In a previous thread I was having issues with a combination surging and electrical gremlin issues probably caused by the starter & relay issues on my Silver 1990 K75. Turned out it was two separate issues. The starter/relay issue was dealt with. But the surging has continued to be an issue any time the bike gets low on fuel. Rapid acceleration or braking will cause the engine to momentarily drop power as fuel sloshing around in the tank will cause momentary loss of fuel pressure. Mostly this is felt in rapid acceleration as obviously in braking, the engine dying momentarily isn't felt.

Looking in the tank the fuel pump appears to be in place correctly. And the hoses are short runs to the fuel filter and fuel filter to the pipe leading to the fuel rail. So I'm wondering if there is some sort of baffling missing or something else not obvious. Has anyone else experienced similar issues?

thanks & best regards
J.

I had a problem with surging and cutting out with low fuel level on hot days in traffic. The fuel in the rail was vaporizing due to excess heat. Other Brick riders have encountered the same problem and fitted fuel coolers. Due to the fact that my fuel rail is exposed I fitted a insulated heat shield which has reduced the temperatures significantly and solved the problem. I also fitted insulation under the tank and insulated the fuel lines. A couple of weeks ago I also ran a hose from the metal return line in the tank over next to the fuel pump pick up.
Regards Martin.

'Low on fuel' is the clue. Under hard acceleration you're uncovering the pump input and there's not a lot you can do about that (except fill up ). Seriously though; once you start getting that you only have around 10 miles / 16 km range left before it strands you at the side of the road.

Excellent advice above.

When you fill the tank after the surging, how much fuel does it take to fill completely? The tank capacity is about 5.3 gallons. If you are putting in more than 4.5 gallons, you're probably running low enough to have fuel pickup issues and possibly fuel boiling not only in the rail, but in the tank as well.

jjefferies wrote:Has anyone else experienced similar issues? 

There's a BMW service bulletin in the portal that could explain the remedy for this condition although you haven't complained about noise, yet. This is it.

You might want to reduce the size of the images to get a clearer view.

Point-Seven-five wrote:Excellent advice above.

When you fill the tank after the surging, how much fuel does it take to fill completely?  The tank capacity is about 5.3 gallons.  If you are putting in more than 4.5 gallons, you're probably running low enough to have fuel pickup issues and possibly fuel boiling not only in the rail, but in the tank as well.

Good points all. I'll check the next time I get it surging.

Well, I near emptied the tank and then tested to see if Laitch's Service Bulletin might have a clue. The PDF says it is applicable to K75's 0253816> .
The ">" I'm interpreting to mean after . My machine is
Yr Model 1990 BMW Year sold 1991
VIN: WB1057400L0133119
So my guess is that this Service Bulletin doesn't apply?? Kindly correct me on that as I could be misinterpreting. And of course as I have several tanks and I'm not absolutely positive which  this tank came off of.

When I looked into the tank during operation, the return fuel was being shot up to the front and as near as I could tell was falling equally on the left (where the pump is ) and on the right.
The Service bulletin says: The back bone of the main frame and the fuel tank were redesigned starting with the above motorcycles. Everything inside of the fuel tank was changed. Some riders have voiced complaints of noise and of running out of fuel before the fuel tank is empty. This can occur because the fuel return aperture dumps the circulated fuel on the right hand side of the tank opposite where the fuel pump is located. 

Naturally the images are as Laitch indicates rather difficult to make out. So, is it essential that the fuel be shot over the front of the tank? I was wondering if this is some sort of cooling mechanism. MartinW mentioned that the fuel rail can get quite hot and also cause problems. I don't believe this tank has insulation between it and the engine. I've found others do.

An aside, I found the filler cap and its associated mount and the gasket ring had gotten quite dirty and for some reason - possibly evaporating fuel - had left deposits around the rubber gasket. So I cleaned the area with a wire brush and some carburetor cleaner. Is there anything to clean and make the rubber affected by the gasoline better?

jjeffries wrote:When I looked into the tank during operation, the return fuel was being shot up to the front and as near as I could tell was falling equally on the left (where the pump is ) and on the right

From your description, this modification would not seem to apply to your moto's VIN or your tank's style. It does apply to my 1995 tank's set up where fuel emerges from the return line mounted above the vapor chamber attached to the ceiling of the tank just forward of the tank's opening, then splashes into the tank slightly right of the tank's forward/rear centerline, when viewed from the rear of the opening.

