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ALIKA T3
Samba Member
Joined: July 30, 2009
Posts: 7165
Location: Honolulu,Hawaii and France
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 Posted: Sat Jul 12, 2025 11:56 pm Post subject: Re: Possible bad O2 sensor? |
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Hi Don!
he did mention replacing the coil a coupe messages up.
Side story about my issue, the van was running acceptably until I parked it for a month.
Mansi finally got his FPR back in stock, so I swapped mine in. It's worse than ever 
So it wasn't the FPR or it's masking an issue.
I'm gonna try installing a Bosch fuel pump and see if that changes anything, but I don't see how it can be good at high speed and not accelerating from idle...
Possessed!!
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Silicone Steering Boots and 930 Cv boots for sale in the classifieds.
Syncro transmission upgrade parts in the Classifieds.
Subaru EJ22+UN1 5 speed transmission
http://www.thesamba.com/vw/forum/viewtopic.php?t=416343
Syncro http://www.thesamba.com/vw/forum/viewtopic.php?t=4...num+gadget |
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AirCooledCurtis
Samba Member
Joined: June 26, 2012
Posts: 99
Location: Portland, OR.
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| Posted: Mon Jul 14, 2025 12:22 pm Post subject: Re: Possible bad O2 sensor? |
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Yep, the only thing I have not replaced is the Temp II, because it tests correctly.
I now have an issue where I don't seem to be getting the o2 bias voltage and the o2 voltage doesn't swing. The o2 signal wire is not grounding out on the shielding, and the wire is not broken from the connector to the ECU pin 2. I have tested 2 o2s on 2 ECU, same deal. I am getting the correct resistance through the ECU when testing the o2 signal wire connector and engine ground, so the wire is making a connection at the ECU.
Is there some wire on the engine harness that tells the ECU to go into a test mode and disregard the o2? |
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RawUmber
Samba Member
Joined: June 23, 2019
Posts: 303
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| Posted: Mon Jul 14, 2025 2:16 pm Post subject: Re: Possible bad O2 sensor? |
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ECU effectively ignores O2 sensor ('open loop') until Coolant Temperature sensor voltage is 'warm' enough for 'closed loop' operation. So, while the O2 sensor voltage may vary during open loop, it won't exhibit that noticeable 'swing' that you may witness during closed loop.
| AirCooledCurtis wrote: |
| Is there some wire on the engine harness that tells the ECU to go into a test mode and disregard the o2? |
A disconnected Coolant Temperature sensor would cause the ECU to disregard O2 sensor. A disconnected O2 sensor would inhibit the 'swing' effect. Many flavors of Vanagon ECU exit closed loop during WOT conditions. |
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DigiMatrix
Samba Member
Joined: April 20, 2010
Posts: 495
Location: Up North Canada
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| Posted: Mon Jul 14, 2025 9:53 pm Post subject: Re: Possible bad O2 sensor? |
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| AirCooledCurtis wrote: |
Yep, the only thing I have not replaced is the Temp II, because it tests correctly.
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So when you tested your Temp II sensor, was it at the sensor or the ECU. I have had the issue where the sensor is fine, but the contacts at the sensor are poor. In my case the van ran like crap and was so rich, it wouldn't restart. I used contact cleaner on the connector and the van fired and ran fine after that. Since I was watching what the ECU was seeing using my Emulator (sends out data from the ECU) at the time I could see what was happening so I went straight to the issue and corrected it.
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'91 Syncro Westfalia
soon to be 2.5 liter stroker WBX 10:1 comp
Digifant ECU with knock sensor and custom
cylinder selective knock control by me |
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AirCooledCurtis
Samba Member
Joined: June 26, 2012
Posts: 99
Location: Portland, OR.
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| Posted: Tue Jul 15, 2025 6:15 am Post subject: Re: Possible bad O2 sensor? |
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| DigiMatrix wrote: |
| AirCooledCurtis wrote: |
Yep, the only thing I have not replaced is the Temp II, because it tests correctly.
