Porsche Repair: 84 Porsche 911 Fuel Injection problem, bosch fuel injection, bosch fuel injection systems

Question
Dave,

I recently purchased a 1984 911 Cab Carrerra. Problem is that when I get down to half a tank of fuel the car begins to choke like it's staving for fuel (Chokes, then accelerates, and sometimes sputters out.)I have already checked all fuel relays, jumpered the pump and watched it run, purged the lines some, and replace the oem style filter.
Problem two maybe related. After a hard rain the tempramental beast won't start. After I let it set a while it will become slightly more agreeable.
It's not that it doesn't look just fine setting in the driveway, but I would like to get in it and fire it up right away.  

Answer
Hi Jim,

Obviously your new baby doesn't like getting its feet wet.

First though, the coughing and sputtering.  If you said the fuel gauge was at 1/4 or so, it would be an easier diagnosis.  But, I suspect the culprit is the fuel return line.  One largely overlooked fact with the Bosch fuel injection systems is the rapidity with which the fuel pump sucks the tank dry.  Some of the fuel (hopefully a tiny amount) is actually used to propel the car, but the vast majority is returned to the tank via the return line...only to be sucked up again for another ride on the fuel line roller coaster.  The fuel moving backwards to your engine is pressurized and moving fast, the fuel moving forward is languishing a bit, taking in some of the sights.

When the tank is relatively low and you acknowledge a lot of fuel is "circling till called upon" there could be some instances where there is nothing for the pump to suck/push.  Hence, a stumble 'cause you are out of gas momentarily.  Now...that's the reason the return line is normally designed to be larger than the feed line.  But if there are any restrictions in the return line the problem can manifest itself when the tank appears to be "fuller".

I can imagine you could have an actual fuel pressure problem, but let's start with that return line.  Ideally, you can open it up (both ends) and clear it with compressed air.  Give that a try.

Dave

P.S.  As for that rainstorm problem, write back and talk to me about any other times there has been hard starting...how long had it been sitting, etc.  Or, I'll sell you some arid land in Arizona.

Dave