[200q20v] Hot starts

[C1J1Miller at aol.com]

In a message dated Fri, 8 Sep 2000  8:54:19 AM Eastern Daylight Time, Peter Schulz <peschulz at cisco.com> writes:

<< Steve:

My TQW started easier for about three weeks after the pump was replaced.
Now it's back to it's old ways, so I have to look elsewhere.  I'm going to 
clean all the ground wires for the ECU and see what happens...

Why? I was out of the country for a week and lent my ECU to Chris 
Miller.  So the car sat for a week.

I re-installed the ECU, and the car started on the first crank.

Go figure...

-Peter
 >>

Removing the ECU would clear any learned/adapted values, such as O2 sensor, and go with the programmed values, modified by barometric pressure, and by temperature from sensors.  

For the O2 sensor:
"The control system is adaptive; it is capable of 
replacing standard control values with modified values 
due to changes in the operating conditions.  For 
example:  the oxygen sensor in the exhaust manifold 
senses that the fuel mixture is too rich (too much 
fuel); the control unit changes the fuel mixture to a 
leaner value by shortening the injector opening time.  
If the mixture is still too rich and the control 
unit's rich limit is exceeded, the control unit will 
adapt to this condition and establish a new basic 
setting.  This new basic setting (pilot value) will
then be used in both open and closed loop engine 
operation.  This eliminates the need for periodic
CO adjustments. 

If the sensor should fail, the engine will run on the
adapted fuel mixture at the point of failure. "

There is also a temperature sensor for the coolant at the rear of the cylinder head:
"A coolant temperature sensor is located at the back 
of the cylinder head.  It is an NTC resistor. 

The sensor is used for ignition timing, injection 
timing during cold and hot starting, warm-up 
enrichment, and idle stabilization. 

If the coolant temperature sensor should fail, a 
substitute value is used based on a signal from the 
air temperature sensor: 
  
     Air temperature greater than 0° C (32° F), 
     the substitute value is 80° C (176° F) 
     Air temperature less than 0° C, the substitute
     value is the intake air temperature for three
     minutes, and then switched to 80° C. "

*I wonder if the carbon canister frequency valve is not staying closed, which adds fuel vapor to the intake manifold (over rich mixture)?

"Carbon Canister Frequency Valve (N80) and Carbon Canister 

Solenoid valve 034 133 517  "Vapor canister purge valve" 

Vapors from the fuel tank are collected in the carbon canister.  A frequency valve is used to regulate the
flow of fuel vapors that are drawn into the intake manifold from the carbon canister.  The carbon canister
is located on the right side of the car in front of the "A" pillar. 

The frequency valve is operated by the ECU.  The valve is controlled by a duty cycle.  The duty cycle
will vary depending on engine temperature, oxygen sensor signal, load and speed.  When the engine is
off, a check valve stops the flow of vapors from entering the engine  This keeps the fuel vapors from
entering the intake manifold and causing a rich mixture on restart. 

If power to the frequency valve is interupted or cut off completely when the engine is running, vacuum
from the engine will open the check valve to allow tank ventilation. "

A last thing to consider is the fuel pressure regulator on the fuel rail: "When the engine is switched off, the diaphram in the pressure regulator is completely closed due to spring
pressure.  This creates a holding pressure between the pressure regulator and the fuel pump. "

This doesn't seem likely in Peter's case, as the engine didn't run for a week; plenty of time to leak down pressure...

Thoughts, comments?  This seems to be a common malady as these cars age.
chris