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<P><FONT SIZE="+2" FACE="Charcoal" COLOR=""><B><I>Cold Start
Stalling and/or Idle Speed "Hunting" In 16V
Engines</I></B></FONT><FONT SIZE="+3" FACE="Charcoal" COLOR=""><B><I>

<HR>

</I></B></FONT><FONT FACE="Charcoal" COLOR=""><B><I>Symptoms:
</I></B><I>&nbsp;Car starts and runs but idle speed jumps to
1200-1400 rpms and then falls to very low levels and then swings back
high. On the low drops, the car may stall but immediately restart. If
you try to drive off, the car may hesitate or stall as soon as you
let off the gas. After a couple of minutes, the symptoms disappear
and the car usually runs fine thereafter with no more symptoms. You
may notice a high idle speed after warm up, setting around 1200-1400
rpms. There are usually no check engine light or LH codes set or
stored. Most common in LH 2.2 cars with Air Mass Meters ending in 005
of the Bosch number. These meters are identifiable by having a plug
for the CO adjustment beside the connector for the wiring
harness.</I></FONT></P>

<P><FONT FACE="Charcoal" COLOR="">This idle hunt at start up
deal is usually NOT the AIC valve. If you have a car that isn't doing
it and one that is, try swapping the air mass meters if they have the
same Bosch numbers and see if the problem doesn't go away. This is
assuming that the car has no vacuum leaks, has a clean AIC and
throttle plate and that the base idle is adjusted
properly.</FONT></P>

<P><FONT FACE="Charcoal" COLOR="">The ECU depends heavily on
the AMM until the oxygen sensor comes on line, taking a few minutes
to reach its operating temp of 600&. I believe the thermistor
in the AMM for ambient temperature is what causes the cold start
blues, but I have found no conclusive test for it on a bad meter.
Factory manuals list pin tests for power to the meter, a ground
check, a burn off function check, and continuity to the LH ECU pins,
but give no test that pinpoints this symptom. I have compared
readings on meters I know where causing the problem to ones that did
not have the problem. It seems that the feedback voltage on the bad
ones is close to 2.0 V DC while the good ones show 3.0 V DC, but I
cannot use this test for certainty. Sometimes a bad AMM will differ
from a good one only by 0.2V, and that's cutting it too close to call
just from a voltage reading.</FONT></P>

<CENTER><FONT SIZE="-1" FACE="Charcoal" COLOR=""><IMG SRC="../engine_folder/engine_images/amm.gif" WIDTH=244 HEIGHT=452 ALIGN=bottom></FONT></CENTER>

<P><FONT FACE="Charcoal" COLOR="">Substitution is a easy test.
If there are </FONT><FONT SIZE="+2" FACE="Charcoal" COLOR=""><B><I><U>NO</U></I></B>
</FONT><FONT FACE="Charcoal" COLOR="">&nbsp;vacuum leaks and
the ignition timing and base idle are good, the AMM usually fixes the
cold start hunters and stallers. After the meter gets worse, it will
run rich, lean, cause black smoke or spark knocking, or cause
hesitation and low power. Hard to distinguish between a bad AMM and
oxygen sensor when the car exhibits symptoms at operating temperature
without testing. I test when I have these problems more to eliminate
the oxygen sensor than to condemn the air mass meter. Before
beginning any tests for this problem, </FONT><FONT SIZE="+2" FACE="Charcoal" COLOR=""><B><I><U>check
ALL the basics first</U></I></B></FONT><FONT FACE="Charcoal" COLOR="">.
Make sure the fuel filter is reasonably new. Make sure you have good
fuel. Spark plug gap should be within specs. Check the secondary
ignition; plug wires, distributor cap, and rotor. Clean the throttle
plate and AIC valve shutter wheel.</FONT><FONT FACE="Charcoal" COLOR="">
</FONT><A HREF="adj_basic_idle_lh2.1.html"><FONT FACE="Charcoal" COLOR=""><B>Set
the base idle</B>.</FONT></A><FONT FACE="Charcoal" COLOR="">
Check the throttle position sensor at closed throttle. Check the
ignition timing. On a warm engine it should be 14 degrees before top
dead center (BTDC) on non turbos and 16 degrees BTDC on turbos at
idle (850 rpm).</FONT></P>

