friedell@harvard.ARPA (Mark Friedell) (11/05/85)
I'm trying to debug the fuel injection on my '78 Rabbit and I've discovered that I need about $10,000 in diagnostic equipment to do it. In particular, I'd like to get a reading of the control pressure between the control-pressure regulator and the fule distributor. The shop manual claims that this is a piece of cake -- just connect your special VW CIS control-pressure guage. And of course... to adjust the fuel mixture... just connect your home exhaust-gas analyzer. It seems that a control-pressure guage and an exhaust-gas analyzer are need to test just about any part of the fuel injection. Does anyone know how to rig a home-brew version of either of these essential diagnostic tools? P.S. My problem seems to be a bad control-pressure regulator; when cold, the car starts and runs fine, but when it warms up the control pressure seems to drop more than it should causing hard starting and a feeble idle.
jwb@mcnc.UUCP (Jack W. Buchanan Jr.) (11/06/85)
I have more experience than I wished in low cost debugging of this type
of injection system. As you implied, you HAVE to measure the pressure to
know what is going on. If you are willing to not measure the pressure to
the accuracy all the manuals say is necessary then the following is one way.
Find one of those cheap foot operated tire pumps with a dial type guage on
it. Unscrew the guage, connect it to a piece of fuel line and, using a
T fitting (I used a plastic one designed for vacuum lines-metal would surely
be better) insert it in the fuel line. The guage covers the right range
(I recall something like 40 or 50 psi). You can read it to about 2 psi and
the accuracy is probably 2 or 3 psi (the manuals say you need to know to 1
psi). In my case the mechanical regulator failed stuck open allowing full
(electric) fuel pump pressure (about 100 psi). Replacing the regulator fixed
the problem. The shop manual implied that failure usually resulted in low
pressure, which seems to be what you have. As you have no doubt found out,
the cheap fuel pressure gauges cover much lower pressures and are designed
for mechanical fuel pumps in non fuel-injected systems. My experience is on
a 280 Z, but the system is simile2to yours. Good luck!
Jack Buchanan
UNC-Chapel Hill
{decvax, akgua}!mcnc!jwb