Disabled in Dayton

[Dohn]

Have a stumper for the brain trust. My coupe has a 3.5 engine from an '87 735. A few months ago I experienced a severe engine stumble when getting on the highway, followed by backfiring and finally stalling out. Flatbedded to the shop, they diagnosed it as dead alternator and rebuilt mine. Also had to replace the depleted battery, as it would not take a charge. All was well for a few months, then I started it one day and it wouldn't hold an idle. Found a seized idle air screw and a faulty MAF sensor, which was replaced with a used one. The car ran fine. Then a month or so later, my fuel tank, which was dented when I bought the car, began to leak (on a trip to the BMW CCA Foundation). Once back home, I ordered a new tank and had it installed. On my next trip on the highway, when I decelerated off the highway and stopped, I heard a boom from the tank like you hit it with a padded stick. A glance at the tank showed a large concave dent in the bottom, apparently caused by suction. After asking on the forum, the suggestion was to check the vent tube, which we did, and discovered the vent tube on the new tank was clogged with paint. Cleaned it out, pressurized the tank to pop out the dent, and all seemed fine. Drove it to Mid-Ohio on Sunday, and as I left the highway and came to an intersection, it nearly stalled, then stumbled much like it had done before the alternator replacement. I was able to get up to speed, as long as I didn't try to do it quickly, but I could still induce a little stumble by accelerating from my cruising speed. Tank is once again booming, and the dent is back. Car is back at shop, and they are checking the vent tube all the way to the front.

Do all these things seem related, or have I just been having a run of bad (read expensive) luck? I was planning to drive this to the Genesis closing, but the chances seem fairly slim at the moment.

 

[Thomas76]

Roughly how many miles have you put on the car with newer engine in it? If quite a bit it rules out any issues you might be fighting from the swap.

 

[Dohn]

The engine was put in by the previous owner, but done at La Jolla Independent, so I'm pretty confident in it. Roughly 35,000 miles since the switch.

 

[Thomas76]

Well dang, I'm not familiar with the fuel system on the later 735. Is it possible the original dent was from the same issue you're seeing now?
Small chance the new tank has a different configuration like smaller vent line seems possible?
After this I run out of rabbit holes for ya.

 

[Bmachine]

Where does your tank vent line go? Does it travel all the way to the engine bay? Does it go to the carbon canister or somewhere else? We need @sfdon to confirm this but there is a small vent pipe you can install in the trunk to vent the tank directly out back into the air. Not ideal from an environmental point of view but at least you wont have negative pressure in the tank in case it was connected to the wrong place up front.

 

[Ohmess]

I still have my carbon canister system laying around here somewhere, but I run my car with a vent to the outside underneath the trunk in order to ensure that I don't create a vacuum in the gas tank, preventing fuel flow, as I drive. The Germans designed the Euro versions of our cars like this, and they don't hate the environment, so the fuel vapors exiting from the car must be minimal. (And I'm running Webers which require a certain amount of atmospheric air pressure and thus venting to operate).

Have your guy do this (but don't use the plastic hose from the carbon canister; you can make this up with some fuel hose, two clamps, a steel pipe and a grommet):

RealOEM.com - Online BMW Parts Catalog

BMW parts 2.5CS-3.0CSL 3.0CSi Metal fuel tank

www.realoem.com

 

[sfdon]

5 bucks

 

[Dohn]

Well dang, I'm not familiar with the fuel system on the later 735. Is it possible the original dent was from the same issue you're seeing now?
Small chance the new tank has a different configuration like smaller vent line seems possible?
After this I run out of rabbit holes for ya.

I had the same thought about the tank dent. Seems awfully coincidental that the original tank had a similar dent. It eventually weathered along a seam and started to leak, which is why I just mortgaged the house for a new one. I believe the vent line runs all the way to the front, but I haven't traced it. Is there a check valve anywhere along the line, or is it just an open 6mm fuel line? Seems so odd they would run it the length of the car and vent it under the battery.

I'll suggest the alternative vent routing to the shop and see if they can do that.

 

[Ohmess]

I had the same thought about the tank dent. Seems awfully coincidental that the original tank had a similar dent. It eventually weathered along a seam and started to leak, which is why I just mortgaged the house for a new one. I believe the vent line runs all the way to the front, but I haven't traced it. Is there a check valve anywhere along the line, or is it just an open 6mm fuel line? Seems so odd they would run it the length of the car and vent it under the battery.

I'll suggest the alternative vent routing to the shop and see if they can do that.

The idea was to capture the vapors in a charcoal canister near the air filter, and have them sucked into the engine intake air upon startup.

This way, they don't escape into the atmosphere.

I suspect your troubles arose when some portion of the system was removed and/or disconnected with the portion toward the rear of the car that connects to the fuel filler neck blocked off.

And if I did this, I'm pretty sure your mechanic can.

 

[sfdon]

Mind that the hose to the nipple on the fuel tank neck is special- 6mm instead of 8mm

#6 below

Metal fuel tank
No. Description Supp. Qty From Up To
Part Number Price Notes
03 Grommet 1 12/1968 11/1975
16121101014 $1.21

Fuel hose 6x11MM for 518, 520/4 all years of manufacturing

Fuel hose 6x11MM for 518, 520/4 all years of manufacturing

www.wallothnesch.com

05 Vent pipe 1 12/1968 11/1975
16114780100 $54.58
06 Fuel hose 6X11MM X 12/1968 11/1975
13311272750 $11.62

 

[Gransin]

Dohn, your problems certainly sounds like a vacuum issue.
I installed a brand new tank in my coupe as well, and while I didn't have a problem with vacuum (vent line goes out under the trunk) there seemed to have been some paint inside the tank which I didn't see when I installed it - so it slowly mixed with the fuel and clogged up filter and fuelpump and gave me fuel starvation problems. Just a thought.

 

[bavbob]

Do a dry run....1/4 tank of gas, leave gas cap off and go for a drive?
Now being from a 735, some say the in-tank fuel pump does nothing, others disagree. Do you have one? Is the filter ok?

 

[Dohn]

Update for all. Yes, when they installed the new tank originally I had the suction issue, and they found the vent on the tank was completely clogged with paint. Clearing it seemed to fix the problem, but then on a later drive I again experienced the bass drum sound, and the tank dented. Today they re-routed the vent line through the bottom of the trunk, but it seems that the effort involved in supplying fuel while simultaneously crushing the tank was too much for the fuel pump, so I need a new one. So I won't be doing any of the cruising and car show activities, as my 2007 Yukon XL might look a bit out of place, but I'm still going, dang it!