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Hot Start Problem Solved (long)
Scott and All,
I finally found the cause of my hot start problems!!! Was out of town alot in
the last few months and haven't been able to do much on the Audi.
Upon hot start the engine required long cranking time and when it did start it
would idle smooth right at 800 rpm. I assumed since it did not act flooded it
was starving for fuel.
To verify this I wired up a manual switch into the cold start valve circuit.
When I gave it a little pulse during cranking it made the cranking times even
longer meaning I had too much fuel (leaking injectors? - they all are new).
Another test I did was start the engine cold and then shut it off right away
and let it sit the required 20-30 mins. It then started right up!!! O.K this
was driving me crazy, It was getting flooding only when the engine was hot
soaked. If it was injectors they should leak hot or cold. Could it be that
they can leak when hot but not cold? Maybe it was vapor forming in the
I then decided I need a better way to test the injectors for leakage (better
than taking them out and inspecting them for drips while messing with the air
plate - an ugly procedure at best). I rigged up a presure gage to the
injectors themselves. I used a junkyard fuel line fitting and tee to a gage.
This was not easy, on the first attempt I had too much fuel line and they
deliver such a small volumn of fuel it took forever for the engine to stop
missing from running on 4 injectors. On the second attempt I shortened up all
the lines as short as possible and within about 30 sec. the engine would be
running on all 5 injectors. I then tested all five injectors one at a time by
letting them pressure up and then monitor the pressures for 15-30 mins. after
the engine was shut off. I was expecting at least one to show the pressure
bleed off after 15 mins.
Here is what I found: All 5 had similar pressures, at idle I had 55psi, as
engine was revved up the pressure would go to 60-65psi (this agrees closely
with the air pressure test where they opened at 60psi), after engine was shut
off I had 47 psi., after 15 mins. most pressures went UP 2 psi. This took
most of the morning to do and the engine wasn't very hot by the last one. I
think with hot soak the injector pressures increase due to expansion of the
fluid. This is a key point.
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).
Now for the rest of the story: how did my residual press. get so high? When
replacing almost my whole fuel injection system a couple of months ago and
subsquently having low fuel pressure problems I modified the pressure
regulator spring trying to fix the low pressure problem. I stretched the
spring about 1/4 inch and that was still in there. Upon comparing it to one of
the 4 or 5 I now have in my grab bag of fuel injection parts I realized I did
With the proper spring in the regulator and system pressure set at 85psi (I
set it low because now I am paranoid about residual pressure) I have instant
starts, hot or cold.
Special thanks to QSHIPQ for his E-mail help over the last couple of months.
I will pay you that beer I owe you in Steamboat.