Re: hard warm start 85 CGT

[quk@isham-research.demon.co.uk (Phil Payne)]

In message <f914432c.36d1be64@aol.com> DGraber460@aol.com writes:

> The car is an 85 CGT  recently turned 100000 miles, second owner. In the last
> four months it has gradually started harder when full warm and left for about
> 10-15 minutes. Cold- it starts fine although has been sputtering a little
> after first firing up. Hot start is fine. Power and acceleration are very
> good.

All of these warm start problems - and there have been over a dozen
discussed in the last month - are almost certainly down to residual
pressure leakdown.

Why does it start when it's hot?  There hasn't been enough time for the
residual pressure to leak away.

Why does it start when it's cold?  Because almost all of us on the list
are in the Northern Hemisphere, where it really _is_ cold right now.
The engine is starting (and running, for up to fifteen seconds) solely
on the fuel provided by the cold start valve.  Why does this work when
the other injectors don't?  Because it's operated electrically and needs
only a very little fuel pressure to provide a spray.

Why _WON'T_ the damn thing start when it's warm?  Because the cold start
valve either doesn't engage, or only sprays for a second or two.  It
takes the pump a fair while to build up pressure enough for the normal
injectors to operate.  Too long, usually, for the engine to 'catch'.

On cars that lose pressure over a period of around ten minutes, you can
often hear the spring in the fuel accumulator creaking as the fuel leaks
away back into the tank.  It's on the right side of the car, just level
with the rear door - turn off the engine, and get down on your knees.
The sound is a little like WWII 'submarine under extreme pressure' sound
effects.  It's even more dramatic if you leave the car for an hour and
get someone else to jumper the pump while you listen.

Where is it leaking?  Three ideas - control pressure regulator,
injectors, fuel pump check valve.  That's also roughly the right
sequence to check in.

The return line to the fuel tank is connected to the drain of the
control pressure regulator, so just disconnecting the return line (with
the engine off) will tell you if it's leaking.  There should normally be
neither pressure nor flow in this line with the engine off.  Blow out
any fuel in the line and hold it vertically - if it fills up again
within a minute - BINGO!

On those cars where the control pressure valve is integral to the
metering head, Audi do a little-known repair kit - 035 198 685.  BTDT
advice - pull the old one out with the system still pressurised - it
'power flushes' the piston seat.  It comes out in several bits - a
high precision plunger follows the big spring, so be ready for it.

Otherwise - pull out the injectors, unscrew the cold start valve from
the manifold, dry them off with paper towels, and jumper the fuel pump.
The one that gets wet at the tip is junk.

If you're then left with the fuel pump check valve - tough.  It's an
absolute _BITCH_ to get out of an old pump without destroying the
pump's terminals.  I have both a ground-down 17mm open-ended wrench and
a special pair of Vise-Grips - I still prefer to remove the entire
subassembly from the car to a bench.

--
 Phil Payne
 Phone: 0385 302803   Fax: 01536 723021
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