[T3] '73 FI troubles, finally fixed!
john at carartbyjohn.com
Mon Oct 3 08:02:01 PDT 2016
Sounds like you will need to update your FI tech talk before the next Invasion.
----- Reply message -----
From: "Jim Adney" <jadney at VWType3.org>
To: <type3 at vwtype3.org>
Subject: [T3] '73 FI troubles, finally fixed!
Date: Sun, Oct 2, 2016 6:08 PM
For about the last year, my '73 FI AT Square has been running poorly.
The problem seemed to be that it would always flood when I tried to
start it, leaving me running, weakly, on just 2 cylinders. I could
eventually get it running on 4 if I could get it up to some speed
then turn the key off with my foot to the floor and the AT in 2nd,
hoping that the AT would drive the engine enough to clear out the
excess gas. This worked most of the time when I was home, and there
were nice hills to go down, but if it happened elsewhere, I was just
a rolling roadblock that couldn't get out of the way.
I checked everything in the ignition system: spark plugs, SP wires
and connectors, cap and rotor, CDI ignition. I used my timing light
to verify that spark was going to each cylinder. Everything was fine.
I checked the charging system for low voltage; it was good.
I adjusted the valves; they were fine.
This car already had one of my fuel primer switches, but those only
turn the pump ON. I needed a way to turn the pump OFF, so I figured
out a way to do that with a different switch, which has 3 positions:
ON, Normal, and OFF. Using the OFF position, I could let the fuel
pressure drift down until the flooded cylinders would start running
on their excess fuel, then turn the pump back ON. If I timed this
just right, I could get 4 cylinders going. This usually took 3-4
tries; not really a solution.
This car doesn't see regular use, so the gas mileage is almost never
good. It only sees a gas fill-up every few months, so the mileage has
been poor for years. For the last year it's been abysmal, but it
wasn't clear that this was because of some ongoing problem or just
because I wasted so much gas each time I went thru that startup
Eventually, I decided that I must have an injector that's sticking
open, so, last spring when I got the '69 out, I put the '73 away and
removed its injectors. Then I took them to a local shop and put them
on their injector tester/cleaner. To my astonishment, they were
perfect. No problem there.
Okay, now we're up to this past week. The injectors are okay, the
ignition is okay. What else could be wrong? What if the cold start
valve was sticking open? How could I test that? What if it was just a
bad temp sensor?
I pulled the brain, installed my Bosch FI tester, and tested all the
sensors. They were fine. I installed a pressure gauge and watched the
pressure fall as I used the tester to fire each injector. (This is
with the pump OFF, after pressurizing the system.) The pressure held
steady unless an injector was firing. If the cold start valve was
sticking open, the pressure would have continued to fall even when no
other injectors were firing.
So, the voltage supply is okay, the ignition is okay, the sensors are
okay, the injectors, including the cold start valve, are okay. Okay,
we're down to not many possibilities. I've never had a brain fail on
me, but I went downstairs and found a correct '73 048 brain and
plugged it in.
It started and ran almost perfectly. The less than perfect running
was probably due to excess gas from the other brain and the testing
process. I took it for a drive; it ran perfectly. I let the car cool
off for a few hours and tried starting again; it started easily. I
left it overnight and it started easily the next day.
So, today, I did one last confirming test: I put the original brain
back in. It flooded again. Reinstalled the replacement and it started
back up nicely. The original brain is clearly the problem, and
replacing it has solved the problem.
Then I went out and filled up the gas tank. I expect the gas mileage
to be better next time.
Okay, now I've seen my very first failed brain. My guess is that one
of the 2 transistors which drive the injectors in pairs has shorted
out, so it's always on. That would flood 2 cylinders when starting,
but it could run, albeit richly, once it got going.
This also brings up another of my long term projects: I want to
install 4 LEDs on my Bosch FI tester that light up when each injector
is fired. Most of the time this would just be a fun light show, but
if I have injectors that are being turned on full time, the LEDs
would make that obvious.
Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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