<P><FONT FACE="Charcoal" COLOR="">Look for any and all
possible vacuum leaks. Cracked vacuum lines are common on older cars.
On turbos, the boost can blow weak lines off their nipples. Check the
intake tubes for cracks in the rubber or in the plastic tubes of
turbos. Even the aluminum tubes on turbos can't be overlooked, the
overheat switch in the top radiator hose has clamps which can wear a
hole in the aluminum pipe from the turbo to the intercooler. If you
find this, loosen the clamp on the top hose and reposition it so it
has clearance from the tube. Epoxy will patch the damaged tube. A
piece of old radiator hose will make a good insulator on the tube to
prevent any future rubbing. Locate the vacuum line going to the
charcoal canister's vacuum actuator ( on top of the canister) and
make sure the actuator holds vacuum. This is a common point for a
leak. On turbos, check the</FONT><A HREF="../turbo_folder/hootervalve.htm"><FONT FACE="Charcoal" COLOR="">
</FONT><FONT FACE="Charcoal" COLOR="">turbo bypass
valve</FONT></A><FONT FACE="Charcoal" COLOR="">
</FONT><FONT FACE="Charcoal" COLOR="">(hooter valve). It lives
in the intake tube from the air mass meter (air filter) to the turbo.
Its vacuum line when disconnected from the intake nipple should hold
vacuum. If the valve doesn't hold vacuum, you may also be
experiencing stalling when decelerating after being in boost. Check
the vacuum advance on the distributor. Though this is ported vacuum,
I find that when the diaphragm there ruptures, the advance in the
distributor can release further and cause incorrect base timing.
</FONT></P>

<P><FONT FACE="Charcoal" COLOR="">An oxygen sensor will read
between .2 and .8 V DC at idle when the car is at operating temp (
after the cooling fan cycles a time or two) if all is well. It will
average just under .5V DC and should cross .5V DC at least 8X per
minute. You take these readings on its single wire connector with a
DVOM. Peel back the boot on the single wire connector where the
connection to the engine harness is and connect the positive lead of
the meter, but leave the sensor and the harness CONNECTED, just jump
the DVOM lead to it. Attach the negative lead of the DVOM to a good
ground, nothing better for this than the negative battery cable. If
you disconnect the single wire lead, the computer no longer receives
a signal from the sensor and falls back to a allowed voltage of .5V
for the sensor. This may lead you to believe from the readings out of
the sensor that the sensor is bad when it is not. I have been told at
least one after-market manual lists a test for the sensor calling for
unplugging it and testing straight off the disconnected lead. This
will give false readings and is not indicative of the actual
conditions when the sensor is plugged up and feeding back to the
computer the mixture status. It will usually give you a idea if the
sensor is alive at all, but will not reflect the state of the true
air/fuel mixture in the engine.</FONT></P>

<P><FONT FACE="Charcoal" COLOR="">Check the preheater circuit
to the sensor. Near the same place the single wire lead from the
sensor joins the harness, the preheater circuit for it does as well.
Disconnect it and with the engine running, check for battery voltage
across its leads from the harness side, not the sensor side. If there
is no voltage there, check the fuse for the sensor. On early 900s it
is in a inline fuse holder on the right side fender, near the
evaporator box. Later 900s have it in the fuse box. Early 9000s have
it under the left plastic cover near the windshield under the wiper
transmission. Later 9000s have it in the fuse box as well. The inline
fuses are the ceramic type. The fuses in the boxes are blade fuses.
You can check the heater circuit in the sensor by unplugging the two
wire lead to it and measuring across the pins on the sensor side of
the harness with an ohm meter. Look for continuity in the 8 ohm range
for a good circuit. If the preheater circuit is faulty, it may take
the sensor longer to reach its operating temperature and could cause
it to foul easier. If you have had head gasket problems where coolant
went into the exhaust, the silicates from the coolant could have
coated and contaminated the sensor. A engine that has run rich for a
long time may also have a contaminated sensor, fouled from the excess
fuel just like a spark plug. An engine that uses oil can foul the
sensor the same way. If the sensor becomes fouled, there is usually
no fix for it other than replacing it.</FONT></P>

<P><FONT FACE="Charcoal" COLOR="">If you are getting a reading
that fluctuates out of the sensor, pop off a vacuum line and see if
the reading out of the sensor changes, it should drop closer to .2V
DC. Revving the engine and then letting the throttle shut should
bring the reading up toward the .8V DC side. Removing and plugging
the vacuum line to the fuel pressure regulator will cause an increase
in the voltage as well as the engine is initially driven richer by
the increased fuel pressure until the ECU reacts to it and leans the
mix. If the sensor responds to these two tests, it is probably ok and
that leaves either the air mass meter or the ECU to blame, if you are
certain there are no vacuum leaks. If the problem seems to be nothing
more than an incorrect mixture, the AMM is more likely to be the
culprit than the ECU.</FONT></P>

<P><FONT FACE="Charcoal" COLOR="">&nbsp;</FONT><A HREF="22coadjust.PDF"><FONT FACE="Charcoal" COLOR="">PDF
File</FONT><FONT FACE="Charcoal" COLOR="">
</FONT></A><FONT FACE="Charcoal" COLOR="">of factory manual
pages describing 2.2 LH CO adjustment. Note that 380 Ohms is
</FONT><FONT SIZE="+2" FACE="Charcoal" COLOR=""><B><I><U>NOT</U></I></B></FONT><FONT FACE="Charcoal" COLOR="">&nbsp;the
preferred setting, but simply a baseline from where to begin to
adjust to the proper level. Without the proper equipment, the CO
screw on the AMM is best left alone. Some manuals erroneously state
380 ohms is the set point. Refer to the PDF files to see the error in
this assumption.</FONT></P>

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TO TECHNICAL INDEX</I></B></FONT></A></P>

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