahead and bought the tool since I own an 18 yr old bmw and I mess around with cars enough that I will probably have other opportunities to use it.

Will report back once I have a chance to test it out.

They are really nice. They are great for finding engine vaccum leaks also.