Latest Mystery - Attention Hardy Boys...

[rreimund]

Ok, as you all know, my car is relatively new to me.. so I'm still discovering it's intricacies, etc. Well, here's a whopper for you all:

Last week, I pumped only my second tank of gas, and the car was running kind of rough, etc. as I was approaching empty (about quarter tank really... that's when I fill up)
Only I wasn't sure if our cars required premium, and didn't have a network connection to look it up (and too lazy to get the manual in the glovebox)..
So, I pumped premium last week, and as soon as I drove away from the gas station, it was running LOTS smoother.. I attributed it to pumping premium instead of regular..

Well, fast forward a week, and since last week the car has gotten progressively rougher running. (I attirbuted it to the exhaust leak that developed recently and to the colder weather) In any event, I pumped gas again as it was approaching a quarter tank. This time, since I'm so conditioned to pump cheap gas, I pumped 87 octane again.. (I also happened to drop in some fuel system cleaner, figured it couldn't hurt)..

So now I've got mostly regular gas in there.. but guess what.. ?
As soon as I drove away from the pumps, the car was LOTS smoother again!!
Also, I may have mentioned before on here that when you got on and off the gas, the car bucked a little bit? (Kangaroo'd) I thought it was because the guibo was shot... well, even THAT improved with the new tank of gas. I can be cruising in third, let's say.. driving down a straight road.. I let off the gas, and coast, then get on and quickly off the gas... before it used to buck a bit afterwards, like aftershocks from the quick accelerate/deccelerate .. but just now after filling up, I just slow down (engine braking) and speed up (when I slam it down in third), but there's no other residual movement.


So... any ideas? My potential guesses right now (and they're just that)..
1.) Something having to do with the fuel system... maybe the filter is dirty, or where the fuel pickup is for the pump it's clogged with crap, and when I pump clean fuel in it stirs it up, clearing the pickup temporarily.. I'll test this one out as I came back to the office for another hour or so, and I'll see if it's still smooth when I leave.

2.) Maybe it's the extra weight of the fuel in back. Not sure how this would work.. but if it's this, then when I leave tonight, the weight will be the same, so it should still drive smooth.. (I'm talking super smooth.. it was a joy to drive it). I may also try throwing some cinder blocks in the back after I go through some more gas..

All I can say is that the difference is dramatic and instantaneous, as soon as I drive away it's a noticable difference, and it's not the fuel octane.


Any other theories?

[rreimund]

Well, Seemed like it was still smooth when I went home after an hour.. We'll see how it is tomorrow.

Is there a precedent for dirty fuel tanks or problems with fuel pumps, etc.? Looks like it since there's a how-to on bmwe34.net

I guess I'll be tackling that. It says that if my fuel pump isn't pushing 45psi of fuel it should be replaced. How would I go about measuring that? Or would that best be left to a pro?

[ProZak]

Brand new to BMW also (wheee, a newbie thread!) but what you're describing sounds consistent with general crud (probably water from condensation and such) in the tank, manufacturer notwithstanding. I had an old Prelude that did exactly what you describe until all the crap I was pushing through the fuel pump seized it and stopped the car dead. Just my $.02.

-Zak-

[rreimund]

so what was involved in fixing that? Did you have to drop the tank and clean it out, replace the tank? have a specialist vacuum it out?

Oh yeah, and welcome to the board...

[Kalevera]

It's probably a fuel filter issue. The pump and pickup snap into a plastic lined well in the tank; I highly doubt that filling the tank would affect the amount of crud around the preliminary strainer.

Change the filter first. Get an OEM one. Install it correctly (arrow goes toward engine). Then suspect the pump.

Ideally, it'd be wise to get fuel pressure readings when the car runs rough versus smooth, but the diagnosis here seems like a no brainer to me. So, change the filter. I replaced the original one on a 525 earlier this week after doing the fuel pump....HMmmm, I wonder why the fuel pump died...

best, whit

[Incantation]

#1 87 octane is in no way enough for that engine

#2 where are you located? in the united states there are a lot of "dime-store" gas stations which buy the shxttiest gas .. and you can have major performance issues as a result..

#3 in cold weather it's best to keep your tank at least half full.. overnight you may get condensation in the tank which will cause rough idle/performance, etc

and.. if this is an ongoing thing try the fuel filter