Man, that was an excellent explanation of how the ecu reacts to the O2
sensor.

John

----- Original Message -----
From: "Mark Drillock" <drillock@EARTHLINK.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Friday, October 13, 2006 6:20 PM
Subject: 89 MYSTERY problem!!

> 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!!!
>>
>