Date: Sun, 22 Apr 2007 17:17:00 -0700
Reply-To: John Bange <jbange@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: John Bange <jbange@GMAIL.COM>
Subject: Vexing Subtle Misfire Issues
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So now that I'm gainfully employed and no longer struggling against the
ravages of pneumonia, I think I'm ready to attack my maddening misfire issue
again. Symptoms are fairly subtle, but consistent. Every 3/4 second or so,
one of the cylinders fails to fire. The interval between misfires is enough
that I can't tell if it's one cylinder in particular-- the plugs all look
clean. The misfire is such that you can hardly tell it's happening unless
you have a way to monitor the O2 sensor. I have one of those 1v LCD
voltmeter gauges on my dash, so I can watch it do it. About 100 times a
minute the voltage on the O2 sensor slams to 0v, indicating the fuelair
charge in one of the cylinders has failed to ignite, flooding the exhaust
with unconsumed oxygen. The problem is that the ECU sees this as "too lean"
and pushes the mixture rich. It then works it's way down from "too rich"
towards stoichiometric, but just as it reaches that, BAM, misfire again,
back to rich. Subsequently, It basically always runs rich. Not rich enough
to hurt the cat (passed smog easily after doing this for a year), but rich
enough to hurt my gas mileage. I'm getting 14 city, 16 highway, where I used
to get 19-21.
What I know it ISN'T:
-Engine. A Boston Bob with 10K miles. Compression good on all cylinders.
-O2 sensor. Sensor is new, misfire happens even when ECU is in open loop,
and wideband O2 sensor indicates same misfire.
-Temp II. New sensor. Checks OK.
-ECU. I have a spare ECU, does the same thing.
-AFM. Swapping in spare AFM yields same.
-Coil. Good spare coil yields same result.
-Connectors & Wiring. Checked EVERYTHING from ECU plug to endpoints. All
good.
-Plugs/plug wires/cap/rotor. Replaced, same misfire.
So now I'm to the point where my diagnostic skills leave me stumped and I'm
ready to start "changing tires one at a time to see which one is flat". The
question is, which "tire" should I start with? Hall sender? Injectors? Fuel
pump? Fuel pressure regulator? I think I'm leaning towards Injectors. A
clogged injector might be running one cylinder lean enough to misfire when
the ECU gets the other three near stoich... Anyone have any thoughts on the
matter? I am all thunk out.
--
John Bange
'90 Vanagon - "Geldsauger"
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