Botus: your response to the effect that all the vehicles you've worked on have constant fuel pressure illustrates the general lack of understanding of BMW's recent bike fueling systems. No offense intended I hope to assure you. All discussion is welcomed. In this way we hope to separate the wheat from the chaff.
Allow me to get on the soapbox a bit, there are three general types of electronic injection schemes that I know of.
First, Manifold Absolute Pressure (MAP) systems typified by Bosch D-Jetronic. In the early 1970s, BMW (3.0CSi) Volkswagen (Type 3 and 4 IIRC), Volvo (164 models) and others used an aneroid, or pressure sensor, to determine engine load by comparing the difference between ambient pressure with that in the intake manifold. Megasquirt and Microsquirt do that now, with supposedly greater accuracy. The ECU of those days were, simply put, transformers.
Second, MAF systems where there is a vane, hot wire or hot film device in the intake and essentially, movement or cooling of that hot element (or the current required to keep it hot), effectively measures the mass of incoming air. Bosch L-Jetronic exemplified this in the early 1980s with BMW. My two 1985 BMW K100RS bikes used L-Jet. In particular, L-Jet relied on a device that ensured the fuel pressure was in a constant ratio with local barometric pressure, and thus injector pulse duration computation was simplified. Vehicles using this system control fuel rail pressure using intake manifold pressure via a hose to a fuel pressure regulator mounted on the fuel rail, and controlling the quantity of fuel returned to the tank. Fuel pressure varies inversely with intake manifold vacuum. We all remember ads for "rising rate fuel pressure regulators" being a common item for hopping up an injected car. Most current EFI systems are some sort of MAF-based arrangement with digital ECU's. Oxygen sensors provide feedback to closely regulate fuel trim.
Third, Alpha-N systems use a potentiometer on the throttle valve and infer the mass of incoming air from the deflection angle of the throttle butterfly, plus temperature and ambient pressure, to determine incoming air mass. Here we are with BMW bikes since apparently 2004 as I infer from the list of bikes that use the same BMSKP ECU. Alpha-N systems use stored maps relating a throttle deflection angle and barometric pressure and a few other sensors (incoming air temperature, coolant temperature, oxygen sensor voltage, etc.) to determine injector duration. Until discovering that recent Beemers used it, I had no idea who did.
All that said, fuel pressure in alpha-n systems CAN vary, and run well, if the fueling map(s) are calibrated for it. Case in point: K1300 engines.
Now question 1: how DOES the fuel pressure vary, and how is it controlled? Again, the suspects are the fuel pump and fuel pump controller on the front of the tank, fuel pressure sensor on the injection rail, and the temp and pressure sensors mounted on the airbox. Question 2: what are the failure modes of these, and how are they manifested when they occur? Answers will point to the fault, and from there it's up to me to dive in with hammer and tongs, wrench, screwdrivers and multimeter.
I are engineer, so as suggested above, possibly I'm trying too hard. It may well be that the problem is simple and nevertheless, knowing Beemer prices, expensive. But it's me to want to be informed. This is how I "own" my stuff - by understanding the principles of operation and thus how to assess/diagnose problems when they occur. The wife just wants a comfy ride in her Benz, but I gotta know what's going on inside to enable that. If you've read "Shop Class as Soul Craft" by Matt Crawford (and BTW no mean Airhead mechanic) the philosophical ramifications of this are presented and discussed.
Anyway, it's a bit late here (10 p.m.) and in 10 hours my bike is supposed to be towed to the dealer for professional diagnosis. I will certainly post the results (and cost) of that episode. I'll sign off with a quote from "Shop Class...": “What sort of personality does one need to have, as a twenty-first-century mechanic, to tolerate the layers of electronic bullshit that get piled on top of machines?”
― Matthew B. Crawford, Shop Class as Soulcraft: An Inquiry Into the Value of Work
Last edited by Honolulu; Jun 13th, 2019 at 3:52 am.