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So when you tested your Temp II sensor, was it at the sensor or the ECU. I have had the issue where the sensor is fine, but the contacts at the sensor are poor. In my case the van ran like crap and was so rich, it wouldn't restart. I used contact cleaner on the connector and the van fired and ran fine after that. Since I was watching what the ECU was seeing using my Emulator (sends out data from the ECU) at the time I could see what was happening so I went straight to the issue and corrected it. |
Thanks for replying DigiMatrix! (Side note, I've dumped the bin file from the 022D chip and I'm having a Jetronic tuner look at it and he's having a tough time finding out how to tweak it, if you're willing to share your wisdom that'd be much appreciated! I'm trying to work on if a hot wire MAF conversion)
I tested the temp at the ECU plug. Considering I swapped the harness and the problem remains I don't believe that is the source of this problem. Disconnecting the Temp II makes it really really hard to start. I think today I'm going to depin 2 and 19 and run the o2 wires (Vanagain repair kit) wires outside of the loom to the o2, along with clean the pins and replace the melted ignition switch.
Last night I also unhooked the o2 and tested pin 2 and 19 at the ECU plug... again... and nothing. The ECU is acting like the wire is grounding out, as in it continues to richen sometimes |
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AirCooledCurtis
Samba Member
Joined: June 26, 2012
Posts: 99
Location: Portland, OR.
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| Posted: Tue Jul 15, 2025 2:24 pm Post subject: Re: Possible bad O2 sensor? |
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I got the o2 to swing today, for whatever reason that was hard to get a definitive reading on was the spade on the signal wire. It gets 0.40V now, not 0.45V like it was with the previous harness. Also 12.2V at the alternator when off, but 14.5V at the battery when running. I had gotten low voltage readings at the coil previously haven't rechecked after the harness swap. I don't like
So it actually did NOT run well at all with the o2 unplugged today. It's extremely tempermental I suppose. One day I ripped it around for 20 minutes and did a highway acceleration without issue (with o2 unplugged), and now today it's running like garbage, between those 2 times only the o2 wire connector had changed.
Everything has been replaced except the temp II, some parts more than once.
The temperament of it, to me, shows that it's not something mechanical like the camshaft off a tooth, or a bad valve seal that doesn't show up on a hot compression test. There's a point where it clears up instantly and runs fine. Somehow the ECU is calculating the wrong injector time, and then suddenly, figures it out.
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do.dah
Samba Member
Joined: August 27, 2015
Posts: 792
Location: Washington
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| Posted: Tue Jul 15, 2025 5:32 pm Post subject: Re: Possible bad O2 sensor? |
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| Didn't read the whole thread; you say you've replace everything but the ts2, that might be a clue, but, did you try a different ecu? Your yr van would originally have an older version of the ecu, that has a habit of going bad and or solder failures.. |
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AirCooledCurtis
Samba Member
Joined: June 26, 2012
Posts: 99
Location: Portland, OR.
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| Posted: Wed Jul 16, 2025 7:30 am Post subject: Re: Possible bad O2 sensor? |
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| do.dah wrote: |
| Didn't read the whole thread; you say you've replace everything but the ts2, that might be a clue, but, did you try a different ecu? Your yr van would originally have an older version of the ecu, that has a habit of going bad and or solder failures.. |
Yep, I've tried both a 022 and silver label 022D from much later.
I threw in a Temp II this morning. Washer was clean, everything looked fine, no change. After about 20 seconds of idling it wont rev. Revs perfectly fine when stone cold and has no problem starting and finding idle.
A complete list of everything changed:
ECU, wiring harness, battery, ignition coil, distributor (timed using max advance @ 3500 rpms), cap, rotor, spark plug wires, spark plugs, fuel pressure regulator, fuel pump, fuel filter, fuel lines, fuel tank resealed, injectors cleaned and flow tested by Injector-Rehab, valves adjusted, 2 bent pushrods replaced, Idle Stabilizer Control Unit, o2 sensor, multiple AFMs, alternator rebuilt, battery wiring suspicious areas fixed, replaced the missing cylinder head to chassis ground strap, and now the Temp II sensor.
