My trusty Westfalia started having drivability problems the other day. It would run like it was missing and then it would suddenly clear up. Run well for a while and then it it would start all over again. Then a day later it started hesitating, would get really rich and backfire. If I turned the key 'off' and then 'on' again while still rolling, it would run as if nothing ever happen for about 2 minutes and then it would start all over again.
I brought it to my mechanic who specializes in volvos and old British stuff (my other vehicle is a volvo) and after some checking came up with the following diagnosis.
the voltage coming off the O2 sensor wire is over 9 volts instead of a trace amount with the key in the 'on' position engine not running. Needs a new ECU. I supplied him with another ECU that I have which I was assured was in working condition. This time the sensor wire voltage was 8.6 volts. I called the guy I got the ECU from a couple of years ago and he said it was probably the ground on the O2 sensor wire shielding. Mechanic figured this would create a 'dead short' and zero voltage.
I called somebody else would specializes in vanagon many hours from me about a rebuilt ECU and he suggested checking the grounds because these units hardly ever go bad. Have any of you got any thought on this.
Well, the rich running and such certainly sounds like an O2 sensor problem, and shorting of the shield to the signal wire is a known problem as these harnesses age, and it would explain the intermittent symptoms. (Regardless of voltage measurement, shorting the sensor lead to ground does cause a rich condition.)
What I'd try as a test would be to clip the sensor signal wire from the ECU connector and see how it runs. With an open circuit to the O2 the engine should run fine if this is the problem. If so you can run a new wire to the O2 as a final fix. (I believe the O2 signal wire goes to pin 2 on this ECU.)
I did have one ECU fail on an 87, but I'd agree ECU failures are very rare.
thanks tom for your info.
the O2 sensor was bench tested with heat and a voltage meter and responded correctly.
As I said at the beginning, with the O2 sensor disconnected it reads around 8-9 volts with key turned on but not running. with the O2 sensor disconnected, the van runs really poorly. with the sensor connected and engine running, it starts at about 0.3 volts and keeps rising, while the mixture keeps getting richer. Turn the key off and on immediately and the cycle begins again at 0.3 volts and gets higher until it won't run over a period of about 1 1/2 minutes.
Since the voltage starts out low each time and rises quickly and the cycle begins again, and the O2 sensor works (tested and is 4 years old), could it be that the ECU is actually bad. I know that the grounds can be bad on these aging beasts, but wouldn't a bad ground cause bad but stable voltage not rising voltage?
The ECU ground is a good tip. Recently found a Mexican aircooled Beetle (Digifant) with similar symptoms, and it was indeed a bad ground to the ECU that caused it. In that case it was poor contact at the ECU connector to the harness.
thanks for your input. I now have to either convince my mechanic, who has served me well for years, to re-check the grounds or possibly find another shop. I myself an at best a backyard mechanic. I can usually remove and replace, but have real difficulty with electrical.
I finally got our Westy back from the shop and decided to check grounds before sending away for a rebuilt ECM.
If anyone is still looking at this thread, could somebody who knows please let me know how many grounds there are and where to look for them? (I've heard numbers up to 4.) I can probably find them easily enough on the Bentley wiring diagram, but finding them in person would be a lot easier with some direction. Also, does reattaching the box to its mount do any grounding or can I leave it loose until I have it running OK? Also any hints on finding the common ground problem areas?