Re: Hot Start Problem (still very long)

["Scott Mo." <scottmo@teleport.com>]

Russ,

EXCELLANT Detective Work! 

I didn't follow your original post, but I will pipe in with a similar
finding and theory I had Back in April 96. Of course I never took the
time to verify this "Theory"....Very glad you did....

Shortly after buying  my used 89 200TQ with 75K miles, I also had a hot
start problem. After sitting for 30-60 min, the car would crank for 5-6
seconds before starting. The later MC engines (dual knock sensor) use
different injectors (Viton tipped, fine thread, that open at a higher
pressure and they also use a remote mounted System pressure regulator
similar to what is used in the CIS E III systems. They did away with the
regulator mounted inside the fuel distributor. Same fuel distributor is
used though...

According to my notes, I checked the system pressure and it was at the
high end of the spec ~6.5 bar (MC spec is shown as 5.8-6.6 bar) but CIS
E III is spec'ed to have a system pressure of 6.1 -6.5 bar. Which one is
correct is not clear, but in general the Bentley doesn't cover this fuel
system change that occurred around 11/88 on the 89 200TQ very well, if
at all.

The residual pressure measured immediately at shut off was found to be
3.3 bar (47.8 psi) BUT after 20 min. the pressure went UP to 4.1 bar
(59.4psi). YIKES!  After 1 hour it dropped back to 3.4 bar (49.3psi). I
had similar thoughts of the injectors opening up during this pressure
rise and "thought" possibly the warm up regulator was sticking and not
allowing the high pressure to disipate, but fitting another warm up reg.
unit had no effect on this pressure rise after shut off. I had the same
thought on the high system pressure causing the problem but because the
pressure regulator was tough to get at I didn't do any more testing.
Replacing the injectors fixed the problem for me 95% of the time.....,
but I would suspect that reducing the system pressure could have also
solved the problem with the old injectors in place. I don't know if the
later pressure regulators are adjustable, but I need to find out....

The CIS E-III specs the residual pressure to be a MAXIMUM of 3.7 bar
within 5 seconds of shut off, after 10min, it should be a MINIMUM of 3.5
bar but it uses the electrical diff/pressure regulator and not the warm
up reg. as on the early/late MC. As you indicated the MC spec is 3.2 bar
at shut off, 3.0 bar after 10min.

Regarding the injector opening specs...
 Now exactly what pressure the later fine thread injectors are spec'ed
to open at is a little vauqe. The Bentley shows they are designed to
open at (4.3 - 4.6 bar, 62-67psi)in the CIS E-III section but the test
section indicates the Viton tipped injector opening pressure changes
depending on how long they have been in contact with fuel. If they are
removed and tested within 3 hours they are spec'ed to open at 3.9bar. It
sure sounds like the 4.1 bar residual pressure I was seeing could very
well open up the injectors and puke fuel into the engine after shut off. 

Well enough rambling from me,
Thanks again for this additional insight...boy, it doesn't take much to
get me excited......where is that medication....
Cheers
Scott Mo.

Russ said:
> All along I knew my residual pressure was a little high at 55psi >shutoff and 50psi after 10 mins.  The spec is 46psi shutoff and >40-43psi after 10 mins. (I was about 10 psi high). I thought a little >high residual pressure should help vapor lock and hot starting.  IF THE >RESIDUAL PRESSURE IS TOO HIGH (higher than the injector leak or open >pressure) GUESS WHERE THE FUEL GOES WHEN THE INJECTOR PRESSURES >INCREASE DURING HOT SOAK  - YOU GUESSED IT, THE FUEL GOES OUT THE >INJECTORS!!!!!
>  This was precisely my problem - residual pressure was too high and >upon hot soak fuel was leaking out the injectors.  There is a fine line >between the proper residual pressure of 40-46psi and the pressure where >the injectors will open and/or leak (about 50psi in my case).