The modification mentioned in Laitches bulletin is also different to my 92 K75s tank configuration. I didn't have to bend  the return line which was situated on the RHF near the filler neck. A bit of fiddling was required to get the hose onto the pipe and a bit more of a fiddle clamping it up, it was then ran over to the fuel pump. 

As I had the only K75s in our group at the time of temperature comparisons I had to do temperature comparisons with StandardK's naked K100. Although now we have another K75s now in our group and I can now do a comparison when summer hits. A comparison with Dan's naked K100 to my modified K75s revealed a temperature difference of 80C to my 40C. As stated mine has reflective insulation under the tank, insulated fuel lines and a fuel rail heat shield.https://www.google.com/url? Even on the hottest Qld days at a slow pace there is no sign of overheating. Unfortunately the heat shield is only applicable to Bricks with an exposed fuel rail.
Regards Martin.

jjefferies wrote:I don't believe this tank has insulation between it and the engine. I've found others do.
Is there anything to clean and make the rubber affected by the gasoline better?

You should insulate the bottom of the tank. It'll make warm-weather riding more comfortable by lessening the rate of heat picked up by the fuel then transferred to the tank wall to radiate into your body. Reflectix can used; it's sold at many hardware stores. Silicone sealant can be applied to it to stick it to the tank, but it really has nowhere to go once the tank is lowered. Adhesive backed insulation is also available. You could also install a johnny-blanket.

I use Scotchbrite abrasive pads instead of wire brushes to clean that area, and aluminum in general. They remove less aluminum and leave a smoother finish.

BMW was aware of the fuel vaporizing problem and did try to alleviate or the reduce the problem. See extract from Haynes workshop manual.
Regards Martin.

Point-Seven-five wrote:Excellent advice above.

When you fill the tank after the surging, how much fuel does it take to fill completely?  The tank capacity is about 5.3 gallons.  If you are putting in more than 4.5 gallons, you're probably running low enough to have fuel pickup issues and possibly fuel boiling not only in the rail, but in the tank as well.

TESTING, removed over a gallon of fuel today and went out riding to test this. After about 10-20 miles and a couple of stops I got the surging. Shortly thereafter I got the low fuel light. Traveled another 6 miles and filled. Tank took 4.4 gallons US. The tank does not have the filler baffle in place. Surging definitely happens before the low fuel light but the level at which the light comes on can be adjusted. So I would guesstimate that I had about 1 gallon left when the surging started. And it was surging only when accelerating from a dead stop. Possibly the situation which would give the most wave action.

Thinking perhaps to hit the problem with both solutions:
1.) direct return flow to the left by the pump.
2.) add insulation under the tank.

Any thing else possible?

I've never had the surging problem but I read that you should try to keep the fuel above the fuel pump if possible to help cool it so I just fill up at 100 miles and all seems good.I do have the factory insulation in place under the tank as well.

It looks like your low fuel light is about right. Mine all come on when there is about 4 liters of 1 gallon left in the tank. That you experience surging before the light is puzzling since with a gallon in the tank there should be no problems with fuel pickup unless the pump is sitting higher than it should in the mount.

I typically run my K75RT well into the low fuel light before stopping for fuel when touring. This usually puts me at a 4.5 to 4.8 gallon fill up. Like you I have removed the flapper and fill to the bottom of the well under the cap. My pump will make a lot of noise for the last 10-20 miles, but the only time I experienced any surging was when I ran totally out of fuel, and the surging only occurred for the last 10 or so miles, so I figure there was less than 1 liter of fuel left in the tank at that point.

One solution to the vaporizing fuel issue is to move the fuel pressure regulator to a spot where it isn't exposed to so much heat. There is a thread with a post and photos by Crazy Frog showing where he relocated his pressure regulator. Not only does his modification put the regulator and it's plumbing in a cooler spot, it makes all the fuel lines easily accessible for service. It's one of the projects on my to do list.