[Diy_efi] Frustrating fuel delivery problem!

[Brian McMahan]

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Brian Mcmahan=20
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-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]On
Behalf Of charlesmorris at direcway.com
Sent: Monday, January 17, 2005 2:08 PM
To: diy_efi at diy-efi.org
Subject: [Diy_efi] Frustrating fuel delivery problem!


This EFI system is driving me crazy. I can't fix it and don't know why. =
Any help greatly appreciated.

Car is an '81 BMW 745i (factory intercooled turbo 3.2 liter, Bosch =
L-Jetronic system, six 270 cc/min @ 36 psi injectors) that has been =
extensively modified. The engine is now 3.5 liters and stock 7 psi boost =
level is now 15 psi. I added two large 600cc/min injectors mounted just =
upstream of the throttle, controlled by a homemade 8-bit microprocessor.

The fuel system details:
#8 power cable directly from battery to trunk for stereo, tapped under =
back seat for fuel pump feed, relay controlled by original fuel pump =
wiring which was way too small.
Bosch 10208 (large) pump to drive original injectors through #6 Teflon =
braided line
Original Bosch pump feeding the supplemental injectors through #4 Teflon =
line
#6 Teflon return line
Earl's "screen" prefilter in suction side of each pump
Earl's sintered bronze filter in pressure side of each line
Eliminated transfer pump. Made "Y" pickup with 1/2" copper tubing and =
original pickup "sock".
Stock tank 100 liter/26 gal.
Increased fuel pressure from 2.5 to 3 bar (43 psi).
Single "Fogger" nitrous system (20-40hp for quicker spool-up, fuel taken =
from main rail).

I used to drive this car daily, sometimes to "closed course only" 140+ =
speeds, and *never* had a fuel delivery problem. Then it sat in a barn =
for three years and everything steel under the trunk rusted out =
completely including the gas tank. I replaced it, unknowingly pinching =
the flex lines from the pickup to the suction filters nearly closed =
(more on this later) - but never drove the car further than the driveway =
until this year.

Once I started driving it again, it was obvious that there was a fuel =
delivery problem. It would not make more than a couple of lbs. boost and =
would start misfiring badly above 3500 rpm under any positive manifold =
pressure. It gradually got worse (can't floor it above 2000 rpm or it =
starts bucking) to the point where I started fixing it. The filters were =
plugged with rust and varnish. Still no good. Replaced big Bosch pump =
with an MSD 2255. No good. Finally found the pinched intake lines and =
replaced them with 1/2" aluminum hardline. STILL no good!

Added a fuel pressure gauge to the dash and consistently whenever it's =
bucking the fuel pressure is dropping like a rock and will go to zero if =
pedal held down far enough and long enough. Discovered the inline 40A =
fuse holder was defective (getting warm to the touch, 0.6 volt drop with =
the car idling and half-melted the solder on one end of the fuse). =
Jumpered it out. Resoldered original control relay ("main relay") =
terminals on its internal PC board, a known weak spot on these cars. =
Now, steady 13.05 volts under the back seat (18" 12ga. wire from there =
to the pair of pumps). Ground wire intact and clean, no drop to body. =
Volts steady regardless of pressure or how severe the bucking. Still not =
fixed!

Tried yet another pump (Walbro 255 LPH). Not fixed. No obvious pinch or =
kink anywhere in the lines from pickup (siphons freely when =
disconnected). Can blow through filters. Screens are visibly clear. =
Confirmed by measuring flow from return line, only has a few hundred cc =
in 30 sec. with engine off, 43 psi at the rail, not nearly the liter per =
20 seconds expected.

I can't think of anything else. The system is only a hose, pump, and =
wires, for crying out loud!
I would have thought that the transfer pump was required except that =
with two fuel pumps going, it wouldn't be big enough anyway and would be =
an obstruction. And it USED TO WORK. It does self-prime, by the way. =
Tank fuel level makes no difference. Additional injector rail is steady =
on 36 psi, haven't bothered to check that system's return rate yet since =
the main system isn't right.

WHAT else can I look for?? I'm pulling out my hair.

thanks
Charles


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