[urq] URQ quirqiness

[urq]

... what is the symptom that led you to find that the OXS FV operation was a 
problem?  How does the operation of the vehicle change when this situation 
is present?

I know you say that you've tried 3 ECUs ... but given the intermittent 
nature of the problem that would still be my main suspicion.  What do the 
IST docs say?  I read that the Oxygen Sensor System uses the input from the 
OXS, WOT switch and coolant temperature sensor to control the duty cycle at 
the OXS Freq Valve ... seems to me you shouldn't need to go any further than 
these components.  Are you able to measure the duty cycle when the car is 
misbehaving?  If you see the duty cycle going to 100% it tells you that the 
OXS FV is probably not the problem ... if the duty cycle signal shows a 
reasonable value when it is misbehaving then you can be pretty certain that 
it *IS* the OXS Freq Valve.  As I read the text more closely I see that the 
stuff controlling the OXS FV is also aware of the engine RPM ... but it 
seems to me if you had a problem with Hall Sender or flywheel sensors you 
wouldn't be complaining about an OXS FV functionality problem.  You didn't 
say anything about checking grounding ... any time we're talking about 
issues relating to the control signals, that's got to be one of the first 
things to investigate.  Where is the ECU getting its ground?  It should be 
the wire attached to the back of the intake manifold.  That connection has 
bitten me more than once.  I also advise folks to check the big ground braid 
at the left engine mount.

I've always felt that it would be a good idea to have a couple bar graph 
displays set up on the dash to monitor the OXS voltage and Freq Valve duty 
cycle at the ECU pins while the engine is running.  Have you measured the 
voltage on the OXS at the ECU?  I've never heard of this happening, but I 
can imagine that the shielded wire the OXS signal goes to the ECU over could 
develop an intermittent short.

So, you've tried multiple ECUs on your car ... have all of the ECUs been 
demonstrated functional long enough to indicate that they are not a problem 
on some other car?  The OXS Freq Valve driver transistor is well known to be 
a common failure on the MAC-02 ... and I have BTDT with how intermittent the 
failure can be.  I had a situation where I drove my car to a meeting point 
when we were going to the Monterey Historics one year.  The car ran 
flawlessly to the meet point, but would not start to save my life for 20 
minutes or so when it was time to go.  Actually, the car would start, but 
any attempt to run off idle would cause the engine to die ... this was back 
before I had realized that it was the transistor in the ECU ... I knew the 
problem was the freq valve somewhere, but I had convinced myself that it was 
the valve or the wiring thereto which was the cause.

Boy Dennis, you've been through a lot with troubleshooting these cars ... I 
am in awe of your tenacity!  Many lesser folks would have just given up and 
moved on to MC engines ... or gotten rid of the car!

Steve B
San Jose, CA (USA)


> My 83 is still being a bit of a puzzle. That's the nicest way I can put 
> it.
> Symptoms: Frequency Valve operation comes and goes _very_  sporadically.
>
> Actions so far:
> New hall sender
> New O2 sensor
> new wire from ECU to FV
> 3 different FV
> 3 different (and verified) temp sensors
> 3 different flywheel sensors
> 3 different ECU's
> Air temp sensor readings within spec
> replaced fuel pump
>
> For my money, that pretty much covers the bases. No amount of wire 
> wiggling
> (including the back of the fuse panel) will re-produce or stop the effect.
> Where do I go from here?
>