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Date:         Fri, 13 Oct 2006 15:20:18 -0700
Reply-To:     Mark Drillock <drillock@EARTHLINK.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Mark Drillock <drillock@EARTHLINK.NET>
Subject:      89  MYSTERY problem!!
Comments: To: vanagon <vanagon@MAIL.ABSOLUTEARTS.COM>
In-Reply-To:  <20061013154952.M42898@mail.absolutearts.com>
Content-Type: text/plain; charset=us-ascii; format=flowed

Your problem is at least related to the O2 sensor. That does not have to mean the sensor is bad and replacing the sensor won't necessarily fix it. Disconnecting it would be a stupid idea as well. Please read carefully.

Every time the ECU is powered on, it ignores the O2 sensor for a short while. How long it ignores it depends on various factors. During the time it ignores it, it fires all the fuel injectors by automatic program. After it starts using the O2 sensor, it stops using the automatic program and instead uses the readings it gets from the O2 sensor to determine how long to fire the injectors. If the readings it gets are bad, false, errant, or just plain wrong, it can make big mistakes in firing the injectors. If it starts making big mistakes, the engine will run like crap or not run at all.

Now the hard part. You need to figure out why the ECU is seeing bad, false, errant, or just plain wrong readings from the O2 sensor. It COULD be that nothing whatsoever is wrong with the O2 sensor or its wiring. It COULD be that the fuel mixture is badly wrong on one or more cylinders. If one injector is pouring too much fuel into it's cylinder, the O2 sensor will detect a overfuel problem in the exhaust and report this to the ECU. This is the tricky part. The O2 sensor does not know which injector is causing a problem or even that it is only one of them that is off. The ECU does not know either and it can only order them ALL to put in either more OR less fuel. All the O2 sensor can do is look at the total exhaust mixture combined from ALL injectors and their cylinders. If only ONE of them is way off, the ECU will try to fix it by adjusting ALL of them, until the COMBINED exhaust reading as reported by the O2 sensor is improved. Unfortunately, this results in the 3 good fuel injectors being told to misbehave. This can cause the engine run like crap or not run at all.

The above is just one example of what could be wrong and it could be something else. It is also possible that the O2 reading AS SEEN by the ECU is wrong. This can be caused by problems with the 2 wires that connect the O2 sensor to the ECU. Notice I said TWO WIRES! One is the signal wire that connects directly to the O2 sensor. The other is a special dedicated O2 GROUND wire that connects to the engine far away from the O2 sensor and not seemingly related, even though IT IS! If either of these wires is bad, flaky, or making poor contact, the O2 sensor reading AS SEEN BY THE ECU, will be wrong. The ECU will then adjust all the fuel injectors in a futile attempt to correct what it sees as a bad fuel mixture in the exhaust. This can cause the engine to run like crap or not run at all.

This is exactly the kind of problem the Digitool was made to solve. The Digitool was invented by a list member and allows you to monitor the various sensors while you drive, as seen at the ECU itself!!!

One of the great aspects of the design of the Digitool is that it shows the readings seen by the ECU at the closest point to the ECU, namely on the ECU side of the main fuel injection harness connector. Another great thing it does is allow LIVE Voltage readings, not just ohm readings of wires with the system turned off. It is a shame that the Digitool is no longer being produced. I have often thought that we should have a couple list owned Digitools that could be sent around as needed when list members have hard to find problems.

To recap, your problem is showing up only AFTER the ECU begins using the reading from the O2 sensor. That means there is either a real or imagined problem with the fuel mixture as seen in the exhaust system, as told to the ECU by the O2 sensor.

Mark

vanagon wrote:

> I am reposting this question because NOBODY has been able to fix it! > > I have been completely clueless for almost a year by now about my 89 Vanagon. > I have never had a problem with this it and rebuilt the engine about 3k ago. > It ran beautifully until one day it just stopped running right. > > It idles and accelerates fine when cold. Just as it gets warm it starts > coughing and backfiring through the air intake when you press on the gas like > if you would accelerate from a red light. It sounds like it is not getting the > right mixture. It smells weird. It just won't smoothly accelerate and rev up. > You obviously cannot drive the van this way. It will hesitate like crazy, > sometimes it will backfire through the air intake other times it will rev up > a little. > > The computer will reset itself and it will run fine for about 30 seconds when > quickly turning off the ignition and back on again. IT IS NOT A mechanical > issue because compression and all are fine. > > All electrical test as per official service manual check out fine, computer > checks out fine, new o2 sensor, fuel pressure fine, injectors brand new, coil > brand new, distributor brand new, temp sensors new, grounds to chasis fine, > all resistance tests are fine, relays like fuel, etc. test fine, new plugs, > wires all new, new gasoline as well... It must be electrical but where??? > > Of course when grounding the o2 sensor the motor will run because it enriches > the mixture... but when hooked up correctly it will not. continuity between o2 > sensor and brain is fine, too. resistance of o2 sensor is fine, too. temp > sensor at intake is fine as well, > > Does anyone have an idea what this problem is??? NOBODY seems to know what > the problem is... I wonder if we have a real expert here!!! >


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