Re: UPdate: 1984 GTI Rabbit Ox Sensor Frequency valve and Control Unit Mystery

Time to attach your fuel pressure guage and check that system out as
per Bentley manual. Pay attention to residual pressure.

Also inspect all fuel injectors for drips when using pressure guage.

On Fri, 20 Jul 2007 18:35:50 -0700, "Tube Audio"
<murphysf@xxxxxxxxxxx> wrote:

Hello

Today I went to the junkyard and got a ox sensor control unit and the
problem still exists. I kinda doubt it is bad and failed in the same way.
I reran the tests for the hell of it and all the results I got yesterday in
my orignal post are the same. I put the original unit back in.


So I need to go back and troubleshoot, things that are running through my
mind, worn fuel dist, ignition dist, ignition control module?.

I am going to remove the ducting for the air intake and clean the throttle
body and the air flow plate. This way I can also inspect the hoses and
boots.










"Tube Audio" <murphysf@xxxxxxxxxxx> wrote in message
news:VFWni.11962$rL1.11711@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hello



Note this is long with lots of details but I believe it is important to
understand what is going on.



I posted a couple of months ago about my 1984 Gti Rabbit. I originally
thought that I had fuel pressure problems.



The car would start immediately and then die 2-3 seconds later, I could
restart it and then again it would die 2-3 seconds later. If I started it
with my foot on the gas pedal it would start and run at about 2000 rpm
however I would need to keep the rpm up for several minutes and then it
would be ok. I ruled out losing fuel pressure because of this situation
where it would take minutes of holding my foot down a bit and keeping the
rpm up at about 2000 rpm. If it was losing fuel pressure like when you
change a fuel filter, the idle stumbles for a few seconds or so upon
restarting but after about 10 seconds or so the system pressure is built
back up.



In my case it takes a couple of minutes of holding the gas pedal down at
2000 rpm before I can release it and then the car can idle by itself. I
also noticed that at this time after about two minutes when the car can
finally idle by itself the rpm is about 850-900 rpm. After driving it for
30 minutes on the freeway the idle is about 1200 rpm. This may be an
indication of the problem, the idle increases as the car comes up to temp.





I checked for vacuum leaks, changed the idle air bypass screw oring, fuel
filter, injector orings and injector holder inserts. I also had a spare
warmup regulator sitting around and I swapped that too. None of this made
a difference.



One it is warmed up it starts and runs fine. I only have this problem
after it sits for a while.



I've been driving with these conditions daily for a few months now,
freeway driving to and from work about 20 miles each way. A couple of
months ago when all this started to happen one day I started it up and the
car had all kinds of power like never before also all kinds of throttle
response, it was undeliverable, this lasted for about a week and then it
went back to normal running conditions, and then my problems started
shortly after. Was this a sign a something failing? The frequency valve
control unit?



Recently I started to think that, the car starts immediately and then
dies, so it must have fuel and then it runs out. I was thinking maybe the
frequency valve was not working upon a cold start up. So today after work
I dove into it and checked out the O2 sensor and the frequency valve.
(Note I do hear the frequency valve buzzing, however I need to check it in
the morning cold when the problem exists and listen to see if the valve
buzzes for the few seconds that it does run.)



So this is what I measured tonight when I got home, car was up to temp,
etc.



I hooked up the dwell meter to the test plug near the cold start valve to
measure the dwell of the frequency valve. Upon starting the car the dwell
went to 45 degrees steady and the car was hot. This is like open loop
conditions however the car was up to temp. After about 1-2 minutes the
dwell then went to 50 degrees and heald steady for 3 seconds then it went
back to 45 degrees and held steady for 2 seconds then it went to 50 and
swung between 50 and 54 degrees. I turned the car off and restarted it
and it did the same thing, went through this weird cycle where it looks
like it goes open loop and then up to 50 (solid) back to 45(solid) then
swings between 50-54 and pretty mush stays here swinging between 50-54. I
was surprised I thought that it would be either open loop or not, is there
some sequence it goes through even if the car is up to temp? The fan was
coming on.



So I took a voltmeter and measured the ox sensor voltage with it still
connected, and I measured .4-.6 Volts as the dwell swung between 50-54
degrees. I then disconnected the ox sensor and grounded the green wire
(the one that goes to the control unit), the dwell slowly increased
(within about 5 sec) went up to 74 degrees, stayed there for a few seconds
then slowly went down to 36 degrees, the ox sensor read .8 V. I read
that, upon grounding the wire to the control unit that this would simulate
a lean condition .8V - 1V and that the dwell should go to 75 degrees or
higher. My dwell increased but then cycled back down? Is this normal?



I read on a Volvo web site a CIS discussion that upon grounding the wire
the system would compensate for the lean / rich condition. As it
compensates the lean rich condition will eventually swing the other way
and the entire procedure will start all over again? Is this true, that if
you disconnect the green wire from the ox sensor and ground it that the
dwell will swing up and then back down? I've read previous posts on this
group stating that it would swing up to 75 degrees and stay there. Why
would the dwell go up and then come back down with the wire ground. I
would think that it would go to one extreme and stay there.



Lastly, if you are still with me, another odd thing I noticed. This is
the last test / experiment I did. The car was up to temp; I disconnected
the ox sensor temp switch. This switch opens above 77F and shorts below
68F. So I ohmed out the switch and it was open as expected therefore
leaving the connector off the switch made no difference. The car was
idling and the dwell was between 50-56 degrees. Just for the hell of it I
took a small piece of wire and I shorted out the connector. This would
simulate a below 68F water temp and put the system into open loop? Well
the wire shorted across the connector the dwell went up to 60 degrees and
stayed there. What give? Shouldn't it have gone open loop to 45 degrees?



Do I have a bad control unit?



I repeated all of the above tests 3 times and got the same results.



Thanks for you time.



Ideas?


.

Relevant Pages

  • Re: 1984 GTI Rabbit Ox Sensor Frequency valve and Control Unit Mystery
    ... Just do a fuel pressure test with a simple pressure gauge. ... The car would start immediately and then die 2-3 seconds later, ... however I would need to keep the rpm up for several minutes and then it ... I hooked up the dwell meter to the test plug near the cold start valve to ...
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