Hard start & rough idle when cold

[Bill Green]

'85 Audi 4000S Quattro...

I hooked up the PSI gauge to the fuel distributor, pressurized the system
(78psi with pump on and 44psi when off) and left it over night. Bentley
specifies that the psi not drop below 38psi within 10 minutes. It stayed
above 38psi for several hours falling to 22psi over night. Beyond the first
ten minutes I have no idea what the residual psi should be. 

I checked the purge valve to the charcoal canister and it's fine. 

I replaced the upper coolant sensor and though my dash gauge now works it
made no difference to how the engine ran. 

I swapped out the ECU with the spare that arrived yesterday but it made no
difference. 

While I had the ECU out I checked the electrical connections according to
the Bentley manual. All checked out except one - There should be a minimum
of 8 volts from the starter to the ECU when the starter is operated. I have
zero volts for some reason. Bentley says that indicates either a faulty
starter solenoid or a broken wire. What exactly that shot of voltage does I
do not know. I don't see any such wire on the schematic in Bentleys
(admittedly not the best). This will require some investigation!  

Many thanks to all who left suggestions! 

Bill :-) 

-----Original Message-----
From: Tony Hoffman [mailto:auditony at gmail.com] 
Sent: Wednesday, February 10, 2010 10:36 PM
To: Bill Green
Cc: audi at humanspeakers.com; audifans
Subject: Re: Hard start & rough idle when cold

The guage reads off the top temp sensor. The ECU reads off the bottom
one. If the bottom one is the one you replaced, that should take care
of that part. To check the guage, simply short those two connections
together at the temp sender. If the guage pegs, the wiring is good.

As mentioned, the OXS is ignored till the car is warm. I'm going to
add a bit of recent experience here. I dd an 86 4kq. Mine has the
opposite problem, running way too rich when warming up. (maybe we can
park them side by side, and mine will give yours the extra fuel, and
they will both run perfectly ;-)

I've also replaced the CTS, but not the plug wires. Mine did the same
before and after. However, when the guage clears the four small marks
at the bottom, it completely smooths out and runs near perfect. That's
obviously when the OXS comes into play. I still have some spall
playing to do with the timing and I need to do the wires, but it's
such an obvious difference (almost like you hit a switch) when it hits
that exact point on the guage that it's clearly a cold running/basic
settings problem. One thing I haven't done with this one yet is to
clean the DPR, and I'm going to do that soon. You may want to try that
as well.

Your CTS is fine, BTW, if the ohms reading is good. That's the only
thing that determines what signal it sends to the ECU. I've used
several different brands, but prefer the Bosch. That said, I have a
nockoff in my car now.

BTW, I also have an 85 that serves mostly as a track car, but is
mostly stock (some suspension and brake work). It also does the same
thing, crummy running till warm. It also runs rich. When I got it I
didi the CTS in it just in case, and it changed nothing on it when
cold. However, the 4-wire O2 did help it a bit when warm. I also have
another 86 that was recently wrecked, but it runs perfect the whole
time. Go figure :(

Tony


On Tue, Feb 9, 2010 at 8:12 PM, Bill Green <wgreen56 at tampabay.rr.com> wrote:
> P.S. I just double checked the temp sensor and thermal time switch as
well.
> The gauge on the dash has been reading way too low for quite a while now.
It
> reads off of the thermal time switch (according to the electrical
schematic)
> so maybe that is also what the ECU uses? That would add up to being the
> problem if that's the case. At any rate it needs to be changed obviously
so
> I'll change it and we'll see.
>
> Thanks!