[T3] '73 FI troubles, finally fixed!

john@carartbyjohn.com 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.

John Jaranson

----- 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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