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Date:         Sun, 22 Apr 2007 23:19:13 -0700
Reply-To:     Michael Elliott <camping.elliott@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Michael Elliott <camping.elliott@GMAIL.COM>
Subject:      Re: Vexing Subtle Misfire Issues
In-Reply-To:  <6da579340704221717n6c9e91a1t218f9c394d608ebf@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Mellow Yellow also passes smog, but only gets 15mpg. Others seem to have better mileage and I have just assumed that because it passes smog, the engine must be running fine. But your post suggests otherwise! What's the scoop on the voltmeter you've got hooked up to monitor the O2? -- Mike "Rocket J Squirrel" Elliott 71 Type 2: the Wonderbus 84 Westfalia: Mellow Yellow ("The Electrical Banana") 74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano KG6RCR

John Bange typed: > 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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