[Diy_efi] Frustrating fuel delivery problem!

[charlesmorris at direcway.com]

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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