Rough idle due to fuel pump?

I have an odd situation: I have a 1985 528e with 145K miles on it. Runs like a champ. However, I noticed when I run the AC for any length of time, the engine will run rough at idle the NEXT time I drive the car and is rough to start...like it's not getting enough gas. If I put it in neutral and give it some gas, it'll keep kicking.

When this occurs, I also hear a hum coming from the gas tank.

I have two theories:

1) Most likely: Fuel pump is going on the fritz and this has nothing to do with the AC. I live in the desert southwest where temps regularly get in the 110s during the day. I've been through 3 fuel pumps in other cars as a result of the heat frying the units.

2) The onboard computer is adjusting the amount of fuel to compensate for the increased load on the engine when the AC is on. It doesn't "reset" the idle when the AC is off until the engine is turned and the car sits for a bit. This one is wacky.

Any thoughts?

John

Have you checked or replaced the auxiliary fuel pump inside the gas tank? because I think there are two fuel pumps in those cars.

No, haven't checked. How hard is it to get to the pump inside the tank?

you can reach it via a panel in the trunk floor on the right side under the carpet. It has 6 screws, unscrew it, the fuel pump with the fuel lines will be visible, but beware! you may discover a nasty rust, test it, if it works properly it should be pushing about 3-6 psi of fuel pressure. If the main pump continuosly fails its probably working too hard because the auxiiary pump is not sending it fuel.

Wonderful! I will take a look tomorrow and see what I can discover. My local BMW car club, unlike most others, is full of people with new cars and all they talk about are chip upgrades, etc. I joined to learned how to keep the old cars running, not talk about performance upgrades that cost more than my car! ;)

What tool do I need to pick up to test the fuel pressure coming from the auxilary pump?

I know what you mean, I was a member at DTM Power.net and they chatted about the same stuff, I got bored so I left. Not a lot of BMW owners work on their own cars, we're a rare species. I have a 84 528e, its a great car.

To check the fuel pressure, get a fuel pressure tester from Sears, or Autozone, they're like $30. Good luck!:smile:

Just a late update. I replaced the in-tank fuel pump last summer and haven't had a problem since. We'll see how it runs through this summer. My attention was focused on keeping an '86 Bronco running this past fall...until the SOB threw a rod two weeks ago and got towed to auction.

Man, the money I wasted on that thing could have completely revamped my 528!

John

webhak, how hard was it to replace the in tank fuel pump? Mt friend has a 1993 525i with a bad fuel pump. If you have a DIY, that'd be awesome.

It was very simple actually. I followed the directions in the Bently manual, which I no longer have since the car was totaled in August. :(

In my case, I lifted up the carpet in the trunk, removed the three screws or bolts on a circular metal plate that was near the passenger side wheel. This exposed the pump assembly. Unbolt that and lift it out. I had to remove the fuel lines, but that wasn't a big deal either. It took a little bit of jiggling to work the thing out, but it was a pretty simple job overall.

you wouldn't happen to know what the presure is on the main fuel pump by chance?

Sorry...haven't a clue. The Bently manual may have had that info. Rather than buy the guage and go through the trouble of testing the pressure, I just replaced the pump. It solved my problem.

The heat coming off the roads in the desert southwest is notorious for cooking in-tank fuel pumps.

What's up? New guy here. I'm having the same problem with my car; difference is that after driving my bmw and turning it off lets say to fill up, after turning it on again it dies repeatedly. I have to put it in neutral and rev it up to 2000 rpm for a minute then i can take off. What i hate is that it will also do it when i'm on bumper to bumper traffic and it leaves me in the middle with traffic behind me waiting for me to move.