Date: Tue, 22 Jun 2004 11:46:22 -0700
Reply-To: Michael Diehr <md03@XOCHI.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Michael Diehr <md03@XOCHI.COM>
Subject: Cold Idle Diagnosis (1.9L)
In-Reply-To: <20040622180524.622225B9AF@gollum.dreamhost.com>
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The Bentley manual has lots of good diagnostic procedures for
individual parts, but it seems rather lacking on the "big picture",
especially for cold/warm idle problems.
Here is my initial stab at an explanation / flowchart for diagnosing
cold/warm idle speed problems on a 1.9l motor. I'm not sure at all
about some of these items, so corrections are greatly appreciated.
On a 1.9L, idle speed is controlled by these systems:
1. The ECU richens the mixture at cold start for about 90 seconds.
(? does it richen the mixture or change the timing?)
(? does it happen at every start, or only when cold?)
2. The Auxiliary Air Regulator / Valve (AAR) is mostly open when
cold, which gives extra air to the engine. This valve is thermally
coupled to the engine block, and also has a 12V internal heater which
closes it in about 5-8 minutes. The air is drawn through the AFM, so
the overall mixture is not changed.
3. There are two more auxiliary air valves: One that opens when you
have the steering wheel all the way to the stop (to avoid RPM drops
due to power steering load), and one that opens when you have the Air
Conditioner turned on.
4. The Digital Idle Stabilizer (DIS) advances or retards the timing
to keep the RPMs at a constant RPM, but is only operational when the
throttle is closed (i.e. the throttle switch is closed).
5. The O2 sensor controls the mixture richness, but is not in the
circuit when cold (??? is this based on time or temperature -- and is
this perhaps the same as #1 above???)
6. The idle adjustment screw (varies the amount of air that gets in
with the throttle closed)
7. Therefore, the engine basically has 4 distinct modes:
A. First 90 seconds
B. First 5-8 minutes
C. Warm w/throttle closed
D. Warm w/throttle open
8. The general tune-up procedure is to set the timing and idle speed
with all of the circuits (except O2) disabled. You do this by:
A. Making sure the engine is warm (disabling the AAR)
B. Physically disconnecting the DIS
Diagnosis:
1. Many problems are caused by air/vacuum leaks. Do a really good
physical inspection of the engine. Replace all suspect vacuum hoses.
Take off the airbox, AFM and intake boot and make sure there are no
cuts or loose screws. Pay particular attention to the AAR tubing,
which is really only visible with the airbox and AFM removed.
2. With the engine warm, set the idle/timing as per Bentley.
3. If the timing/idle are set properly when warm, but you have
problems when cold, then pay particular attention to the Time Course
of the problems, i.e.
A. Problems in first 5-8 minutes, then suspect the AAR, especially
if the problem is better for the first 60-90 seconds after starting.
B. Problems in the first 60-90 seconds -- ECU problem?
4. Testing / repairing the AAR
Bentley suggests that you test the AAR by pinching the hose leading
from the AAR to the manifold. If you pinch in the first 5 minutes,
the idle should slow, and after 5 minutes there should be no change.
Beware that this is a gross test -- on my particular van, the AAR was
working (and would pass the pinch test) but it was not working well
enough. It was not opening enough when cold. Some (all?) AARs are
adjustable, but it's a pain in the butt to get to it.
5. Testing the DIS -- with a timing light, you should see the timing
jump around 5 degrees or so at idle. With the DIS out of circuit,
the timing should be much more steady.
6. Testing the O2 sensor -- unplug before cold start. If it runs
better when cold (but worse when hot) then suspect your O2 sensor or
wiring.
7. Test the throttle-closed switch. It should be closed when the
throttle is closed, but open with just the slightest bit of throttle.
8. (???) How to diagnose the Temp I and Temp II sensors???