Good grief!! The weirdness never stops lately. Doesn’t really make sense to me either.
LLoyd
“Prediction is very difficult, especially if it’s about the future.”
Physicist Niels Bohr
ALL modern cars, for decades now, have had “evaporative emissions systems”, which means the entire fuel system is sealed, to prevent gas vapors from escaping to the outside world. Instead they are stored in an activated charcoal canister on the car. When the engine is started, intake manifold vacuum is used to pull the vapor from the charcoal, and feed it into the engine. And, when the engine is running, there will be a slight vacuum applied to the gas tank, to keep fumes from escaping while the car is running. So, there IS a path from the fuel tank, to the intake manifold. When the cap is not present, the EVAP system cannot maintain the desired vacuum in the fuel tank, but that doesn’t keep it from trying. The result is, effectively, a manifold vacuum leak, and that leak is “un-metered air” as far as the computer is concerned, which means it is entering the intake system, WITHOUT being metered by the MAF (Mass Air Flow meter). The engine runs lean as a result (the computer knows this because of the O2 sensors), so it has to pump in more fuel to compensate, to maintain proper mixture. Airflow at idle is small, so the extra aid introduced by the leaking filler cap is a very significant amount of the total airflow, hence the very high fuel trim values.
It’s a wonderful system when it works properly…
Regards,
Ray L.
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OK. I can imagine air getting into the "top"of the tank, but not far enough into the system to trigger a vacuum leak on the intake manifold side. But, then, I are just a rithmetician, ewe no.
“Prediction is very difficult, especially if it’s about the future.”
Physicist Niels Bohr
Only if the purge valve remained opened after the initial startup, IMHO, which it shouldn’t after a minute or two. The purge valve could enable quite a bit of air to enter through the charcoal cannister
Good morning,
One morning about a year and a half ago I was in the garage and I heard a noise from our Honda CRV, took me awhile to figure it out but it turns out that the car has a vacuum pump that periodically tests the integrity of the tank. My understanding is that this will trigger the check engine light… Just thought you’d like to know 
Cheers,
LLynn
I welcome our new microelectronic overlords.
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Lloyd,
Manifold vacuum is used to pull the vapors stored in the system (gas tank, charcoal canister, etc.) into the engine to be burned. So a leaky gas cap will absolutely create, in effect, an intake manifold leak. On ANY modern car, leaving the gas cap off WILL result in a Check Engine light. The mystery here is that the fault codes I got did NOT include any EVAP system faults, but just a couple of lean mixture faults. Had I gotten any EVAP fault, with or without the lean faults, I would have immediately looked at the gas cap, since I know the other parts of the EVAP system are working properly, as I’d recently serviced them.
Of course, the real problem is these cheap German cars just don’t hold up. I mean, this car is only 18-1/2 years old, with a mere 230K miles on it, and the gas cap is ALREADY failing? What a piece of junk! 
Regards,
Ray L.
Audis (at least the one owned by SWMBO) solved that problem with an auto-eject gas cap design. She would report the check engine light, and the fault was EVAP as you say. I sourced new caps on eBay so they never got old. Our next Audi had a little plastic thingy that kept the ejected cap from disappearing altogether. Same EVAP code, but the cap was right there to screw back on. Still have that car, but only 88K miles; guess the cap will fail in a while.
BTW Ray, you seemed to allude to one of those code readers that uses a phone and Bluetooth? How do you like it, and what brand is it?
Robert,
The tool I use most is OBDFusion, a ~$5 android app, available on the Android Play store. I use a VeePeak OBD bluetooth adaptor, ~$11 on Amazon. It is great for reading and clearing faults, but can also monitor dozens engine parameters, and has a nice feature that tells you whether your car will pass an emissions check, and if not, why. It’s a generic device, so does not give access to any manufacturer specific information, but is enough 90+% of the time, and really handy since you can keep an adaptor in each car, and just whip out your phone when you need it.
For more comprehensive coverage, I have a Foxwell NT510, which has downloadable modules that give access to a lot of OEM-specific information, and it allows you to see into ALL of the controllers on the car - engine, airbag, brakes, transmission, radio, HVAC, etc., etc… I currently have the BMW module, but they support most manufacturers, and the additional modules are only ~$60-70 each. For ~$150, including one OEM module, it is a bargain.
Regards,
Ray L.
Thanks Ray. Very useful information. I would need the version for IOS.
That Foxwell unit is nice, too. I currently have a pricey VAGCOM device for our Audi, but it requires Windows and my Windows laptop died. Plus I have nothing for the girls’ Mini and Subaru.
Can you replace a MINI (similar to BMW) battery with the Foxwell, or must you buy if from the dealer (like I just did)?
I use one called Bluedriver. About $100. The app is free. So far has worked on all of my cars, except the Etype of course. Toyota, Jeep, GM, and Ford. Scans the barcode VIN and pulls up all of the specific vehicle info, brings up all the service bulletins, data on what has been the most probable cause of the fault etc. will also provide real-time information on engine data parameters. Only problem is trying to read that while driving😎
I would guess it will do that, but I’d have to look at it to see, and I don’t have it here at the moment. The BMW module does include Minis. The Foxwell site does list all the functions it supports. I really cannot understand the need to tell the car you’be changed batteries. That just seems insane to me. And I’ve never had to do it on our BMWs.
Regards,
Ray L.
THIS is why cars we see on the road new today will not be driven when they are 30-40-50 years old.
MGAs, old Jags, flat-head Fords, Model As and Ts… Mad Max fodder.
Oh, they will be: take a look at any number of young folk, modding electronics-heavy cars, into working race cars.
A newer generation doesn’t find modern electrickery as daunting as is old farts!
Hi Ray -
Just getting back to this - thanks for posting up these details. I want to run by you what I found, to make sure I’m selecting the right parts before I pull the trigger:.
Coilpack - Motorcraft DG535 (e.g., here)
Ignition Control Module - Looks like the original Motorcraft part is discontinued/NLA. Would you consider either of these alternatives (this, or this) to be acceptable?
Pickup - Motorcraft P/N DY1014 (4F2Z-6C315-BA) (e.g., here)
Thanks again for your feedback.
Dave
Dave,
Those all look correct. The module I got from was a genuine Ford/Motorcraft part, but most places do carry the Standard Motor Parts module, which should be fine, notwithstanding the one I got being DOA. If you’re going to carry them as spares, I suggest you first install them and drive them enough to ensure they all work properly.
Regards,
Ray L.