That is every single part associated with the ignition and fuel injection minus IAC.
Compression is poorer than I'd like at 120psi evenly across cylinders but as I've driven it I believe it's come up, the van had been dormant a long time.
So I'm at the end of the road here. It's unfixable, and 100% not mechanical.
I'm going to use the old wiring harness to convert to Bosch LH-Jetronic 2.2. Both use 2.5 bar FPRs, both injectors flow the same rate, both use the same o2 and Temp II sensor (Syncro) part numbers. I will have to find a solution for the TPS since Digifant uses the same signal wire for idle and WOT and LH uses independent wires, I may not utilize the WOT part of the circuit, LH2.2 IAC is the same L shape, and most importantly... a hot wire mass airflow sensor. I'm going to use Digijet parts for the ignition. I already have piles of the LH parts since I primarily work on redblock Volvos. |
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zimflux
Samba Member
Joined: September 30, 2015
Posts: 226
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| Posted: Wed Jul 16, 2025 7:43 am Post subject: Re: Possible bad O2 sensor? |
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Darn. I really want to see this solved! Please don't give up.
-Perhaps get your hands on one of these testers to see life data: https://www.shoebox-electronix.com/
-Have you posted in other Facebook groups?
-100% no exhaust leaks?
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84 Westy w/ 2.1 MV |
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AirCooledCurtis
Samba Member
Joined: June 26, 2012
Posts: 99
Location: Portland, OR.
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| Posted: Wed Jul 16, 2025 12:12 pm Post subject: Re: Possible bad O2 sensor? |
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100% no exhaust leaks. When I plug the tail pipe the engine goes quiet and then dies.
I've had another '86 that ran totally fine with the cylinder 1&3 pipe cracked in half. |
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zimflux
Samba Member
Joined: September 30, 2015
Posts: 226
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| Posted: Thu Jul 17, 2025 5:03 am Post subject: Re: Possible bad O2 sensor? |
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| AirCooledCurtis wrote: |
100% no exhaust leaks. When I plug the tail pipe the engine goes quiet and then dies.
I've had another '86 that ran totally fine with the cylinder 1&3 pipe cracked in half. |
Plug with your hand or something more substantial?
From another forum:
*Look for something so obvious you didn't see it. Like a carbon track on the ignition coil. The resistance in the combustion chamber changes when you hit the throttle. It becomes more difficult for the spark to jump the plug gap, so the high voltage current takes an easier path. Don't always ASSume it's an injection issue."
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84 Westy w/ 2.1 MV |
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AirCooledCurtis
Samba Member
Joined: June 26, 2012
Posts: 99
Location: Portland, OR.
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| Posted: Thu Jul 17, 2025 5:21 am Post subject: Re: Possible bad O2 sensor? |
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| zimflux wrote: |
| AirCooledCurtis wrote: |
100% no exhaust leaks. When I plug the tail pipe the engine goes quiet and then dies.
I've had another '86 that ran totally fine with the cylinder 1&3 pipe cracked in half. |
Plug with your hand or something more substantial?
From another forum:
*Look for something so obvious you didn't see it. Like a carbon track on the ignition coil. The resistance in the combustion chamber changes when you hit the throttle. It becomes more difficult for the spark to jump the plug gap, so the high voltage current takes an easier path. Don't always ASSume it's an injection issue." |
My hand, but I've done that test on multiple vehicles and I know what it would sound like if it was leaking.
I have long considered it could be ignition related, cylinder fill (VE) is highest at peak engine torque, and looking at the dyno results that GoWesty have posted, peak torque is 3000-3500, and the engine runs fine at WOT in those areas. Immediately off idle, with 1/3 throttle, it doesn't act like spark blow out. I've checked for moisture in the distributor cap, swapped spark plugs out, and all of the ignition related components are different from when I got the van. I am not running the little guard on the distributor cap, I know that VW put that there for noise issues etc. I've seen hall effect sensors give a poor wave form that seemed to clear up with RPM, that's something that I'm thinking now. Like at idle the rpm is stable enough to see the wave form, then on a throttle blip at low RPM is drops signal. |
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do.dah
Samba Member
Joined: August 27, 2015
Posts: 792
Location: Washington
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| Posted: Thu Jul 17, 2025 5:27 am Post subject: Re: Possible bad O2 sensor? |
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Oh yeah! I forgot about this diagnoses trick. In a garage/at night or large blanket to make the engine compartment very dark, run the engine and look for sparks or a faint blueish glow of electrical 'bleed' thru.
CAREFULLY wiggle wires.
Use a flashlight to see where you're putting your hand, then turn flashlight off.
WATCH OUT FOR MOVING SPINNING PARTS!!!
| zimflux wrote: |
| AirCooledCurtis wrote: |
100% no exhaust leaks. When I plug the tail pipe the engine goes quiet and then dies.
I've had another '86 that ran totally fine with the cylinder 1&3 pipe cracked in half. |
Plug with your hand or something more substantial?
From another forum:
*Look for something so obvious you didn't see it. Like a carbon track on the ignition coil. The resistance in the combustion chamber changes when you hit the throttle. It becomes more difficult for the spark to jump the plug gap, so the high voltage current takes an easier path. Don't always ASSume it's an injection issue." |
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zimflux
Samba Member
Joined: September 30, 2015
Posts: 226
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| Posted: Thu Jul 17, 2025 9:13 am Post subject: Re: Possible bad O2 sensor? |
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| AirCooledCurtis wrote: |
| My hand, but I've done that test on multiple vehicles and I know what it would sound like if it was leaking. |
Interesting, perhaps because of the IAC? I can't stall my engine with my hand FWIW.
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84 Westy w/ 2.1 MV |
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xflyer
Samba Member
Joined: June 15, 2006
Posts: 311
Location: SOCAL
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| Posted: Yesterday 4:13 pm Post subject: Re: Possible bad O2 sensor? |
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Has the spark quality been checked? Pull the big lead wire from the center of the distributor cap. Hold it near a clean metal part that is grounded to the chassis/engine. Have someone crank the engine. A good spark will jump 10-12mm (or more) and be blue/white color. If the spark jumps 5-6mm and is yellow/orange that is not good.
According to the thread most all of the ignition parts have been tested or replaced on the OPs Vanagon, so if there is inadequate spark the ECU is suspect. The spark control circuit in the Digifant ECU is a common failure point, and it does not necessarily stop working completely. That is no spark at all.
I went through this on my 89 Vanagon camper with auto trans. I was chasing engine running problems for at least a year. It would often hesitate on acceleration and sometimes idle rough. I had checked/replaced spark plugs, plug wires, adjusted valves, checked for vacuum leaks. Sometimes it seemed to run better for a while.
Eventually the engine quit and would not restart. A test of the spark revealed the 5-6mm red/orange spark. I did all the ignition checks in the Bentley and ended up replacing the ECU with a "tested good" one. Engine started immediately and ran properly for the first time in a long time.
Ordered a "rebuilt/repaired" ECU and installed.
BTW I have worked on vehicles, mostly German, since the early 1970s and went to some of the Bosch EFI training schools. This poor quality spark was a new one for me, on engines with electronic ignition. The first failure of an ECU ignition circuit, that I saw, was in the mid 1990s on a customer's Vanagon. In that case there was no spark at all. There have been a few others since then. All with zero spark. Also I bought the "rebuilt/repaired" ECU from VanAgain.
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1989 Campmobile, 1984 7 passenger beater Vanagon
Both with cool A/C |
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